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Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting

Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in... acoustics Article Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting 1 2 , 1 1 , 3 4 Anica Otilia Placinta , Felix Borleanu * , Emilia Popescu , Mircea Radulian and Ioan Munteanu Department of Research and Development Innovation in Earth Sciences, National Institute for Earth Physics, 12 Calugareni Street, P.O. Box MG2, Magurele, 077125 Ilfov, Romania; anca@infp.ro (A.O.P.); epopescu@infp.ro (E.P.); mircea@infp.ro (M.R.) National Data Center, National Institute for Earth Physics, 12 Calugareni Street, P.O. Box MG2, Magurele, 077125 Ilfov, Romania Academy of Romanian Scientists, 54 Splaiul Independentei, 050094 Bucharest, Romania Faculty of Geology and Geophysics, University of Bucharest, 6 Traian Vuia Street, 020956 Bucharest, Romania; ioan.munteanu@gmail.com * Correspondence: felix@infp.ro Abstract: Romanian seismicity is mainly confined to the Eastern Carpathians Arc bend (ECAB), where strong subcrustal earthquakes (magnitude up to 7.9) are generated in a narrow lithospheric body descending into the mantle. The seismic activity in the overlying crust is spread over a larger area, located mostly toward the outer side of the ECAB. It is significantly smaller than subcrustal seismicity, raising controversies about possible upper mantle-crust coupling. A significant earthquake sequence took place in the foreland of the ECAB triggered on 22 November 2014 by a mainshock of Citation: Placinta, A.O.; Borleanu, F.; magnitude 5.7 (the greatest instrumentally recorded earthquake in this region) located in the lower Popescu, E.; Radulian, M.; Munteanu, crust. The mainshock triggered a significant increase in the number of small-magnitude events spread I. Earthquake Source Properties of a over an unusually large area in the ECAB. The paper ’s goal is to compute the source parameters Lower Crust Sequence and of the earthquakes that occurred during the aforementioned sequence, by empirical application of Associated Seismicity Perturbation in the SE Carpathians, Romania, Green’s function and spectral ratio techniques. Fault plane solutions are determined using multiple Collisional Setting. Acoustics 2021, 3, methods and seismicity evolution at regional scale is investigated. Our results highlight a still active 270–296. https://doi.org/10.3390/ deformation regime at the edge of the EE Craton, while the source parameters reveal a complex acoustics3020019 fracture of the mainshock and a very high-stress drop. Academic Editor: C. W. Lim Keywords: earthquake sequence; source parameters; seismic source scaling; SE Carpathians foredeep Received: 12 March 2021 Accepted: 8 April 2021 Published: 19 April 2021 1. Introduction The crustal seismicity developed in front of the SE Carpathians bending zone (SEC) is Publisher’s Note: MDPI stays neutral spread over the entire area between the Neogene Trotus ¸ and Intra-Moesian faults (Figure 1). with regard to jurisdictional claims in Crustal earthquakes are generated into a complex structure with significant lateral inhomo- published maps and institutional affil- geneities involving different kinematics [1]. This is also reflected in the variability of the iations. focal mechanisms that show consistent stress regime with different localization: normal faulting on the eastern and southern flank of the Focsani ¸ Basin (FB), thrust faulting in the area adjacent to the Vrancea subcrustal earthquakes and strike-slip solutions preferably observed along the Peceneaga-Camena Fault [2]. The crustal seismicity in the SEC partly Copyright: © 2021 by the authors. overlaps with the intermediate-depth earthquakes generated in a depth range of 60–180 km, Licensee MDPI, Basel, Switzerland. beneath the Vrancea Zone (Figures 1 and 2). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Acoustics 2021, 3, 270–296. https://doi.org/10.3390/acoustics3020019 https://www.mdpi.com/journal/acoustics Acoustics 2021, 3 271 Acoustics 2021, 3 FOR PEER REVIEW   2  Figure 1. (a) Main geological units within the epicentral area (modified after [3]) and epicenters distribution (blue dots) of  Figure 1. (a) Main geological units within the epicentral area (modified after [3]) and epicenters distribution (blue dots) of the seismic events associated with the 2014 Mărăşeşti sequence. The inset map shows the study area (pink square) dis‐ the seismic events associated with the 2014 Mărăse ¸ sti ¸ sequence. The inset map shows the study area (pink square) displayed played at the regional scale. The red line represents three vertical cross‐sections crossing the epicentral area. (b) Seismic  at the regional scale. The red line represents three vertical cross-sections crossing the epicentral area. (b) Seismic activity activity at the SE Carpathians Arc bend according to the ROMPLUS catalogue (between 2010 and 2014, [4]) colored func‐ at the SE Carpathians Arc bend according to the ROMPLUS catalogue (between 2010 and 2014, [4]) colored function of tion of depth (intermediate‐depth events in black and crustal earthquakes within the color scale—right side), seismic sta‐ depth (intermediate-depth events in black and crustal earthquakes within the color scale—right side), seismic stations tions distribution (blue triangles) and neighboring cities. The white dashed lines represent the local fault system; the black  distribution (blue triangles) and neighboring cities. The white dashed lines represent the local fault system; the black line line shows the Pericarpathian Fault. The inset map shows a zoom of the epicentral area. The red star shows the mainshock  shows the Pericarpathian Fault. The inset map shows a zoom of the epicentral area. The red star shows the mainshock epicenter.  The  abbreviations  are  as  follows:  VR—Vrancea  Zone,  IMF—Intra‐Moesian  Fault,  PCF—Peceneaga‐Camena  epicenter Fault, AMRR . The—Amara, abbreviations  PGOar Re—Pogoanele, as follows: VR—V  TLB—Topalu rancea Zone,  seismi IMF—Intra-Moesian c stations.  Fault, PCF—Peceneaga-Camena Fault, AMRR—Amara, PGOR—Pogoanele, TLB—Topalu seismic stations.   Acoustics 2021, 3 272 Acoustics 2021, 3 FOR PEER REVIEW   3  Figure 2. (a) Epicenters distribution (colored as a function of local magnitude ML) within the epicentral area (0.75 deg.  Figure 2. (a) Epicenters distribution (colored as a function of local magnitude M ) within the epicentral area distance around the mainshock) for the seismic activity recorded during a time interval of 100 days (22 November 2014–2  (0.75 deg. distance around the mainshock) for the seismic activity recorded during a time interval of 100 days March 2015) from the mainshock (red star) occurrence; (b) hypocenters distribution displayed in the same area and for the  (22 November 2014–2 March 2015) from the mainshock (red star) occurrence; (b) hypocenters distribution displayed in the same time interval mentioned above versus latitude and function of ML and (c) versus longitude and function of ML.  same area and for the same time interval mentioned above versus latitude and function of M and (c) versus longitude and function of M . The intermediate‐depth seismicity is located in a narrow seismogenic volume which  The intermediate-depth seismicity is located in a narrow seismogenic volume which descends almost vertically in the mantle, able to generate 2–3 strong events (Mw ≥ 7) per  descends almost vertically in the mantle, able to generate 2–3 strong events (Mw  7) per century. The seismicity in the overlying crust is limited to moderate‐size events (Mw < 6)  century. The seismicity in the overlying crust is limited to moderate-size events (Mw < 6) and spreads over a larger area following some specific alignments. One of the significant  and spreads over a larger area following some specific alignments. One of the significant seismically active faults placed in front of the SEC is the Peceneaga‐Camena (PCF) fault  seismically active faults placed in front of the SEC is the Peceneaga-Camena (PCF) fault zone  and  related  deformation,  which  delineates  (Figure  1)  the  boundary  between  the  zone and related deformation, which delineates (Figure 1) the boundary between the Moesian Platform (MP) and North Dobrogea Orogen (NDO, [5]). Along its path towards  Moesian Platform (MP) and North Dobrogea Orogen (NDO, [5]). Along its path towards NW from the Black Sea Basin to the SEC, the fault shows two segments with distinct seis‐ NW from the Black Sea Basin to the SEC, the fault shows two segments with distinct seismic mic activities: (1) from the Black Sea Basin to the Danube River, and (2) from the Danube  activities: (1) from the Black Sea Basin to the Danube River, and (2) from the Danube River River to the NE edge of the Vrancea epicentral area. The first segment is less active with a  to the NE edge of the Vrancea epicentral area. The first segment is less active with a stress stress regime of thrusting faulting [6,7], while the second segment displays a significant  regime of thrusting faulting [6,7], while the second segment displays a significant seismic seismic activity with predominant strike‐slip faulting along the major fault line and its  activity with predominant strike-slip faulting along the major fault line and its satellites, satellites, oriented SE—NW. Towards the NW edge, where the PCF intersects the Peri‐ Carpathian Fault, the seismicity configuration becomes more complex (Figure 1).     Acoustics 2021, 3 273 oriented SE—NW. Towards the NW edge, where the PCF intersects the Peri-Carpathian Fault, the seismicity configuration becomes more complex (Figure 1). Several studies [7–11] indicate this area as the place where the descending lithospheric slab started to break off, beneath the SEC, at the time of the continental collision between the Carpathian (as part of Tisza-Dacia) plate overriding the East-European, Scythian and Moesian plates. In this complex tectonic framework, a significant earthquake sequence occurred in November 2014 close to Mărăse ¸ sti ¸ city. According to the Romanian earthquake catalogue ROMPLUS [4] permanently updated by the Romanian Data Centre (RONDC) of the Na- tional Institute for Earth Physics (NIEP), the mainshock occurred on 22 November 2014 at 19:14 (GMT), located in the lower crust (H = 41 km), in the range of the Moho disconti- nuity [12] and had a local magnitude (M ) of 5.7. The occurrence of this sequence in the Mărăse ¸ sti ¸ area is the most significant episode of seismic energy release in this region since instrumental recordings were used to monitor the seismic activity in Romania. A similar event is mentioned in the ROMPLUS catalogue but occurred more than one century ago (01.03.1894, M = 6.1). The poly-kinetic character of these events has been pointed out by previous stud- ies [13,14] as well as through the analysis of several significant earthquake sequences [15,16] generated along the Eastern Carpathians [17–19] However, the 2014 Mărăsesti seismic se- , , quence differs from past ones, because of the location of a large part of the hypocenters close to the crust-mantle discontinuity and because of the subsequent increase of seismic activity spread over an unusually large area in the adjacent regions (Figures 1 and 2). The analysis of the source parameters for the earthquakes of the seismic sequence will certainly bring relevant elements to the understanding of the tectonic processes and of the physical mechanism driving seismicity in the region. Moreover, the seismic hazard assess- ment strongly depends on estimation accuracy. One of the most debated parameters in such analysis is the stress drop. This gives indications about the amount of released energy and also may estimate the destructive potential of an earthquake. A major disadvantage is given by the estimation of accuracy since its uncertainties are rather high, due to the corner frequency determinations that could increase or decrease the stress drop value by at least three times [20]. Large earthquakes with magnitudes of M > 6 occurred in inter-plate or intraplate regions [21] and pointed out stress drops in a range of 3–10 MPa, with an increasing trend for intraplate earthquakes [22,23]. For crustal earthquakes occurring in the East- ern Carpathians’ foredeep, previous studies [24,25] highlight stress drops in a range of 2.5–50 MPa. Another notable feature highlighted by previous research [26–28] reveals simi- lar properties for small and large earthquakes, changing only in scale, which would imply a constant stress drop for the whole range of magnitudes. However, several studies have shown [20,29] that for small earthquakes (M < 3) there is an increasing trend towards stress drop at the seismic moment. One of the features that could cause this behavior is the strong attenuation effect within the upper crust, which acts as a filter, removing the high frequencies (f > 10 Hz) and limiting the corner frequencies’ resolution [30], especially for small earthquakes, leading to an underestimation of the stress drop. In this study, we determine source parameters of 21 earthquakes that occurred during the 2014 Mărăse ¸ sti ¸ sequence (22 November 2014–1 February 2015), as well as for several events that have occurred at higher distances or were delayed in time, but considered as having been triggered by the aforementioned seismic sequence. To achieve this goal, we apply empirical Green’s function (EGF) and spectral ratio (SR) techniques, since they allow suitable constraints independent of the structure between the source and receiver, which is difficult to know with sufficient accuracy. In addition, we determine the fault plane solutions for some of these events and investigate the evolution of seismic activity in the region to draw an interpretation in connection with the tectonic structures highlighted by the previous studies. Acoustics 2021, 3 274 The previous investigations dealing with the 2014 Mărăs ¸es ¸ti sequence limited themselves to the evaluation of the macro-seismic effects [31] and the computation of the focal mechanisms for the main event and 32 aftershocks using P-wave polarities and amplitude ratios [32]. Sequence Description The seismic activity in front of the SEC bending zone along the PCF up to the north- western edge at the contact with the Vrancea epicentral area and Scythian Platform has continuously developed in time with sporadic weak-to-moderate earthquakes or clusters of events (Figure 1). These events are probably related to some extent to the geodynamics in the mantle beneath the Vrancea region. However, this is still a controversial issue [6,7,33] and therefore understanding the interdependence of the tectonic stress in the mantle and coupling and crustal seismicity requires further investigation. The mainshock of 22 November was followed by 230 aftershocks located by the RONDC using the LOCSAT routine [34] embedded within the Antelope software and using IASP91 as 1-D global velocity model [35]. A noteworthy increase of seismicity in the region was recorded after the mainshock for at least three months which is significantly more than the duration typical for the aftershock activity of moderate-size mainshocks. The aftershocks were numerous but unusually weak in size relative to the magnitude of the mainshock (Figure 2). A single stronger event was recorded on 7 December 2014 (M = 4.5), and only two events with M > 3 were recorded on 22 November 2014 (M = 3.1) L L L and 19 January 2015 (M = 3.8). The seismic activity generated after the mainshock occurrence (22 November 2014) manifests through a significant increase in the seismicity rate first in the Mărăse ¸ sti ¸ area and second over an extended region to the south and to the north (the Râmnicu Sărat and Adjud areas, see Figure 2). The triggered seismicity developed over a large region and for a long-time interval. 2. Methods and Data To determine the source parameters of the earthquakes that occurred during the 2014 seismic sequence close to the Mărăse ¸ sti, ¸ and also for the events triggered by this seismic activity, we searched in the ROMPLUS catalogue (November 2014–July 2015) for clusters of events (co-located events with similar waveforms). Each cluster has one or several main events and associated EGFs. The selection of clusters is carried out in two steps. To select the potential EGF events, we started with the mainshock (M1/5.7 in Figure 3) and searched among the events within a magnitude able to solve the source parameters within the frequency range of the available data. First, for each main event (M) we searched in the aforementioned catalogue for potential EGF events if the inter-hypocenter distance is less than 0.3 deg. and the difference in magnitude around one magnitude unit or more. As a second step, we applied the waveform cross-correlation technique to select as EGF events those with large cross-correlation coefficients (CC  0.7). A high cross-correlation for a pair of events is considered to consistently reflect the waveform similarity among them [28,36]. For the waveform correlation analysis, we used time windows based on the magnitude of the M [21], a length of 0.5–1 s being enough for the small size events. The waveforms are bandpass filtered to an appropriate frequency band, using a two-pole Butterworth filter. We low-pass filter both the M and each potential EGF at a frequency related to the expected corner frequency of the M. This is because large and small earthquakes are not expected to cross-correlate well at high frequencies (e.g., [36]). We applied cross-correlation separately for P and S waves. Acoustics 2021, 3 275 Table 1. Selected main (bold characters) and associated EGF events. Date OriginTime No Latitude ( N) Longitude ( E) Depth (km) M DDMMYYYY hh:mm:ss M1 22112014 19:14:17 45.8683 27.1517 41 5.7 M2 22112014 20:30:56 45.8530 27.1859 36 3.1 M3 07122014 21:04:05 45.8836 27.1714 41 4.5 M4 12012015 06:08:31 45.5420 27.0448 19 4.2 M5 19012015 23:53:07 45.8782 27.1483 40 3.8 M6 29062015 22:20:56 46.0114 27.1754 19 4.0 1 22112014 19:27:39 45.8688 27.1185 40 2.0 2 22112014 20:24:47 45.8626 27.1644 34 2.8 3 22112014 20:30:56 45.8530 27.1859 36 3.1 4 23112014 01:14:39 45.8819 27.1610 35 2.5 5 23112014 02:21:05 45.8509 27.1852 34 2.5 6 23112014 04:01:58 45.8230 27.1750 31 2.6 7 23112014 05:27:58 45.8450 27.1889 35 2.4 8 23112014 23:22:49 45.8477 27.1852 33 2.3 9 24112014 00:45:06 45.8700 27.1660 34 2.4 10 25112014 01:52:25 45.8480 27.1612 39 3.2 11 07122014 21:04:05 45.8836 27.1714 41 4.5 12 14122014 17:24:47 45.6054 27.1018 14 3.1 13 14122014 18:24:34 45.6036 27.1164 16 2.6 14 18012015 22:41:40 45.4962 27.0243 31 1.8 Acoustics 2021, 3 FOR PEER REVIEW   6  15 19012015 23:53:07 45.8782 27.1483 40 3.8 16 20012015 02:52:46 45.8850 27.1585 33 2.2 17 01072015 04:34:24 46.0217 27.1893 17 3.2 Figure 3. Hypocenters distribution of main (filled dots) and Green’s function events (empty cir‐ Figure 3. Hypocenters distribution of main (filled dots) and Green’s function events (empty circles). cles). The colors of the main event (M) match the colors of empirical Green’s function (EGF)  The colors of the main event (M) match the colors of empirical Green’s function (EGF) events. The events. The M are counted from M1 to M6 and their magnitudes (ML) are written to the right side.  M are counted from M1 to M6 and their magnitudes (M ) are written to the right side. Note that Note that some EGF events can be associated with more M. M2, M3 and M5 are selected both as M  some EGF events can be associated with more M. M2, M3 and M5 are selected both as M and EGF and EGF (see Table 1).  (see Table 1). We found 6 individual master events (M1 to M6 given below) and plot them in Figure  3 as dots filled in different colors and 17 EGF (empty circles, with the outline colors match‐ ing one of the master events, see Figure 3), meeting our requirements imposed by the  method.  M1: 22 November 2014, 19:14, ML = 5.7.  M2: 22 November 2014, 20:30, ML = 3.1.  M3: 7 December 2014, 21:04, ML = 3.1.  M4: 12 January 2015, 06:01, ML = 4.2.  M5: 19 January 2015, 23:53, ML = 3.8.  M6: 29 June 2015, 22:20, ML = 4.0.  To determine the source parameters of the selected events (Figure 3 and Table 1),  only the broadband recordings of the RSN were considered.   Table 1. Selected main (bold characters) and associated EGF events.  Date  OriginTime  No  Latitude (°N)  Longitude (°E)  Depth (km)  ML  DDMMYYYY  hh:mm:ss  M1  22112014  19:14:17  45.8683  27.1517  41  5.7  M2  22112014  20:30:56  45.8530  27.1859  36  3.1  M3  07122014  21:04:05  45.8836  27.1714  41  4.5  M4  12012015  06:08:31  45.5420  27.0448  19  4.2  M5  19012015  23:53:07  45.8782  27.1483  40  3.8  M6  29062015  22:20:56  46.0114  27.1754  19  4.0  1  22112014  19:27:39  45.8688  27.1185  40  2.0  2  22112014  20:24:47  45.8626  27.1644  34  2.8    Acoustics 2021, 3 276 We found 6 individual master events (M1 to M6 given below) and plot them in Figure 3 as dots filled in different colors and 17 EGF (empty circles, with the outline colors matching one of the master events, see Figure 3), meeting our requirements imposed by the method. M1: 22 November 2014, 19:14, M = 5.7. M2: 22 November 2014, 20:30, M = 3.1. M3: 7 December 2014, 21:04, M = 3.1. M4: 12 January 2015, 06:01, M = 4.2. M5: 19 January 2015, 23:53, M = 3.8. M6: 29 June 2015, 22:20, M = 4.0. To determine the source parameters of the selected events (Figure 3 and Table 1), only the broadband recordings of the RSN were considered. The fault plane solution has a significant relevance since it is directly used to determine the stress regime of a region. To obtain a high accuracy in determining the focal mechanisms, we selected only the events whose recordings (broadband or short-period stations) allowed us to read with enough accuracy the P-waves’ first motion at a minimum of 12 stations. Two methods provided by the SEISAN algorithm [37] are applied. Both FOCMEC [38] and HASH [39,40] algorithms use the first P-wave polarities together with the amplitudes measured on all three components (vertical, transversal and radial). The FOCMEC method is more rigorous since it removes the erroneous polarities and ratios above an imposed limit (we adopted 0.3 in our case). HASH uses weights for the polarity and amplitude ratio errors and provides averaged values, without eliminating the stations with large errors. For both methods, we filtered the seismograms, removing the frequencies above the corner frequency, and measured the maximum amplitudes at intervals of maximum 2s starting at first P, then respectively S arrivals. The spectral ratios and empirical Green’s function methods have been used in various areas of the world [36,41–46] showing reliable results in retrieving source parameters. These are efficient in eliminating the path, site and instrument effects, which distort to a large extent the ground motion observed at the surface when pairs of earthquakes are available and satisfy the conditions imposed for the selection of the events. Examples of earthquake pairs recorded by the Topalu (TLB) station are given in Figure 4. The SRs are approximated by a nonlinear least-squares procedure with theoretical spectral ratios, depending mostly on the source properties. The fit parameters are the ratios of the low-frequency asymptotes and corner frequencies of the large and small shocks, respectively. The least-squares best fit is estimated by iteration from an initial guess using a simplex algorithm. The relationship which best approximates the Fourier spectral ratio R for a pair of collocated events is: 1/2 M G W [1 + ( f / f ) ] 0 C R f = (1) ( ) 1/2 G M W [1 + ( f / f ) ] 0 C M G where W , W are the low-frequency spectral amplitudes of main and EGF earthquakes, 0 0 M G and f , f are the corresponding corner frequencies. We used the hypothesis of the c c Brune’s model [47,48] with spectral decay at the high frequencies, . The ratio of the low-frequency spectral levels is equal to the ratio of the seismic moments for the collocated earthquakes if they have similar fault plane solutions. Acoustics 2021, 3 277 Acoustics 2021, 3 FOR PEER REVIEW   9  Figure 4. Waveforms recorded by Topalu (TLB) seismic station, for: (a) M (red) of 22 November 2014, 19:14; (b) EGF (green)  Figure 4. Waveforms recorded by Topalu (TLB) seismic station, for: (a) M (red) of 22 November 2014, 19:14; (b) EGF (green) of 22 November 2014, 20:30; (c) M (red) of 7 December 2014, 21:04; (d) EGF (green) of 22 November 2014, 20:24.  of 22 November 2014, 20:30; (c) M (red) of 7 December 2014, 21:04; (d) EGF (green) of 22 November 2014, 20:24. The Thesize  seism ofithe c moment rupturse fo ar r ea EGF is events computed,  were taking determine intod consideration using the spectthe ral ra cirti cular o value fault   assumption, directly from the corner frequency [47,48]: at low frequency as is given by the following equations:  𝑙𝑜𝑔𝑀 𝑙𝑜𝑔Ω P,S r = K (2) P,S (5) 𝑎   f P, S 𝑙𝑜𝑔𝑀 𝑙𝑜𝑔Ω where r represents the source radius, K is a constant (K = 0.32 for P waves and K = 0.21 P S for S waves), f is the corner frequency (for P and S waves) and V is the velocity of P and S c (6) 𝑀   waves in the epicentral region. The determinations were performed for both P and S waves using vertical, or respectively horizontal, components of velocity recordings. Once the seismic moment and source radius estimates are obtained, we compute the  Because the spectral ratio technique is a relative method, it cannot estimate the absolute stress drop values using the relation of [47]:   values of the seismic moments for the two earthquakes, computing only their ratio. Based 7𝑀 on the spectral analysis we determined P and S displacement spectra separately and seismic (7)    moments of the main events by using the following equation: 16𝑟 To determine the source time function (STF) 3  we applied in parallel the method of  4p r v W R M = (3) empirical Green’s functions, deconvolv 0 ing EGF functions from M events. The deconvolu‐ F R qj tion functions were obtained by spectral division for different time windows (function of  the size of the event) for both P and S waves, using a tapering of 10%. To stabilize the  where r is the density at the source depth, v is the velocity of P or S waves at source depth, spectral division, the amplitude spectrum of EGF was smoothed out, before division, with  W is the long period displacement spectral level, R is the hypo-central distance, F (=2) is a five‐point running moving window. In addition, to reduce high‐frequency instabilities,  the free-surface parameter and R is the source radiation pattern (average values of 0.52 qj the deconvolved source time function was band‐pass filtered between 0.5 and 10 Hz with  for P waves and 0.63 for S waves, according to [49]). The density and velocities adopted for the study area were taken from [12] (V = 7.0 km/s, V = 4.04 km/s and r = 2.85 g/cm ). P S The seismic moments determined from P and S wave recordings are rather close, therefore,   Acoustics 2021, 3 278 we estimated M for each station as the mean value and as a final value the median of all selected stations was considered. Based on M we also compute the moment magnitude (M ) using the equation of [50]: log M M = 6.03 (4) 1.5 The seismic moments for EGF events were determined using the spectral ratio value at low frequency as is given by the following equations: M M log M logW 0 0 a = = (5) G G log M logW 0 0 G 0 M = (6) Once the seismic moment and source radius estimates are obtained, we compute the stress drop values using the relation of [47]: 7M Ds = (7) 16r To determine the source time function (STF) we applied in parallel the method of empirical Green’s functions, deconvolving EGF functions from M events. The deconvo- lution functions were obtained by spectral division for different time windows (function of the size of the event) for both P and S waves, using a tapering of 10%. To stabilize the spectral division, the amplitude spectrum of EGF was smoothed out, before division, with a five-point running moving window. In addition, to reduce high-frequency instabilities, the deconvolved source time function was band-pass filtered between 0.5 and 10 Hz with a Butterworth filter [45]. To increase the accuracy of the results, we used EGF pairs recorded by broadband stations by each component and stacked them up. The source rise time and source duration (t, t1/2) for the main events were computed from the STFs each time it had a pulse-like shape. The corner frequency (fc) was determined from the source duration using equation (8) from [20] and finally we computed the source radius using formula (2). f = (8) tp In addition, to have a better image of the tectonic processes in the region we computed the Gutenberg-Richter relation (see [51] and Equation (9)) before, during and after the seismic sequence, for an area extending to a distance of 0.75 deg. around the epicenter of the mainshock. The analysis was performed with Zmap software [52] using the maximum likelihood method [53]. For computation, we take into account local magnitude provided by the ROMPLUS catalogue, estimated as a maximum Wood-Anderson amplitude of one of the horizontal components corrected for distance and compiled only for stations with a good signal/noise ratio (SNR  3). log N( M) = a bM (9) N(M) is the number of earthquakes with magnitude larger or equal to M, a is a constant and b is the scaling parameter. 3. Results The results obtained for fault plane solutions, source parameters and evolution of seismic activity within the study area are in agreement with the findings pointed out by previous studies. By increasing the number of seismic stations in the area, we succeeded in increasing the results accuracy and also in adding complementary information that could improve the seismotectonic characteristics of the region. Acoustics 2021, 3 279 3.1. Focal Mechanisms The fault plane solutions determined by applying FOCMEC and HASH techniques are displayed in Figure 5 and listed in Table 2, where we also list the associated errors and the amount of data used. Therefore, for the FOCMEC method the amplitude ratio fit AF, no. of bad polarities NBP and no. of bad amplitude ratios NBAR are provided; while for the HASH routine the fault plane uncertainty FPU, auxiliary fault plane uncertainty AFPU, fit errors F-fit and station distribution ratio STDR are listed. All the fault plane Acoustics 2021, 3 FOR PEER REVIEW   12  solutions are well constrained by the selected seismic recordings. In addition, the focal mechanisms determined using both methods are comparable, increasing their reliability degree, and are also in agreement with the results pointed out within the study area by past studies [54–56]. For the mainshock, the fault plane solution indicates normal 319  67  77  169  27  119  faulting with both nodal planes NW- SE oriented, in the direction of the Peceneaga-Camena 17  19012015  23:53:07  3.8  0.16  2  1  3.30  2.20  0.05  42/6  RF  Fault zone. Our determination shows a good similarity with the solutions computed by          0.73  several international centers (INGV, USGS, GFZ), as well as with the solution of [32]. The 311  74  20  214  72  161  compression axis is plunging almost vertically, while the extension axis is almost horizontal 18  29062015  22:20:56  4.0  0.21  2  7  9.20  3.00  0.05  43/11  RLLO  W-E oriented. According to [57], the fault-plane solutions can be grouped into six types of          0.69  faulting (Table 2): normal (NF—4 events), strike-slip (SS—1 event), thrust (RF- 2 events), 257  64 −16  354  69 −170  normal left lateral (NLLO—6 events), thrust left lateral (RLLO—5 events) and strike-slip 19  01072015  04:34:24  3.2  0.25  1  2  14.70  4.30  0.07  26/6  LLSS  left lateral (LLSS—1 event).          0.46  Figure 5. The fault plane solutions computed using (a) FOCMEC and (b) HASH algorithms, for 19 earthquakes of the  Figure 5. The fault plane solutions computed using (a) FOCMEC and (b) HASH algorithms, for 19 earthquakes of the sequence that are listed in Table 2. Beach‐balls are displayed as a function of magnitude. The circles show earthquake  sequence that are listed in Table 2. Beach-balls are displayed as a function of magnitude. The circles show earthquake epicenters with outline color as a function of depth.  epicenters with outline color as a function of depth. 3.2. Spectral Ratios  The spectral ratios method was applied to all earthquake pairs, the main event‐em‐ pirical Green’s function, listed in Table 3. The source parameters in Equation (1) are the  asymptote at low frequency (in logarithmic scale), a, the corner frequencies of main (fc )  and EGF (fc ) events. The corner frequencies are determined by approximating the com‐ puted spectral ratio with the theoretical ratio and averaging the results for all the available  earthquake pairs and stations. The analysis was performed for both P and S‐waves, taking  into account only the recordings where the signal‐to‐noise ratio was above 2. The results  obtained for M and EGF events are given in Table 4 and examples of spectral ratios com‐ puted for pairs of earthquakes are displayed in Figure 6. Note that the obtained results  show that low‐frequency level, a, and the corner frequency of the EGF (fc ) show a higher  variation since they depend on the earthquake pair, while for the main event the corner  frequency (fc ) should not be influenced by the pair of the events if source directivity ef‐ fects are negligible. This feature is similar to the results pointed out by previous studies  [58].       Acoustics 2021, 3 280 Table 2. Fault plane solutions computed both FOCMEC and HASH techniques (crt. no. corresponds with the numbers in Figure 5). FOCMEC HASH No. of Origin Polarities/ Type of Crt. Date Strike Dip Rake Strike Dip Rake Time L No. of Fault No. DDMMYYYY hh:mm:ss Amplitude Plane F-Fit AF NBP NBAR FPU AFPU Ratios STDR 152 55 75 305 27 110 1 22112014 19:14:17 5.7 0.08 3 5 1.10 1.80 0.06 48/11 NF 0.40 77 71 69 207 30 134 2 22112014 19:27:39 2.0 0.19 3 4 2.00 2.40 0.28 12/6 NF 0.69 96 72 81 100 72 109 13/7 3 22112014 19:32:37 2.1 0.18 1 4 11.30 13.50 0.19 NF 0.65 315 40 61 106 60 110 4 22112014 20:24:47 2.8 0.20 1 1 2.80 0.00 0.09 24/13 NLLO 0.67 287 1 90 109 75 91 5 22112014 20:30:56 3.1 0.17 0 2 2.80 3.10 0.05 24/6 NF 0.51 70 36 26 192 65 120 21/5 6 23112014 01:14:39 2.5 0.11 1 3 6.50 5.30 0.21 NLLO 0.71 327 33 62 102 68 116 7 23112014 02:21:05 2.5 0.15 3 2 0.60 1.00 0.11 22/4 NLLO 0.72 344 64 0 248 71 146 8 23112014 04:01:58 2.6 0.12 1 2 6.40 5.00 0.04 19/6 SS 0.6 298 73 58 244 26 103 9 23112014 05:27:58 16/2 RLLO 2.4 0.16 2 1 4.70 9.10 0.15 0.68 128 85 60 215 40 174 10 23112014 10:16:15 2.7 0.13 1 3 10.40 17.20 0.07 19/7 NLLO 0.48 127 18 26 8 20 21 11 25112014 01:52:25 3.2 0.04 1 1 5.10 15.90 0.15 23/2 RLLO 0.72 315 54 37 177 44 117 02122014 04:19:29 2.5 12/6 RLLO 12 0.17 1 3 17.60 3.20 0.20 0.65 141 81 47 260 43 144 45/11 13 07122014 21:04:05 4.5 0.11 4 6 10.40 8.10 0.17 NLLO 0.70 158 44 22 48 62 121 14 14122014 17:24:47 3.1 0.19 2 1 5.10 2.10 0.16 28/7 RLLO 0.63 129 55 24 234 68 143 15 14122014 18:24:34 2.6 17/4 NLLO 0.14 1 1 5.20 1.90 0.09 0.57 220 45 82 226 45 93 36/5 16 12012015 06:08:31 4.2 0.14 6 2 1.50 1.00 0.18 RF 0.62 Acoustics 2021, 3 281 Table 2. Cont. FOCMEC HASH No. of Origin Polarities/ Type of Crt. Date Strike Dip Rake Strike Dip Rake Time L No. of Fault No. DDMMYYYY hh:mm:ss Amplitude Plane F-Fit AF NBP NBAR FPU AFPU Ratios STDR 319 67 77 169 27 119 17 19012015 23:53:07 3.8 0.16 2 1 3.30 2.20 0.05 42/6 RF 0.73 311 74 20 214 72 161 18 29062015 22:20:56 4.0 43/11 RLLO 0.21 2 7 9.20 3.00 0.05 0.69 257 64 16 354 69 170 19 01072015 04:34:24 3.2 0.25 1 2 14.70 4.30 0.07 26/6 LLSS 0.46 3.2. Spectral Ratios The spectral ratios method was applied to all earthquake pairs, the main event- empirical Green’s function, listed in Table 3. The source parameters in Equation (1) are the asymptote at low frequency (in logarithmic scale), a, the corner frequencies of main M G (f ) and EGF (f ) events. The corner frequencies are determined by approximating c c the computed spectral ratio with the theoretical ratio and averaging the results for all the available earthquake pairs and stations. The analysis was performed for both P and S-waves, taking into account only the recordings where the signal-to-noise ratio was above 2. The results obtained for M and EGF events are given in Table 4 and examples of spectral ratios computed for pairs of earthquakes are displayed in Figure 6. Note that the obtained results show that low-frequency level, a, and the corner frequency of the EGF (f ) show a higher variation since they depend on the earthquake pair, while for the main event the corner frequency (f ) should not be influenced by the pair of the events if source directivity effects are negligible. This feature is similar to the results pointed out by previous studies [58]. P S The corner frequencies ratio of P to S waves (f /f ) averaged over all stations for c c the main events varies between 0.94 and 1.06 for the M1 and M2 events, respectively, with a mean value of 1.05 computed for all selected events (see Table 4). These results show similarities with the ones pointed out for various regions of the world [45,46,59–62], and follow the circular source model proposed by [63]. Commonly, the P source pulse shows a shorter duration than the S source pulse, which leads to higher corner frequencies for P waves because the propagation time of P waves P S is faster compared with S waves. Thus, an average value of 1.5 is expected for the f /f c c when the waves leave the fault at angles of incidence greater than 30 from the normal. For cases where the propagation of the rays is close to the normal direction of the fault, the P and S waves leave the source in-phase and interference due to the source limitation is P S negligible (f  f ). c c Using equations from (1) to (7), we determined the seismic moment, source radius and stress drop for the main and EGF events (Tables 5 and 6). The source radius estimated from corner frequency is an average between the estimations using P-wave and S-wave P S corner frequencies. Since relation (2) assumes a ratio of about 1.5 for f /f and for our data c c P S P f  f , the radius computed from f is systematically greater than the radius computed c c c from f roughly by a factor of 1.5 (see Tables 5 and 6). c Acoustics 2021, 3 282 Table 3. Event pairs, the inter-distance and inter-magnitude separation parameters. The correspond- ing events are given in Table 1. Epicenters Hypocenters Depths Earthquake Pair Inter-Distance Inter-Distance Distance M Difference (km) (km) (km) M1-1 2.6 2.9 1.4 3.7 M1-2 1.2 6.6 6.5 2.9 M1-3 3.1 5.6 4.6 2.6 M1-4 1.7 6.3 6.1 3.2 M1-5 3.2 7.4 6.7 3.2 M1-6 5.3 11.2 9.9 3.1 M1-8 5.3 9.3 7.6 3.4 M1-10 2.4 3.2 2.1 2.5 M1-11 2.3 2.3 0.3 1.2 M1-15 1.1 1.6 1.1 1.9 M1-16 1.9 7.7 7.5 3.5 M2-1 5.5 6.4 3.2 1.1 M3-1 4.4 4.5 1.1 2.5 M3-2 2.4 6.6 6.2 1.7 M3-3 3.6 5.6 4.3 1.4 M3-4 0.8 5.9 5.8 2.0 M3-5 3.8 7.4 6.4 2.0 M3-6 6.7 11.7 9.6 1.9 M3-7 4.5 7.5 6.0 2.1 M3-8 6.7 9.9 7.3 2.2 M3-9 1.6 6.4 6.2 2.1 M3-10 4.0 4.4 1.8 1.3 M3-16 1.0 7.3 7.2 2.3 M4-12 8.6 9.7 4.5 1.1 M4-13 8.8 9.3 3.1 1.6 M4-14 5.3 15.8 14.9 2.4 M5-1 2.5 2.5 0.3 1.8 M5-2 2.1 5.8 5.4 1.0 M5-7 4.8 7.1 5.2 1.4 M5-9 1.6 5.6 5.4 1.4 M5-16 1.1 6.5 6.4 1.6 M6-17 1.4 1.9 1.3 0.8 P S P S Table 4. Corner frequencies of P (f ) and S (f ) waves with associated ratio (f /f ) determined for c c c c main (bold characters) and associated EGF events (as were listed in Table 1). P S P S No. of ev. f (Hz) f (Hz) f /f c c c c M1 1.92 2.04 0.94 M2 9.25 8.72 1.06 M3 3.81 3.98 0.96 M4 4.46 4.69 0.95 M5 5.67 5.38 1.05 M6 5.13 5.20 0.99 1 16.44 17.98 0.91 2 10.07 8.62 1.17 3 3.81 3.98 0.96 4 11.14 8.86 1.26 5 11.75 10.12 1.16 6 9.21 8.96 1.03 7 11.47 10.97 1.05 8 15.00 10.61 1.41 9 11.15 12.08 0.92 10 6.48 6.79 0.95 11 3.81 3.98 0.96 12 8.25 6.10 1.35 13 7.30 9.69 0.75 14 17.06 16.45 1.04 15 5.67 5.38 1.05 16 15.83 14.91 1.06 17 7.20 6.80 1.06 Acoustics 2021, 3 283 Acoustics 2021, 3 FOR PEER REVIEW   15  Figure 6. Spectral ratios determined (for vertical‐left and horizontal‐right components) for the following pairs: (a) M of 22  Figure 6. Spectral ratios determined (for vertical-left and horizontal-right components) for the following pairs: (a) M of November 2014, 19:14 and EGF of 22 November 2014, 20:30; (b) M of 19 January 2015, 23:53 and EGF of 20 December,  22 November 2014, 19:14 and EGF of 22 November 2014, 20:30; (b) M of 19 January 2015, 23:53 and EGF of 20 December, 02:52. 02:52.  Table 5. Final source parameters for main events determined using the SR technique. Using equations from (1) to (7), we determined the seismic moment, source radius  and stress drop for the main and EGF events (Tables 5 and 6). The source radius estimated  Seismic Source Radius (m) Stress Drop (MPa) Event Mw P S P S from corner frequency Moment (Nm)  is an average between f (Hz)  the estimations f (Hz) using Pf‐wave and Sf‐wave  c c c c P S corner frequencies. Since relation (2) assumes a ratio of about 1.5 for fc /fc  and for our data  M1 1.76  10 5.4 1236 914 41.0 101.0 P S P fc  ≈ fc , the radius computed 14  from fc  is systematically greater than the radius computed  M2 1.14  10 2.7 234 166 3.5 11.0 from fc  roughly by a factor of 1.5 (see Tables 5 and 6).  M3 2.39  10 3.4 622 397 4.4 16.7 M4 3.3 402 256 5.0 19.4    7.43  10 M5 7.55  10 3.1 417 294 4.6 13.0 M6 4.28  10 3.2 366 241 3.8 13.4   Acoustics 2021, 3 284 Table 6. Final source parameters for EGFs determined using the SR technique. Source Radius (m) Stress Drop (MPa) Date Origin Time Seismic Mw P S P S DDMMYYYY hh:mm:ss Moment (Nm) from f from f c c 22112014 19:27:39 1.80  10 2.1 144 88 2.6 11.6 22112014 20:24:47 5.92  10 2.6 215 168 2.6 5.5 22112014 20:30:56 2.7 234 166 3.5 11.0 1.14  10 23112014 01:14:39 4.63  10 2.4 194 163 2.8 4.7 23112014 02:21:05 3.11  10 2.4 184 143 2.2 4.7 23112014 04:01:58 2.5 213 147 1.9 5.8 4.23 10 23112014 05:27:58 3.93  10 2.3 189 132 2.5 7.5 23112014 23:22:49 2.14  10 2.3 144 136 3.1 3.7 24112014 00:45:06 2.48  10 2.3 194 120 1.5 6.3 25112014 01:52:25 2.21  10 2.8 303 193 3.5 13.5 07122014 21:04:05 3.4 622 397 4.4 16.7 2.39 10 14122014 17:24:47 1.30  10 2.7 236 213 4.3 5.9 14122014 18:24:34 4.83  10 2.5 266 134 1.1 8.8 18012015 22:41:40 2.0 115 80 2.5 7.4 8.71  10 19012015 23:53:07 7.55  10 3.1 417 294 4.6 13.0 20012015 02:52:46 5.12  10 2.2 137 98 8.7 24.5 01072015 04:34:24 6.24 10 2.8 249 176 1.8 5.0 3.3. Empirical Green’s Function Analysis For the same pairs of events considered in the SR method, we applied in parallel the method of deconvolution with EGF to compute for M the STFs, rise time (t1/2), source duration (t), corner frequency (fc) and source radius (r) using Equations (2) and (8). To remove any biases, we computed the relative source time function for a given station by summing the functions resulting from deconvolution from all three components of that station. The obtained values are listed in Table 7 and examples of STFs resulting from the deconvolution method are displayed in Figure 7. The source radius and stress drop values are equivalent to the same values inferred by the SR method. The only exception is the case of the mainshock where the differences are ~30% smaller for source radius and ~40% larger for stress drop. Table 7. Final source parameters for main events determined using the EGF technique. Source Radius (m) Stress Drop (MPa) Seismic Moment t (s) f (Hz) Event Mw c t t (Nm) M1 1.76  10 5.4 0.5 1.27 666 260.7 M2 1.14  10 2.7 0.16 3.98 213 5.2 M3 2.39  10 3.4 0.26 2.45 346 25.2 M4 3.3 0.18 3.54 240 23.6 7.43  10 M5 7.55  10 3.1 0.23 2.77 306 11.5 M6 4.28  10 3.2 0.22 2.90 293 7.4 Acoustics 2021, 3 FOR PEER REVIEW   17  Acoustics 2021, 3 M4  7.43 × 10   3.3  0.18  3.54  240  23.6  285 M5  7.55 × 10   3.1  0.23  2.77  306  11.5  M6  4.28 × 10   3.2  0.22  2.90  293  7.4  Figure 7. Examples of source time functions computed for the sequence occurred in the Mărăsesti area: (a) deconvolution of Figure 7. Examples of source time functions computed for the sequence occurred in the Mărășești area: (a) deconvolution  , , of M of 22 November 2014, 19:14 with EGF of 22 November 2014, 20:30 at Pogoanele (PGOR) and (b) Amara (AMRR)  M of 22 November 2014, 19:14 with EGF of 22 November 2014, 20:30 at Pogoanele (PGOR) and (b) Amara (AMRR) stations. stations. (c) deconvolution of M of 19 January 2015, 23:53 with EGF of 20 January 2015, 02:52 at all stations (left side) and  (c) deconvolution of M of 19 January 2015, 23:53 with EGF of 20 January 2015, 02:52 at all stations (left side) and the average the average of these pulses (right side). The dashed lines represent the standard error.  of these pulses (right side). The dashed lines represent the standard error. 3.4. Scaling Relationships  3.4. Scaling Relationships The source parameters determined in the present analysis and their scaling relation‐ The source parameters determined in the present analysis and their scaling relation- ships are investigated as indicators of geotectonic peculiarities of the Carpathians fore‐ ships are investigated as indicators of geotectonic peculiarities of the Carpathians foredeep deep region as in previous papers [16,24,25,45,46,64]. Up to now, there has been no sys‐ region as in previous papers [16,24,25,45,46,64]. Up to now, there has been no systematic tematic investigation of the source parameters for the earthquakes that occurred close to  investigation of the source parameters for the earthquakes that occurred close to Mărăsesti , , Mărășești city. The scaling of M0 with ML is displayed in Figure 8. The data are well con‐ city. The scaling of M with M is displayed in Figure 8. The data are well constrained 0 L strained (correlation coefficient of 0.97) by the regression line:  (correlation coefficient of 0.97) by the regression line: (10) 𝑙𝑜𝑔 𝑀 0.97 0.06 𝑀 11.11 0.19   log M = (0.97  0.06) M + (11.11  0.19) (10) 0 L R = 0.97, σ = 0.25.  The scaling shows a similar trend with those obtained for other areas. Thus, the slope  R = 0.97, s = 0.25. value of 0.97 is similar to the values of ~1.1, previously obtained by other studies [65–67].  The scaling shows a similar trend with those obtained for other areas. Thus, the slope The deviation highlighted by the mainshock (ML = 5.7) can be explained by a possible  value of 0.97 is similar to the values of ~1.1, previously obtained by other studies [65–67]. underestimation  of  ML  for  large  earthquakes  since  the  value  of  M0  determined  in  this  The deviation highlighted by the mainshock (M = 5.7) can be explained by a possible underestimation of M for large earthquakes since the value of M determined in this study L 0 is similar to those computed by the international seismological centers (GFZ, EMSC, CSM, NEIC). A second possible explanation for this deviation is given by an overestimation of seismic moments of small earthquakes (M < 4). Several studies achieved in different L Acoustics 2021, 3 FOR PEER REVIEW   18  Acoustics 2021, 3 FOR PEER REVIEW   18  Acoustics 2021, 3 286 study is similar to those computed by the international seismological centers (GFZ, EMSC,  CSM, NEIC). A second possible explanation for this deviation is given by an overestima‐ study is similar to those computed by the international seismological centers (GFZ, EMSC,  regions [68,69] pointed out, for these magnitudes, seismic moments with values smaller by tion of seismic moments of small earthquakes (ML < 4). Several studies achieved in differ‐ CSM, NEIC). A second possible explanation for this deviation is given by an overestima‐ up to an order of magnitude. ent  regions  [68,69]  pointed  out,  for  these  magnitudes,  seismic  moments  with  values  tion of seismic moments of small earthquakes (ML < 4). Several studies achieved in differ‐ Scaling of the seismic moment with source radius is shown in Figure 9 for both P and smaller by up to an order of magnitude.  ent  regions  [68,69]  pointed  out,  for  these  magnitudes,  seismic  moments  with  values  S waves and is approximated by the regression lines: Scaling of the seismic moment with source radius is shown in Figure 9 for both P and  smaller by up to an order of magnitude.  S waves and is approximated by the regression lines:  Scaling of the seismic moment with source radius is shown in Figure 9 for both P and  P waves log M = (3.85  0.26) log r + (4.83  0.642) (11) S waves and is approximated by the regression lines:  (11) 𝑃 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.85 0.26 𝑙𝑜𝑔 𝑟 4.83 0.642   R = 0.96, s = 0.27. 𝑃 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.85 0.26 𝑙𝑜𝑔 𝑟 4.83 0.642   (11) R = 0.96, σ = 0.27.  S waves log M = (3.89  0.24) log r + (5.35  0.565) (12) R = 0.96, σ = 0.27.  (12) 𝑆 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.89 0.24 𝑙𝑜𝑔 𝑟 5.35 0.565   (12) R = 0.96, s = 0.26. 𝑆 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.89 0.24 𝑙𝑜𝑔 𝑟 5.35 0.565   R = 0.96, σ = 0.26.  R = 0.96, σ = 0.26.  Figure 8. Scaling of the seismic moment with local magnitude.  Figure 8. Scaling of the seismic moment with local magnitude. Figure 8. Scaling of the seismic moment with local magnitude.  Figure 9. Scaling of the seismic moment with source radius for P (blue) and S (purple) waves. The  dashed lines are the fitting lines, while solid lines are the fitting lines imposing the theoretical  Figure 9. Scaling of the seismic moment with source radius for P (blue) and S (purple) waves. The  Figure 9. Scaling of the seismic moment with source radius for P (blue) and S (purple) waves. The slope (3).  dashed lines are the fitting lines, while solid lines are the fitting lines imposing the theoretical  dashed lines are the fitting lines, while solid lines are the fitting lines imposing the theoretical slope (3). slope (3).    Acoustics 2021, 3 287 Acoustics 2021, 3 FOR PEER REVIEW   19  The slope of the regression line is higher than the theoretical value for scaling the The slope of the regression line is higher than the theoretical value for scaling the  seismic source with a homogeneous rupture process (compare dashed with solid lines in seism Figur ic eso9urc ). e Since  within a hom all these ogeneous cases rup theture same  pro relative cess (compa deconvolution re dashed with techniques  solid line wer s in e   applied, Figure 9). we Sin assume ce in althat l these the case devis athe tion sam from e relative a theor dec etical onvolution scaling (slope  techniques ~ 4 instead  wereof applied, 3) is a   consequence we assume th ofatthe  theunder  deviaestimation tion from aof the the oreti corner cal sca frequency ling (slope for ~the  4 in smaller stead of earthquakes.  3) is a conse‐ quence of the underestimation of the corner frequency for the smaller earthquakes.   P waves logDs = (0.28  0.05) log M (3.46  0.67) (13) P 0 𝑃 𝑣𝑎𝑒𝑠𝑤 𝑙𝑜𝑔 ∆𝜎 0.28 0.05 𝑙𝑜𝑔 𝑀 3.46 0.67   (13) R = 0.80, s = 0.20. R = 0.80, σ = 0.20.  S waves logDs = (0.28  0.04) log M (2.99  0.63) (14) 𝑆 𝑣𝑎𝑒𝑠𝑤 𝑙𝑜𝑔 ∆𝜎 S 0.28 0.04 𝑙𝑜𝑔 𝑀 0 2.99 0.63   (14) R = 0.82, σ = 0.19.  R = 0.82, s = 0.19. The scaling of stress drop with the seismic moment (Figure 10) indicates an increas‐ The scaling of stress drop with the seismic moment (Figure 10) indicates an increasing ing value stress drop over the entire magnitude range. The stress drop values are in the  value stress drop over the entire magnitude range. The stress drop values are in the range range 1–41 MPa for P waves and 4–101 MPa for S waves. However, the increasing trend  1–41 MPa for P waves and 4–101 MPa for S waves. However, the increasing trend of the of the distribution may be apparent as the decisive element is given by the comparatively  distribution may be apparent as the decisive element is given by the comparatively higher higher value of the stress drop in the case of the mainshock of 22 November 2014 (for the  value of the stress drop in the case of the mainshock of 22 November 2014 (for the rest rest of the events, if we remove the largest event, a constant stress drop scaling seems to  of the events, if we remove the largest event, a constant stress drop scaling seems to be rbe easonable).  reasonable The ). The statistics  statistic ars e are too too small  small for for larger  larger earthquakes  earthquakes to have  to have an eloquent an eloque image nt im‐ age of scaling. In Figure 10 we can notice both for P and S‐wave determinations an in‐ of scaling. In Figure 10 we can notice both for P and S-wave determinations an increasing trcreasing end of str tress enddr of op stres with s dr theop seismic  with th moment e seismiwhich c moment may wh suggest ich may a higher  sugge slip st arather  higher than  slip  arath higher er than fault a higher dimension  fault [ 19 di].mension [19].  Figure 10. Scaling of stress drop with the seismic moment for P (blue) and S (purple) waves. Figure 10. Scaling of stress drop with the seismic moment for P (blue) and S (purple) waves.        Acoustics 2021, 3 288 Acoustics 2021, 3 FOR PEER REVIEW   20  3.5. Frequency–Magnitude Distribution 3.5. Frequency–Magnitude Distribution  The analysis of the frequency–magnitude distribution at the scale of the entire region The analysis of the frequency–magnitude distribution at the scale of the entire region  (0.75  0.75 deg.) outlines a typical distribution with the b slope close to 1 for ‘normal’ time (0.75 × 0.75 deg.) outlines a typical distribution with the b slope close to 1 for ‘normal’ time  intervals (see Figure 11a,d). intervals (see Figure 11a,d).  Figure 11. Frequency–magnitude distribution computed for four‐time intervals: (a) 2005–2011; (b) 2011–2014; (c) 100 days  Figure 11. Frequency–magnitude distribution computed for four-time intervals: (a) 2005–2011; (b) 2011–2014; (c) 100 days since 22 November 2014; (d) 2015–2020.  since 22 November 2014; (d) 2015–2020. We performed the analysis for four times intervals, two of them situated outside the  We performed the analysis for four times intervals, two of them situated outside perturbed activity before the mainshock of 22 November 2014 (2005–2011), one for a three  the perturbed year intervalactivity  before the befor  mainesh the ock mainshock occurrence (2of 01122 –20November 14), one for 10 2014 0 day(2005–2011), s starting with one for a the mainshock occurrence and the last one for a five year interval after the sequence (2015– three year interval before the mainshock occurrence (2011–2014), one for 100 days starting 2020). To compute the b value, we use the maximum likelihood method [53]. The magni‐ with the mainshock occurrence and the last one for a five year interval after the sequence tude of completeness (Mc) of 2.1 and the b slope of 1.14 characterize the first interval  (2015–2020). To compute the b value, we use the maximum likelihood method [53]. The magnitude of completeness (Mc) of 2.1 and the b slope of 1.14 characterize the first interval (2005–2011). Mc keeps the value 2.1 for the next interval, before the sequence triggering (2011–2014), while the b slope has a significant increase to 1.4. During the post-sequence anomalous interval, the b value comes back to ‘normal’, close to 1 value (b = 0.99), while the Mc parameter drops down to 1.2. During the last interval (2015–2020) the seismic- ity regime largely preserves the characteristics observed during the aftershock activity (Mc = 1.3, b = 1.04) due to the expansion of the RSN (Figure 12). Acoustics 2021, 3 FOR PEER REVIEW   21  (2005–2011). Mc keeps the value 2.1 for the next interval, before the sequence triggering  (2011–2014), while the b slope has a significant increase to 1.4. During the post‐sequence  anomalous interval, the b value comes back to ‘normal’, close to 1 value (b = 0.99), while  Acoustics 2021, 3 289 the Mc parameter drops down to 1.2. During the last interval (2015–2020) the seismicity  regime largely preserves the characteristics observed during the aftershock activity (Mc =  1.3, b = 1.04) due to the expansion of the RSN (Figure 12).  Figure 12. The number of seismic stations distributed function of the time in the study region and  Figure 12. The number of seismic stations distributed function of the time in the study region and the adjacent areas.  the adjacent areas. 4. Discussion  4. Discussion We discuss in the following the results of the present work taking into account the  We discuss in the following the results of the present work taking into account the tectonic and geophysical characteristics of the study region as well as comparison with  tectonic and geophysical characteristics of the study region as well as comparison with the the results of previous studies.  results of previous studies. The space–time and frequency–magnitude analyses on the crustal seismic events that  The space–time and frequency–magnitude analyses on the crustal seismic events that occurred within the study region in 2014–2015, during and after the seismic sequence of  occurred within the study region in 2014–2015, during and after the seismic sequence of Mărășești, show that the seismic activity is distributed in several clusters of earthquakes  Mărăsesti, show that the seismic activity is distributed in several clusters of earthquakes , , (Figure 2). The mainshock (ML = 5.7), located close to the Moho boundary (Figure 13),  (Figure 2). The mainshock (M = 5.7), located close to the Moho boundary (Figure 13), triggered a significant increase in seismic activity, not only in the source neighborhood  triggered a significant increase in seismic activity, not only in the source neighborhood but but spread over a larger area, as compared with its source dimension. It was also unusual  spread over a larger area, as compared with its source dimension. It was also unusual that that this intensification consisted of a relatively large number of small‐magnitude earth‐ Acoustics 2021, 3 FOR PEER REVIEW   22  this intensification consisted of a relatively large number of small-magnitude earthquakes quakes spanning a long‐time interval.  spanning a long-time interval. Figure 13. Cross‐sections (modified after [70,71]) through the Eastern Carpathian Orogen, crossing (a) the Mărășești area,  Figure 13. Cross-sections (modified after [70,71]) through the Eastern Carpathian Orogen, crossing (a) the Mărăsesti , , (b) the Râmnicu Sărat region and (c) parallel with the Eastern Carpathian Orogen (d) 3D view of the selected profiles  area, (b) the Râmnicu Sărat region and (c) parallel with the Eastern Carpathian Orogen (d) 3D view of the selected overlayed on the geological map (see Figure 1 for cross‐sections locations). Moho depth is taken from [72]. The hypocenters  profiles overlayed on the geological map (see Figure 1 for cross-sections locations). Moho depth is taken from [72]. The distribution of the 2014 Mărășești seismic sequence is given by blue dots with the mainshock colored in red. The abbrevi‐ hypocenters distribution of the 2014 Mărăsesti seismic sequence is given by blue dots with the mainshock colored in red. The , , ations are as follows: BF‐Bistrita Fault, PCF‐ Peceneaga‐Camena Fault, NDO‐ North Dobrogea Orogen, TF‐Trotuș Fault,  abbreviations are as follows: BF-Bistrita Fault, PCF- Peceneaga-Camena Fault, NDO- North Dobrogea Orogen, TF-Trotus VF‐Vaslui Fault.  Fault, VF-Vaslui Fault. The mainshock occurrence first generated an aftershock activity located in the epi‐ central area, near Mărășești city. Then an intensification of seismic activity was observed  in the Râmnicu Sărat area (~50 km south of Mărășești, see Figures 1 and 2). Apart from  the small size seismic activity triggered there, a moderate earthquake occurred in the same  area (12 January 2015, 06:08 GMT, ML = 4.2). An increase in seismic activity has also been  emphasized towards the north (~30km relative to Mărășești), close to Adjud city (Figure  2). Similar to the previous case, but delayed in time, a moderate size earthquake occurred  close to Adjud city (29 June 2015, 22:20 GMT, ML = 4.0). Intensification of the seismic ac‐ tivity was pointed out to the East and South‐East of Mărășești, but here only small‐mag‐ nitude events were recorded. Overall, the epicenters distribution within the study region  follows a direction parallel to the Eastern Carpathian Bend zone, as was also shown by  previous studies [2,7,24,25,73]. However, if the seismicity is investigated at a smaller scale  (for specific areas) the shape of the epicenters’ distribution may change (Figure 5).   The depth distribution (Figures 2 and 13) emphasizes higher depths for the events  that occurred close to the Mărășești city with most of them located within the lower crust,  while the ones located in the adjacent regions were placed in the middle or upper crust  (Figures 2, 3 and 13). The depth estimations of these events are rather confident, even for  the low size events, since all the locations were manually performed and the seismic sta‐ tion coverage is good (Figure 1) as a consequence of a continuous improvement of the  seismic network in the study area [74] (Figure 12). The depth distribution gives us valua‐ ble indications about the depth of the faults. The mainshock of the sequence is located in  the Focșani Basin, at ~5 km, west of the Peceneaga‐Camena Fault, a deep crustal fault with  a Moho offset of ~5 km [75]. It separates the Moesian Platform from the buried part of the  North Dobrogea orogen since Neogene [76]. In our analysis, the epicenters distribution  seems to follow the characteristics described by [70,77] which showed, based on reflection  seismic lines, that in the eastern side of the Focșani Basin the deformations occur along  numerous normal faults in a wide area with clear topographic expression (see also [1]).  The frequency–magnitude distribution for the earthquakes of the Mărășești sequence  (Figure 11c) shows a magnitude of completeness of ML = 1.2 and a slope of the distribution  (0.99) similar to the normal value (b ~ 1.0) [78]. This is a good indication that these earth‐ quakes are of tectonic origin. On the other hand, we notice important variations of b value    Acoustics 2021, 3 290 The mainshock occurrence first generated an aftershock activity located in the epicen- tral area, near Mărăsesti city. Then an intensification of seismic activity was observed in , , the Râmnicu Sărat area (~50 km south of Mărăsesti, see Figures 1 and 2). Apart from the , , small size seismic activity triggered there, a moderate earthquake occurred in the same area (12 January 2015, 06:08 GMT, M = 4.2). An increase in seismic activity has also been emphasized towards the north (~30km relative to Mărăsesti), close to Adjud city (Figure 2). , , Similar to the previous case, but delayed in time, a moderate size earthquake occurred close to Adjud city (29 June 2015, 22:20 GMT, M = 4.0). Intensification of the seismic activity was pointed out to the East and South-East of Mărăsesti, but here only small-magnitude , , events were recorded. Overall, the epicenters distribution within the study region follows a direction parallel to the Eastern Carpathian Bend zone, as was also shown by previous studies [2,7,24,25,73]. However, if the seismicity is investigated at a smaller scale (for specific areas) the shape of the epicenters’ distribution may change (Figure 5). The depth distribution (Figures 2 and 13) emphasizes higher depths for the events that occurred close to the Mărăsesti city with most of them located within the lower crust, , , while the ones located in the adjacent regions were placed in the middle or upper crust (Figures 2, 3 and 13). The depth estimations of these events are rather confident, even for the low size events, since all the locations were manually performed and the seismic station coverage is good (Figure 1) as a consequence of a continuous improvement of the seismic network in the study area [74] (Figure 12). The depth distribution gives us valuable indications about the depth of the faults. The mainshock of the sequence is located in the Focsani Basin, at ~5 km, west of the Peceneaga-Camena Fault, a deep crustal fault with a Moho offset of ~5 km [75]. It separates the Moesian Platform from the buried part of the North Dobrogea orogen since Neogene [76]. In our analysis, the epicenters distribution seems to follow the characteristics described by [70,77] which showed, based on reflection seismic lines, that in the eastern side of the Focsani Basin the deformations occur along numerous normal faults in a wide area with clear topographic expression (see also [1]). The frequency–magnitude distribution for the earthquakes of the Mărăsesti sequence , , (Figure 11c) shows a magnitude of completeness of M = 1.2 and a slope of the distribu- tion (0.99) similar to the normal value (b ~ 1.0) [78]. This is a good indication that these earthquakes are of tectonic origin. On the other hand, we notice important variations of b value in time, for the first period (2005–2011), the b value is about 1.14, increasing up to ~1.4 in the second period (2011–2014), and returning close to the normal (b ~ 1.04) for the last period (2015–2020). The changes of b value could be caused by several factors such as geological conditions, degree of heterogeneity of cracked medium, and strain and stress variation in the region [79,80]. The higher b value before the sequence could be explained by a relative blocking of the stress on moderate-size resisting areas (asperities) and intensi- fication instead of small-size earthquakes, taking into account the high heterogeneity of the region [1,81]. The decrease of Mc after 2014 is directly related to the seismic network improvements to monitor seismicity in the region (Figure 12). Our results for the fault plane solutions reveal three new mechanisms determined with rather good accuracy for events 14, 18 and 19 (see Table 2). The overall mechanisms highlight a wide variety for earthquakes associated or triggered by the 2014 Mărăsesti , , seismic sequence, although most of these events occurred in the Focsani Basin, a thick Neogene foredeep sediment layer as thick as 13 km located near the Carpathian bend zone [70]. Although the region is relatively narrow, this variety of fault plane solutions is not uncommon for the SE Carpathians foredeep area as shown by other studies [7,31,32,54,55] due to the geological complexity of this region, pointed out on one hand by the collision of three major units with different geometries and characteristics, all representing cratonic continental platforms [82,83] and on the other hand by the significant subsidence, although it is still uncertain whether these faults influenced the lateral variations in the thin-skinned thrusting kinematics [83]. We noticed particular characteristics for each group of events. For the moderate-size earthquakes that occurred near Mărăsesti city, normal fault plane solutions are prevalent , , Acoustics 2021, 3 291 with the nodal planes oriented in the NW–SE direction, corresponding to the PCF direc- tion. The small size earthquakes that occurred in the same region and in the same depth range have in general the same type of faults, highlighting acceptable deviations of the focal planes, probably caused by a reduced amount of data used in their determination. Conversely, earthquakes that occurred in the upper depth segments (but still at the level of the lower or middle crust) tend to change their fault plane solutions towards a normal left lateral mechanism, which may involve activation of secondary faults of the same system (PCF). Given that the epicenters are distributed parallel with the advancing orogen, a flexural rupture of the lower plate with partial reactivation of inherited normal faults might be an interpretation for the earthquakes located further south of the PCF (Figure 13b). The normal faulting highlighted by our results for this area was also emphasized by the results of previous studies [6,77]. In addition, [1] pointed out a large number of normal faults with offsets in the order of tens of meters at the base Quaternary level which belong to the system of the PCF. The moderate size earthquakes generated near Adjud city (~30 km northern part of Mărăsesti city) were located in the upper crust (Figures 1, 2 and 13), show- ing a tendency to change the mechanism to strike-slip. At least one of the nodal planes is oriented in the same direction (NW–SE) with PCF, which may indicate an activation of a secondary faults system located towards its northern side and at the upper crust level dominated by horizontal movements. The faults pattern observed and described above can be explained by the differences in localization deformation transferred from the advancing Carpathian Orogen to the plate. These differences are generated by the sharp changing rheology and mechanical behavior along inherited lower plate blocks and their obliquity to the advancing orogen [1], such as the case of the major border faults TF and PCF separating the Scythian Platform/NDO from Moesian Platform (Figure 13). In the case of the TF, which is almost perpendicular to the advancing orogen, the inherited structures are not inverted, but instead form a new left lateral strike-slip system (with associated normal and reverse faults) located above the contact between strong Scythian vs weak Moesian Platform. The geometry of the PCF, orientated roughly at a 45 angle from the advancing orogen, makes it susceptible for inversion during orogenic loading and flexure, with the normal faults located at the contact between NDO and Moesian Platform. The earthquakes from the Râmnicu Sărat region (~50 km south relative to Mărăsesti) , , were also located in the upper crust, with a predominant reverse mechanism, with the nodal planes oriented in the NE–SW direction, parallel to the Eastern Carpathians’ bending. This type of mechanism was observed in a large number of earthquakes that occurred in the past in the same area [6,25,84], the existence of a possible fault along the Eastern Carpathians’ bending being a debated topic [85]. These NE–SW change in fault type pattern can be explained by the ongoing compres- sional deformation of the SE Carpathians fold and thrust belt which is responsible for both low angles thrusting on the leading edge orogen fault (Pericarpathian thrust Fault, see Figures 1 and 13) and high angle reverse faults, probably related to the inversion of previous Mesozoic normal faults as suggested by [71]; however, the exact geometry of the faults is not resolvable with the current available refraction seismic data (see also [76]). It is worth noticing that for the event that occurred in 12012015, 06:08 (GMT), M = 4.2, our results indicate a reverse focal mechanism, with the nodal planes NE–SW oriented, which contradicts the normal fault plane solution determined by [32]. We assume that the fault mechanism determined by the above study is flawed due to the event location, which in their study is reported as being located ~40 km towards the north, and at a greater depth (with ~20 km) as compared to the location provided by the ROMPLUS catalogue. Previous results have shown that source parameters determined using SR and EGF techniques have a better accuracy since the determination uncertainties decrease by averag- ing the data [23,35,45]. Our data set consists on average of five EGFs for each main event, allowing an uncertainty decrease of up to 20%. The results obtained by using two different techniques (SR and EGF deconvolution) highlight rather close values with an average Acoustics 2021, 3 292 difference of ~30% (for the stress drop) which indicates an adequate degree of accuracy. The largest event in our dataset has an average stress drop of 134 MPa. Similar high-stress drop values were observed in other regions for normal-faulting earthquakes [86] or for the deeper location of the events [87]. At the same time, we noticed an increasing trend of stress drop with seismic moment. According to our analysis, the STF has a simple pulse-like form for all the studied events. However, the apparent pulse-like form for the source of the largest event as obtained by EGF deconvolution is not so well constrained as for the other smaller main events. This could be attributed to some complexity of the rupture process in the source, visible in the waveform recordings of the mainshock. If we assume a multi-shock release of seismic energy for this event, the resulting stress drop is a dynamic value rather than static, which explains the unusually high-stress drop values obtained for the mainshock (Tables 5 and 7). Source parameter scaling relationships fit well with the results previously obtained for the Carpathian foredeep region [24,25,45,64]. The deviations observed for the seismic moment–source radius scaling (slope ~3.9 instead of 3) and seismic moment–stress drop (deviation from a constant stress drop scaling) from the theoretical values could be due to the limitation of the empirical Green’s function and spectral ratios techniques to constrain the corner frequency for the weakest earthquakes and the anomalous high-stress drop value of the mainshock. On the other hand, the high attenuation effects within the upper crust might act as a filter, removing the high frequencies content (f > 10 Hz) and limiting the corner frequencies resolution [28] affecting especially small earthquakes and leading to an underestimation of their stress drop. It is also likely that the usage of these methods for the small earthquakes to lead to a saturation of the corner frequency due to the significant increase of the recorded noise at high frequencies. Because of this, we can expect that our analysis is no longer able to discern the higher corner frequencies and, for this reason, the source dimension becomes overestimated and, correspondingly, the stress drops underestimated. We need more high-quality data for different size earthquakes to validate or not the scaling deviations obtained in the present work. 5. Conclusions The seismic sequence triggered on 22 November 2014 close to Mărăsesti city offered , , the possibility of evaluating the source parameters and the focal mechanism solution for the strongest crustal event (M = 5.7) ever recorded in the area since the instrumental recordings are available. The focal mechanism solution of the mainshock, obtained from local, regional or global data, indicates normal faulting with the compression axis almost vertical and expansion axis almost horizontal, in agreement with the seismotectonic of the eastern and southern flanks of the Focsani Basin. Both nodal planes are oriented NW–SE, in the direction of the PCF. Going toward the south, in the Râmnicu Sărat area, reverse faulting is also obtained, which is characteristic of this area as highlighted in previous investigations. Applying SR and EGF methods the source parameters for 15 earthquakes of the sequence of November 22, 2014 (2.0  M  5.7) and for five earthquakes produced in the first seven months of 2015 (1.8  M 4.2) were determined. The source parameter values determined through SR and EGF methods are close, indicating an increasing trend of stress drop with seismic moment, induced especially by the high value of the mainshock. This seems to explain the not very well constrained pulse-like shape obtained by EGF deconvolution for the mainshock which could indicate a multi-shock release of seismic energy for this event. Investigation of the frequency–magnitude distribution at regional scale outlines a ‘nor- mal’ regime with b-slope value around 1, if we are situated sufficiently far from the perturbed activity related to the occurrence of the strong crustal earthquake of 22 November 2014, and a possible anomaly (b = 1.4) before the occurrence of this event. A behavior with pre-shock increase of b from about 1 to 1.5 and sudden decrease during the aftershock activity and Acoustics 2021, 3 293 further on is typical for acoustic experiment tests and for critical systems in which the population of cracks concentrates after the critical phase along preferential paths. Author Contributions: Conceptualization, A.O.P. and F.B.; methodology, E.P.; software, M.R.; val- idation, A.O.P., F.B., M.R. and I.M.; formal analysis, A.O.P.; investigation, A.O.P.; resources, M.R., A.O.P.,.; data curation, A.O.P.; writing—original draft preparation, A.O.P.; writing—review and editing, A.O.P., F.B., M.R., I.M.; visualization, F.B. and I.M.; supervision, M.R.; project administration, M.R.; funding acquisition, M.R., A.O.P., F.B. All authors have read and agreed to the published version of the manuscript. Funding: The research was partially supported by the “NUCLEU” programs (CREATOR and MULTIRISC) of the Romanian Ministry of Research and Innovation through the projects PN16350307 and PN19080102 and by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI) through the projects PN-III-P2-2.1-PED-2019-1693, 480 PED/2020 (PHENOMENAL) and PN-III-P4-ID-PCE- 2020-1361, 119 PCE/2021 (AFROS). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: We would like to thank two anonymous reviewers for their constructive com- ments which improved the original version of this manuscript. Several figures were made using GMT software. Conflicts of Interest: The authors declare no conflict of interest. References 1. Matenco, L.; Bertotti, G.; Leever, K.; Cloetingh, S.; Schmid, S.; Tărăpoancă, M.; Dinuc, C. Large-scale deformation in a locked collisional boundary: Interplay between subsidence and uplift, intraplate stress, and inherited lithospheric structure in the late stage of the SE Carpathians evolution. Tectonics 2007, 26, TC4011. [CrossRef] 2. Bălă, A.; Toma-Danila, D.; Radulian, M. 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Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting

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acoustics Article Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting 1 2 , 1 1 , 3 4 Anica Otilia Placinta , Felix Borleanu * , Emilia Popescu , Mircea Radulian and Ioan Munteanu Department of Research and Development Innovation in Earth Sciences, National Institute for Earth Physics, 12 Calugareni Street, P.O. Box MG2, Magurele, 077125 Ilfov, Romania; anca@infp.ro (A.O.P.); epopescu@infp.ro (E.P.); mircea@infp.ro (M.R.) National Data Center, National Institute for Earth Physics, 12 Calugareni Street, P.O. Box MG2, Magurele, 077125 Ilfov, Romania Academy of Romanian Scientists, 54 Splaiul Independentei, 050094 Bucharest, Romania Faculty of Geology and Geophysics, University of Bucharest, 6 Traian Vuia Street, 020956 Bucharest, Romania; ioan.munteanu@gmail.com * Correspondence: felix@infp.ro Abstract: Romanian seismicity is mainly confined to the Eastern Carpathians Arc bend (ECAB), where strong subcrustal earthquakes (magnitude up to 7.9) are generated in a narrow lithospheric body descending into the mantle. The seismic activity in the overlying crust is spread over a larger area, located mostly toward the outer side of the ECAB. It is significantly smaller than subcrustal seismicity, raising controversies about possible upper mantle-crust coupling. A significant earthquake sequence took place in the foreland of the ECAB triggered on 22 November 2014 by a mainshock of Citation: Placinta, A.O.; Borleanu, F.; magnitude 5.7 (the greatest instrumentally recorded earthquake in this region) located in the lower Popescu, E.; Radulian, M.; Munteanu, crust. The mainshock triggered a significant increase in the number of small-magnitude events spread I. Earthquake Source Properties of a over an unusually large area in the ECAB. The paper ’s goal is to compute the source parameters Lower Crust Sequence and of the earthquakes that occurred during the aforementioned sequence, by empirical application of Associated Seismicity Perturbation in the SE Carpathians, Romania, Green’s function and spectral ratio techniques. Fault plane solutions are determined using multiple Collisional Setting. Acoustics 2021, 3, methods and seismicity evolution at regional scale is investigated. Our results highlight a still active 270–296. https://doi.org/10.3390/ deformation regime at the edge of the EE Craton, while the source parameters reveal a complex acoustics3020019 fracture of the mainshock and a very high-stress drop. Academic Editor: C. W. Lim Keywords: earthquake sequence; source parameters; seismic source scaling; SE Carpathians foredeep Received: 12 March 2021 Accepted: 8 April 2021 Published: 19 April 2021 1. Introduction The crustal seismicity developed in front of the SE Carpathians bending zone (SEC) is Publisher’s Note: MDPI stays neutral spread over the entire area between the Neogene Trotus ¸ and Intra-Moesian faults (Figure 1). with regard to jurisdictional claims in Crustal earthquakes are generated into a complex structure with significant lateral inhomo- published maps and institutional affil- geneities involving different kinematics [1]. This is also reflected in the variability of the iations. focal mechanisms that show consistent stress regime with different localization: normal faulting on the eastern and southern flank of the Focsani ¸ Basin (FB), thrust faulting in the area adjacent to the Vrancea subcrustal earthquakes and strike-slip solutions preferably observed along the Peceneaga-Camena Fault [2]. The crustal seismicity in the SEC partly Copyright: © 2021 by the authors. overlaps with the intermediate-depth earthquakes generated in a depth range of 60–180 km, Licensee MDPI, Basel, Switzerland. beneath the Vrancea Zone (Figures 1 and 2). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Acoustics 2021, 3, 270–296. https://doi.org/10.3390/acoustics3020019 https://www.mdpi.com/journal/acoustics Acoustics 2021, 3 271 Acoustics 2021, 3 FOR PEER REVIEW   2  Figure 1. (a) Main geological units within the epicentral area (modified after [3]) and epicenters distribution (blue dots) of  Figure 1. (a) Main geological units within the epicentral area (modified after [3]) and epicenters distribution (blue dots) of the seismic events associated with the 2014 Mărăşeşti sequence. The inset map shows the study area (pink square) dis‐ the seismic events associated with the 2014 Mărăse ¸ sti ¸ sequence. The inset map shows the study area (pink square) displayed played at the regional scale. The red line represents three vertical cross‐sections crossing the epicentral area. (b) Seismic  at the regional scale. The red line represents three vertical cross-sections crossing the epicentral area. (b) Seismic activity activity at the SE Carpathians Arc bend according to the ROMPLUS catalogue (between 2010 and 2014, [4]) colored func‐ at the SE Carpathians Arc bend according to the ROMPLUS catalogue (between 2010 and 2014, [4]) colored function of tion of depth (intermediate‐depth events in black and crustal earthquakes within the color scale—right side), seismic sta‐ depth (intermediate-depth events in black and crustal earthquakes within the color scale—right side), seismic stations tions distribution (blue triangles) and neighboring cities. The white dashed lines represent the local fault system; the black  distribution (blue triangles) and neighboring cities. The white dashed lines represent the local fault system; the black line line shows the Pericarpathian Fault. The inset map shows a zoom of the epicentral area. The red star shows the mainshock  shows the Pericarpathian Fault. The inset map shows a zoom of the epicentral area. The red star shows the mainshock epicenter.  The  abbreviations  are  as  follows:  VR—Vrancea  Zone,  IMF—Intra‐Moesian  Fault,  PCF—Peceneaga‐Camena  epicenter Fault, AMRR . The—Amara, abbreviations  PGOar Re—Pogoanele, as follows: VR—V  TLB—Topalu rancea Zone,  seismi IMF—Intra-Moesian c stations.  Fault, PCF—Peceneaga-Camena Fault, AMRR—Amara, PGOR—Pogoanele, TLB—Topalu seismic stations.   Acoustics 2021, 3 272 Acoustics 2021, 3 FOR PEER REVIEW   3  Figure 2. (a) Epicenters distribution (colored as a function of local magnitude ML) within the epicentral area (0.75 deg.  Figure 2. (a) Epicenters distribution (colored as a function of local magnitude M ) within the epicentral area distance around the mainshock) for the seismic activity recorded during a time interval of 100 days (22 November 2014–2  (0.75 deg. distance around the mainshock) for the seismic activity recorded during a time interval of 100 days March 2015) from the mainshock (red star) occurrence; (b) hypocenters distribution displayed in the same area and for the  (22 November 2014–2 March 2015) from the mainshock (red star) occurrence; (b) hypocenters distribution displayed in the same time interval mentioned above versus latitude and function of ML and (c) versus longitude and function of ML.  same area and for the same time interval mentioned above versus latitude and function of M and (c) versus longitude and function of M . The intermediate‐depth seismicity is located in a narrow seismogenic volume which  The intermediate-depth seismicity is located in a narrow seismogenic volume which descends almost vertically in the mantle, able to generate 2–3 strong events (Mw ≥ 7) per  descends almost vertically in the mantle, able to generate 2–3 strong events (Mw  7) per century. The seismicity in the overlying crust is limited to moderate‐size events (Mw < 6)  century. The seismicity in the overlying crust is limited to moderate-size events (Mw < 6) and spreads over a larger area following some specific alignments. One of the significant  and spreads over a larger area following some specific alignments. One of the significant seismically active faults placed in front of the SEC is the Peceneaga‐Camena (PCF) fault  seismically active faults placed in front of the SEC is the Peceneaga-Camena (PCF) fault zone  and  related  deformation,  which  delineates  (Figure  1)  the  boundary  between  the  zone and related deformation, which delineates (Figure 1) the boundary between the Moesian Platform (MP) and North Dobrogea Orogen (NDO, [5]). Along its path towards  Moesian Platform (MP) and North Dobrogea Orogen (NDO, [5]). Along its path towards NW from the Black Sea Basin to the SEC, the fault shows two segments with distinct seis‐ NW from the Black Sea Basin to the SEC, the fault shows two segments with distinct seismic mic activities: (1) from the Black Sea Basin to the Danube River, and (2) from the Danube  activities: (1) from the Black Sea Basin to the Danube River, and (2) from the Danube River River to the NE edge of the Vrancea epicentral area. The first segment is less active with a  to the NE edge of the Vrancea epicentral area. The first segment is less active with a stress stress regime of thrusting faulting [6,7], while the second segment displays a significant  regime of thrusting faulting [6,7], while the second segment displays a significant seismic seismic activity with predominant strike‐slip faulting along the major fault line and its  activity with predominant strike-slip faulting along the major fault line and its satellites, satellites, oriented SE—NW. Towards the NW edge, where the PCF intersects the Peri‐ Carpathian Fault, the seismicity configuration becomes more complex (Figure 1).     Acoustics 2021, 3 273 oriented SE—NW. Towards the NW edge, where the PCF intersects the Peri-Carpathian Fault, the seismicity configuration becomes more complex (Figure 1). Several studies [7–11] indicate this area as the place where the descending lithospheric slab started to break off, beneath the SEC, at the time of the continental collision between the Carpathian (as part of Tisza-Dacia) plate overriding the East-European, Scythian and Moesian plates. In this complex tectonic framework, a significant earthquake sequence occurred in November 2014 close to Mărăse ¸ sti ¸ city. According to the Romanian earthquake catalogue ROMPLUS [4] permanently updated by the Romanian Data Centre (RONDC) of the Na- tional Institute for Earth Physics (NIEP), the mainshock occurred on 22 November 2014 at 19:14 (GMT), located in the lower crust (H = 41 km), in the range of the Moho disconti- nuity [12] and had a local magnitude (M ) of 5.7. The occurrence of this sequence in the Mărăse ¸ sti ¸ area is the most significant episode of seismic energy release in this region since instrumental recordings were used to monitor the seismic activity in Romania. A similar event is mentioned in the ROMPLUS catalogue but occurred more than one century ago (01.03.1894, M = 6.1). The poly-kinetic character of these events has been pointed out by previous stud- ies [13,14] as well as through the analysis of several significant earthquake sequences [15,16] generated along the Eastern Carpathians [17–19] However, the 2014 Mărăsesti seismic se- , , quence differs from past ones, because of the location of a large part of the hypocenters close to the crust-mantle discontinuity and because of the subsequent increase of seismic activity spread over an unusually large area in the adjacent regions (Figures 1 and 2). The analysis of the source parameters for the earthquakes of the seismic sequence will certainly bring relevant elements to the understanding of the tectonic processes and of the physical mechanism driving seismicity in the region. Moreover, the seismic hazard assess- ment strongly depends on estimation accuracy. One of the most debated parameters in such analysis is the stress drop. This gives indications about the amount of released energy and also may estimate the destructive potential of an earthquake. A major disadvantage is given by the estimation of accuracy since its uncertainties are rather high, due to the corner frequency determinations that could increase or decrease the stress drop value by at least three times [20]. Large earthquakes with magnitudes of M > 6 occurred in inter-plate or intraplate regions [21] and pointed out stress drops in a range of 3–10 MPa, with an increasing trend for intraplate earthquakes [22,23]. For crustal earthquakes occurring in the East- ern Carpathians’ foredeep, previous studies [24,25] highlight stress drops in a range of 2.5–50 MPa. Another notable feature highlighted by previous research [26–28] reveals simi- lar properties for small and large earthquakes, changing only in scale, which would imply a constant stress drop for the whole range of magnitudes. However, several studies have shown [20,29] that for small earthquakes (M < 3) there is an increasing trend towards stress drop at the seismic moment. One of the features that could cause this behavior is the strong attenuation effect within the upper crust, which acts as a filter, removing the high frequencies (f > 10 Hz) and limiting the corner frequencies’ resolution [30], especially for small earthquakes, leading to an underestimation of the stress drop. In this study, we determine source parameters of 21 earthquakes that occurred during the 2014 Mărăse ¸ sti ¸ sequence (22 November 2014–1 February 2015), as well as for several events that have occurred at higher distances or were delayed in time, but considered as having been triggered by the aforementioned seismic sequence. To achieve this goal, we apply empirical Green’s function (EGF) and spectral ratio (SR) techniques, since they allow suitable constraints independent of the structure between the source and receiver, which is difficult to know with sufficient accuracy. In addition, we determine the fault plane solutions for some of these events and investigate the evolution of seismic activity in the region to draw an interpretation in connection with the tectonic structures highlighted by the previous studies. Acoustics 2021, 3 274 The previous investigations dealing with the 2014 Mărăs ¸es ¸ti sequence limited themselves to the evaluation of the macro-seismic effects [31] and the computation of the focal mechanisms for the main event and 32 aftershocks using P-wave polarities and amplitude ratios [32]. Sequence Description The seismic activity in front of the SEC bending zone along the PCF up to the north- western edge at the contact with the Vrancea epicentral area and Scythian Platform has continuously developed in time with sporadic weak-to-moderate earthquakes or clusters of events (Figure 1). These events are probably related to some extent to the geodynamics in the mantle beneath the Vrancea region. However, this is still a controversial issue [6,7,33] and therefore understanding the interdependence of the tectonic stress in the mantle and coupling and crustal seismicity requires further investigation. The mainshock of 22 November was followed by 230 aftershocks located by the RONDC using the LOCSAT routine [34] embedded within the Antelope software and using IASP91 as 1-D global velocity model [35]. A noteworthy increase of seismicity in the region was recorded after the mainshock for at least three months which is significantly more than the duration typical for the aftershock activity of moderate-size mainshocks. The aftershocks were numerous but unusually weak in size relative to the magnitude of the mainshock (Figure 2). A single stronger event was recorded on 7 December 2014 (M = 4.5), and only two events with M > 3 were recorded on 22 November 2014 (M = 3.1) L L L and 19 January 2015 (M = 3.8). The seismic activity generated after the mainshock occurrence (22 November 2014) manifests through a significant increase in the seismicity rate first in the Mărăse ¸ sti ¸ area and second over an extended region to the south and to the north (the Râmnicu Sărat and Adjud areas, see Figure 2). The triggered seismicity developed over a large region and for a long-time interval. 2. Methods and Data To determine the source parameters of the earthquakes that occurred during the 2014 seismic sequence close to the Mărăse ¸ sti, ¸ and also for the events triggered by this seismic activity, we searched in the ROMPLUS catalogue (November 2014–July 2015) for clusters of events (co-located events with similar waveforms). Each cluster has one or several main events and associated EGFs. The selection of clusters is carried out in two steps. To select the potential EGF events, we started with the mainshock (M1/5.7 in Figure 3) and searched among the events within a magnitude able to solve the source parameters within the frequency range of the available data. First, for each main event (M) we searched in the aforementioned catalogue for potential EGF events if the inter-hypocenter distance is less than 0.3 deg. and the difference in magnitude around one magnitude unit or more. As a second step, we applied the waveform cross-correlation technique to select as EGF events those with large cross-correlation coefficients (CC  0.7). A high cross-correlation for a pair of events is considered to consistently reflect the waveform similarity among them [28,36]. For the waveform correlation analysis, we used time windows based on the magnitude of the M [21], a length of 0.5–1 s being enough for the small size events. The waveforms are bandpass filtered to an appropriate frequency band, using a two-pole Butterworth filter. We low-pass filter both the M and each potential EGF at a frequency related to the expected corner frequency of the M. This is because large and small earthquakes are not expected to cross-correlate well at high frequencies (e.g., [36]). We applied cross-correlation separately for P and S waves. Acoustics 2021, 3 275 Table 1. Selected main (bold characters) and associated EGF events. Date OriginTime No Latitude ( N) Longitude ( E) Depth (km) M DDMMYYYY hh:mm:ss M1 22112014 19:14:17 45.8683 27.1517 41 5.7 M2 22112014 20:30:56 45.8530 27.1859 36 3.1 M3 07122014 21:04:05 45.8836 27.1714 41 4.5 M4 12012015 06:08:31 45.5420 27.0448 19 4.2 M5 19012015 23:53:07 45.8782 27.1483 40 3.8 M6 29062015 22:20:56 46.0114 27.1754 19 4.0 1 22112014 19:27:39 45.8688 27.1185 40 2.0 2 22112014 20:24:47 45.8626 27.1644 34 2.8 3 22112014 20:30:56 45.8530 27.1859 36 3.1 4 23112014 01:14:39 45.8819 27.1610 35 2.5 5 23112014 02:21:05 45.8509 27.1852 34 2.5 6 23112014 04:01:58 45.8230 27.1750 31 2.6 7 23112014 05:27:58 45.8450 27.1889 35 2.4 8 23112014 23:22:49 45.8477 27.1852 33 2.3 9 24112014 00:45:06 45.8700 27.1660 34 2.4 10 25112014 01:52:25 45.8480 27.1612 39 3.2 11 07122014 21:04:05 45.8836 27.1714 41 4.5 12 14122014 17:24:47 45.6054 27.1018 14 3.1 13 14122014 18:24:34 45.6036 27.1164 16 2.6 14 18012015 22:41:40 45.4962 27.0243 31 1.8 Acoustics 2021, 3 FOR PEER REVIEW   6  15 19012015 23:53:07 45.8782 27.1483 40 3.8 16 20012015 02:52:46 45.8850 27.1585 33 2.2 17 01072015 04:34:24 46.0217 27.1893 17 3.2 Figure 3. Hypocenters distribution of main (filled dots) and Green’s function events (empty cir‐ Figure 3. Hypocenters distribution of main (filled dots) and Green’s function events (empty circles). cles). The colors of the main event (M) match the colors of empirical Green’s function (EGF)  The colors of the main event (M) match the colors of empirical Green’s function (EGF) events. The events. The M are counted from M1 to M6 and their magnitudes (ML) are written to the right side.  M are counted from M1 to M6 and their magnitudes (M ) are written to the right side. Note that Note that some EGF events can be associated with more M. M2, M3 and M5 are selected both as M  some EGF events can be associated with more M. M2, M3 and M5 are selected both as M and EGF and EGF (see Table 1).  (see Table 1). We found 6 individual master events (M1 to M6 given below) and plot them in Figure  3 as dots filled in different colors and 17 EGF (empty circles, with the outline colors match‐ ing one of the master events, see Figure 3), meeting our requirements imposed by the  method.  M1: 22 November 2014, 19:14, ML = 5.7.  M2: 22 November 2014, 20:30, ML = 3.1.  M3: 7 December 2014, 21:04, ML = 3.1.  M4: 12 January 2015, 06:01, ML = 4.2.  M5: 19 January 2015, 23:53, ML = 3.8.  M6: 29 June 2015, 22:20, ML = 4.0.  To determine the source parameters of the selected events (Figure 3 and Table 1),  only the broadband recordings of the RSN were considered.   Table 1. Selected main (bold characters) and associated EGF events.  Date  OriginTime  No  Latitude (°N)  Longitude (°E)  Depth (km)  ML  DDMMYYYY  hh:mm:ss  M1  22112014  19:14:17  45.8683  27.1517  41  5.7  M2  22112014  20:30:56  45.8530  27.1859  36  3.1  M3  07122014  21:04:05  45.8836  27.1714  41  4.5  M4  12012015  06:08:31  45.5420  27.0448  19  4.2  M5  19012015  23:53:07  45.8782  27.1483  40  3.8  M6  29062015  22:20:56  46.0114  27.1754  19  4.0  1  22112014  19:27:39  45.8688  27.1185  40  2.0  2  22112014  20:24:47  45.8626  27.1644  34  2.8    Acoustics 2021, 3 276 We found 6 individual master events (M1 to M6 given below) and plot them in Figure 3 as dots filled in different colors and 17 EGF (empty circles, with the outline colors matching one of the master events, see Figure 3), meeting our requirements imposed by the method. M1: 22 November 2014, 19:14, M = 5.7. M2: 22 November 2014, 20:30, M = 3.1. M3: 7 December 2014, 21:04, M = 3.1. M4: 12 January 2015, 06:01, M = 4.2. M5: 19 January 2015, 23:53, M = 3.8. M6: 29 June 2015, 22:20, M = 4.0. To determine the source parameters of the selected events (Figure 3 and Table 1), only the broadband recordings of the RSN were considered. The fault plane solution has a significant relevance since it is directly used to determine the stress regime of a region. To obtain a high accuracy in determining the focal mechanisms, we selected only the events whose recordings (broadband or short-period stations) allowed us to read with enough accuracy the P-waves’ first motion at a minimum of 12 stations. Two methods provided by the SEISAN algorithm [37] are applied. Both FOCMEC [38] and HASH [39,40] algorithms use the first P-wave polarities together with the amplitudes measured on all three components (vertical, transversal and radial). The FOCMEC method is more rigorous since it removes the erroneous polarities and ratios above an imposed limit (we adopted 0.3 in our case). HASH uses weights for the polarity and amplitude ratio errors and provides averaged values, without eliminating the stations with large errors. For both methods, we filtered the seismograms, removing the frequencies above the corner frequency, and measured the maximum amplitudes at intervals of maximum 2s starting at first P, then respectively S arrivals. The spectral ratios and empirical Green’s function methods have been used in various areas of the world [36,41–46] showing reliable results in retrieving source parameters. These are efficient in eliminating the path, site and instrument effects, which distort to a large extent the ground motion observed at the surface when pairs of earthquakes are available and satisfy the conditions imposed for the selection of the events. Examples of earthquake pairs recorded by the Topalu (TLB) station are given in Figure 4. The SRs are approximated by a nonlinear least-squares procedure with theoretical spectral ratios, depending mostly on the source properties. The fit parameters are the ratios of the low-frequency asymptotes and corner frequencies of the large and small shocks, respectively. The least-squares best fit is estimated by iteration from an initial guess using a simplex algorithm. The relationship which best approximates the Fourier spectral ratio R for a pair of collocated events is: 1/2 M G W [1 + ( f / f ) ] 0 C R f = (1) ( ) 1/2 G M W [1 + ( f / f ) ] 0 C M G where W , W are the low-frequency spectral amplitudes of main and EGF earthquakes, 0 0 M G and f , f are the corresponding corner frequencies. We used the hypothesis of the c c Brune’s model [47,48] with spectral decay at the high frequencies, . The ratio of the low-frequency spectral levels is equal to the ratio of the seismic moments for the collocated earthquakes if they have similar fault plane solutions. Acoustics 2021, 3 277 Acoustics 2021, 3 FOR PEER REVIEW   9  Figure 4. Waveforms recorded by Topalu (TLB) seismic station, for: (a) M (red) of 22 November 2014, 19:14; (b) EGF (green)  Figure 4. Waveforms recorded by Topalu (TLB) seismic station, for: (a) M (red) of 22 November 2014, 19:14; (b) EGF (green) of 22 November 2014, 20:30; (c) M (red) of 7 December 2014, 21:04; (d) EGF (green) of 22 November 2014, 20:24.  of 22 November 2014, 20:30; (c) M (red) of 7 December 2014, 21:04; (d) EGF (green) of 22 November 2014, 20:24. The Thesize  seism ofithe c moment rupturse fo ar r ea EGF is events computed,  were taking determine intod consideration using the spectthe ral ra cirti cular o value fault   assumption, directly from the corner frequency [47,48]: at low frequency as is given by the following equations:  𝑙𝑜𝑔𝑀 𝑙𝑜𝑔Ω P,S r = K (2) P,S (5) 𝑎   f P, S 𝑙𝑜𝑔𝑀 𝑙𝑜𝑔Ω where r represents the source radius, K is a constant (K = 0.32 for P waves and K = 0.21 P S for S waves), f is the corner frequency (for P and S waves) and V is the velocity of P and S c (6) 𝑀   waves in the epicentral region. The determinations were performed for both P and S waves using vertical, or respectively horizontal, components of velocity recordings. Once the seismic moment and source radius estimates are obtained, we compute the  Because the spectral ratio technique is a relative method, it cannot estimate the absolute stress drop values using the relation of [47]:   values of the seismic moments for the two earthquakes, computing only their ratio. Based 7𝑀 on the spectral analysis we determined P and S displacement spectra separately and seismic (7)    moments of the main events by using the following equation: 16𝑟 To determine the source time function (STF) 3  we applied in parallel the method of  4p r v W R M = (3) empirical Green’s functions, deconvolv 0 ing EGF functions from M events. The deconvolu‐ F R qj tion functions were obtained by spectral division for different time windows (function of  the size of the event) for both P and S waves, using a tapering of 10%. To stabilize the  where r is the density at the source depth, v is the velocity of P or S waves at source depth, spectral division, the amplitude spectrum of EGF was smoothed out, before division, with  W is the long period displacement spectral level, R is the hypo-central distance, F (=2) is a five‐point running moving window. In addition, to reduce high‐frequency instabilities,  the free-surface parameter and R is the source radiation pattern (average values of 0.52 qj the deconvolved source time function was band‐pass filtered between 0.5 and 10 Hz with  for P waves and 0.63 for S waves, according to [49]). The density and velocities adopted for the study area were taken from [12] (V = 7.0 km/s, V = 4.04 km/s and r = 2.85 g/cm ). P S The seismic moments determined from P and S wave recordings are rather close, therefore,   Acoustics 2021, 3 278 we estimated M for each station as the mean value and as a final value the median of all selected stations was considered. Based on M we also compute the moment magnitude (M ) using the equation of [50]: log M M = 6.03 (4) 1.5 The seismic moments for EGF events were determined using the spectral ratio value at low frequency as is given by the following equations: M M log M logW 0 0 a = = (5) G G log M logW 0 0 G 0 M = (6) Once the seismic moment and source radius estimates are obtained, we compute the stress drop values using the relation of [47]: 7M Ds = (7) 16r To determine the source time function (STF) we applied in parallel the method of empirical Green’s functions, deconvolving EGF functions from M events. The deconvo- lution functions were obtained by spectral division for different time windows (function of the size of the event) for both P and S waves, using a tapering of 10%. To stabilize the spectral division, the amplitude spectrum of EGF was smoothed out, before division, with a five-point running moving window. In addition, to reduce high-frequency instabilities, the deconvolved source time function was band-pass filtered between 0.5 and 10 Hz with a Butterworth filter [45]. To increase the accuracy of the results, we used EGF pairs recorded by broadband stations by each component and stacked them up. The source rise time and source duration (t, t1/2) for the main events were computed from the STFs each time it had a pulse-like shape. The corner frequency (fc) was determined from the source duration using equation (8) from [20] and finally we computed the source radius using formula (2). f = (8) tp In addition, to have a better image of the tectonic processes in the region we computed the Gutenberg-Richter relation (see [51] and Equation (9)) before, during and after the seismic sequence, for an area extending to a distance of 0.75 deg. around the epicenter of the mainshock. The analysis was performed with Zmap software [52] using the maximum likelihood method [53]. For computation, we take into account local magnitude provided by the ROMPLUS catalogue, estimated as a maximum Wood-Anderson amplitude of one of the horizontal components corrected for distance and compiled only for stations with a good signal/noise ratio (SNR  3). log N( M) = a bM (9) N(M) is the number of earthquakes with magnitude larger or equal to M, a is a constant and b is the scaling parameter. 3. Results The results obtained for fault plane solutions, source parameters and evolution of seismic activity within the study area are in agreement with the findings pointed out by previous studies. By increasing the number of seismic stations in the area, we succeeded in increasing the results accuracy and also in adding complementary information that could improve the seismotectonic characteristics of the region. Acoustics 2021, 3 279 3.1. Focal Mechanisms The fault plane solutions determined by applying FOCMEC and HASH techniques are displayed in Figure 5 and listed in Table 2, where we also list the associated errors and the amount of data used. Therefore, for the FOCMEC method the amplitude ratio fit AF, no. of bad polarities NBP and no. of bad amplitude ratios NBAR are provided; while for the HASH routine the fault plane uncertainty FPU, auxiliary fault plane uncertainty AFPU, fit errors F-fit and station distribution ratio STDR are listed. All the fault plane Acoustics 2021, 3 FOR PEER REVIEW   12  solutions are well constrained by the selected seismic recordings. In addition, the focal mechanisms determined using both methods are comparable, increasing their reliability degree, and are also in agreement with the results pointed out within the study area by past studies [54–56]. For the mainshock, the fault plane solution indicates normal 319  67  77  169  27  119  faulting with both nodal planes NW- SE oriented, in the direction of the Peceneaga-Camena 17  19012015  23:53:07  3.8  0.16  2  1  3.30  2.20  0.05  42/6  RF  Fault zone. Our determination shows a good similarity with the solutions computed by          0.73  several international centers (INGV, USGS, GFZ), as well as with the solution of [32]. The 311  74  20  214  72  161  compression axis is plunging almost vertically, while the extension axis is almost horizontal 18  29062015  22:20:56  4.0  0.21  2  7  9.20  3.00  0.05  43/11  RLLO  W-E oriented. According to [57], the fault-plane solutions can be grouped into six types of          0.69  faulting (Table 2): normal (NF—4 events), strike-slip (SS—1 event), thrust (RF- 2 events), 257  64 −16  354  69 −170  normal left lateral (NLLO—6 events), thrust left lateral (RLLO—5 events) and strike-slip 19  01072015  04:34:24  3.2  0.25  1  2  14.70  4.30  0.07  26/6  LLSS  left lateral (LLSS—1 event).          0.46  Figure 5. The fault plane solutions computed using (a) FOCMEC and (b) HASH algorithms, for 19 earthquakes of the  Figure 5. The fault plane solutions computed using (a) FOCMEC and (b) HASH algorithms, for 19 earthquakes of the sequence that are listed in Table 2. Beach‐balls are displayed as a function of magnitude. The circles show earthquake  sequence that are listed in Table 2. Beach-balls are displayed as a function of magnitude. The circles show earthquake epicenters with outline color as a function of depth.  epicenters with outline color as a function of depth. 3.2. Spectral Ratios  The spectral ratios method was applied to all earthquake pairs, the main event‐em‐ pirical Green’s function, listed in Table 3. The source parameters in Equation (1) are the  asymptote at low frequency (in logarithmic scale), a, the corner frequencies of main (fc )  and EGF (fc ) events. The corner frequencies are determined by approximating the com‐ puted spectral ratio with the theoretical ratio and averaging the results for all the available  earthquake pairs and stations. The analysis was performed for both P and S‐waves, taking  into account only the recordings where the signal‐to‐noise ratio was above 2. The results  obtained for M and EGF events are given in Table 4 and examples of spectral ratios com‐ puted for pairs of earthquakes are displayed in Figure 6. Note that the obtained results  show that low‐frequency level, a, and the corner frequency of the EGF (fc ) show a higher  variation since they depend on the earthquake pair, while for the main event the corner  frequency (fc ) should not be influenced by the pair of the events if source directivity ef‐ fects are negligible. This feature is similar to the results pointed out by previous studies  [58].       Acoustics 2021, 3 280 Table 2. Fault plane solutions computed both FOCMEC and HASH techniques (crt. no. corresponds with the numbers in Figure 5). FOCMEC HASH No. of Origin Polarities/ Type of Crt. Date Strike Dip Rake Strike Dip Rake Time L No. of Fault No. DDMMYYYY hh:mm:ss Amplitude Plane F-Fit AF NBP NBAR FPU AFPU Ratios STDR 152 55 75 305 27 110 1 22112014 19:14:17 5.7 0.08 3 5 1.10 1.80 0.06 48/11 NF 0.40 77 71 69 207 30 134 2 22112014 19:27:39 2.0 0.19 3 4 2.00 2.40 0.28 12/6 NF 0.69 96 72 81 100 72 109 13/7 3 22112014 19:32:37 2.1 0.18 1 4 11.30 13.50 0.19 NF 0.65 315 40 61 106 60 110 4 22112014 20:24:47 2.8 0.20 1 1 2.80 0.00 0.09 24/13 NLLO 0.67 287 1 90 109 75 91 5 22112014 20:30:56 3.1 0.17 0 2 2.80 3.10 0.05 24/6 NF 0.51 70 36 26 192 65 120 21/5 6 23112014 01:14:39 2.5 0.11 1 3 6.50 5.30 0.21 NLLO 0.71 327 33 62 102 68 116 7 23112014 02:21:05 2.5 0.15 3 2 0.60 1.00 0.11 22/4 NLLO 0.72 344 64 0 248 71 146 8 23112014 04:01:58 2.6 0.12 1 2 6.40 5.00 0.04 19/6 SS 0.6 298 73 58 244 26 103 9 23112014 05:27:58 16/2 RLLO 2.4 0.16 2 1 4.70 9.10 0.15 0.68 128 85 60 215 40 174 10 23112014 10:16:15 2.7 0.13 1 3 10.40 17.20 0.07 19/7 NLLO 0.48 127 18 26 8 20 21 11 25112014 01:52:25 3.2 0.04 1 1 5.10 15.90 0.15 23/2 RLLO 0.72 315 54 37 177 44 117 02122014 04:19:29 2.5 12/6 RLLO 12 0.17 1 3 17.60 3.20 0.20 0.65 141 81 47 260 43 144 45/11 13 07122014 21:04:05 4.5 0.11 4 6 10.40 8.10 0.17 NLLO 0.70 158 44 22 48 62 121 14 14122014 17:24:47 3.1 0.19 2 1 5.10 2.10 0.16 28/7 RLLO 0.63 129 55 24 234 68 143 15 14122014 18:24:34 2.6 17/4 NLLO 0.14 1 1 5.20 1.90 0.09 0.57 220 45 82 226 45 93 36/5 16 12012015 06:08:31 4.2 0.14 6 2 1.50 1.00 0.18 RF 0.62 Acoustics 2021, 3 281 Table 2. Cont. FOCMEC HASH No. of Origin Polarities/ Type of Crt. Date Strike Dip Rake Strike Dip Rake Time L No. of Fault No. DDMMYYYY hh:mm:ss Amplitude Plane F-Fit AF NBP NBAR FPU AFPU Ratios STDR 319 67 77 169 27 119 17 19012015 23:53:07 3.8 0.16 2 1 3.30 2.20 0.05 42/6 RF 0.73 311 74 20 214 72 161 18 29062015 22:20:56 4.0 43/11 RLLO 0.21 2 7 9.20 3.00 0.05 0.69 257 64 16 354 69 170 19 01072015 04:34:24 3.2 0.25 1 2 14.70 4.30 0.07 26/6 LLSS 0.46 3.2. Spectral Ratios The spectral ratios method was applied to all earthquake pairs, the main event- empirical Green’s function, listed in Table 3. The source parameters in Equation (1) are the asymptote at low frequency (in logarithmic scale), a, the corner frequencies of main M G (f ) and EGF (f ) events. The corner frequencies are determined by approximating c c the computed spectral ratio with the theoretical ratio and averaging the results for all the available earthquake pairs and stations. The analysis was performed for both P and S-waves, taking into account only the recordings where the signal-to-noise ratio was above 2. The results obtained for M and EGF events are given in Table 4 and examples of spectral ratios computed for pairs of earthquakes are displayed in Figure 6. Note that the obtained results show that low-frequency level, a, and the corner frequency of the EGF (f ) show a higher variation since they depend on the earthquake pair, while for the main event the corner frequency (f ) should not be influenced by the pair of the events if source directivity effects are negligible. This feature is similar to the results pointed out by previous studies [58]. P S The corner frequencies ratio of P to S waves (f /f ) averaged over all stations for c c the main events varies between 0.94 and 1.06 for the M1 and M2 events, respectively, with a mean value of 1.05 computed for all selected events (see Table 4). These results show similarities with the ones pointed out for various regions of the world [45,46,59–62], and follow the circular source model proposed by [63]. Commonly, the P source pulse shows a shorter duration than the S source pulse, which leads to higher corner frequencies for P waves because the propagation time of P waves P S is faster compared with S waves. Thus, an average value of 1.5 is expected for the f /f c c when the waves leave the fault at angles of incidence greater than 30 from the normal. For cases where the propagation of the rays is close to the normal direction of the fault, the P and S waves leave the source in-phase and interference due to the source limitation is P S negligible (f  f ). c c Using equations from (1) to (7), we determined the seismic moment, source radius and stress drop for the main and EGF events (Tables 5 and 6). The source radius estimated from corner frequency is an average between the estimations using P-wave and S-wave P S corner frequencies. Since relation (2) assumes a ratio of about 1.5 for f /f and for our data c c P S P f  f , the radius computed from f is systematically greater than the radius computed c c c from f roughly by a factor of 1.5 (see Tables 5 and 6). c Acoustics 2021, 3 282 Table 3. Event pairs, the inter-distance and inter-magnitude separation parameters. The correspond- ing events are given in Table 1. Epicenters Hypocenters Depths Earthquake Pair Inter-Distance Inter-Distance Distance M Difference (km) (km) (km) M1-1 2.6 2.9 1.4 3.7 M1-2 1.2 6.6 6.5 2.9 M1-3 3.1 5.6 4.6 2.6 M1-4 1.7 6.3 6.1 3.2 M1-5 3.2 7.4 6.7 3.2 M1-6 5.3 11.2 9.9 3.1 M1-8 5.3 9.3 7.6 3.4 M1-10 2.4 3.2 2.1 2.5 M1-11 2.3 2.3 0.3 1.2 M1-15 1.1 1.6 1.1 1.9 M1-16 1.9 7.7 7.5 3.5 M2-1 5.5 6.4 3.2 1.1 M3-1 4.4 4.5 1.1 2.5 M3-2 2.4 6.6 6.2 1.7 M3-3 3.6 5.6 4.3 1.4 M3-4 0.8 5.9 5.8 2.0 M3-5 3.8 7.4 6.4 2.0 M3-6 6.7 11.7 9.6 1.9 M3-7 4.5 7.5 6.0 2.1 M3-8 6.7 9.9 7.3 2.2 M3-9 1.6 6.4 6.2 2.1 M3-10 4.0 4.4 1.8 1.3 M3-16 1.0 7.3 7.2 2.3 M4-12 8.6 9.7 4.5 1.1 M4-13 8.8 9.3 3.1 1.6 M4-14 5.3 15.8 14.9 2.4 M5-1 2.5 2.5 0.3 1.8 M5-2 2.1 5.8 5.4 1.0 M5-7 4.8 7.1 5.2 1.4 M5-9 1.6 5.6 5.4 1.4 M5-16 1.1 6.5 6.4 1.6 M6-17 1.4 1.9 1.3 0.8 P S P S Table 4. Corner frequencies of P (f ) and S (f ) waves with associated ratio (f /f ) determined for c c c c main (bold characters) and associated EGF events (as were listed in Table 1). P S P S No. of ev. f (Hz) f (Hz) f /f c c c c M1 1.92 2.04 0.94 M2 9.25 8.72 1.06 M3 3.81 3.98 0.96 M4 4.46 4.69 0.95 M5 5.67 5.38 1.05 M6 5.13 5.20 0.99 1 16.44 17.98 0.91 2 10.07 8.62 1.17 3 3.81 3.98 0.96 4 11.14 8.86 1.26 5 11.75 10.12 1.16 6 9.21 8.96 1.03 7 11.47 10.97 1.05 8 15.00 10.61 1.41 9 11.15 12.08 0.92 10 6.48 6.79 0.95 11 3.81 3.98 0.96 12 8.25 6.10 1.35 13 7.30 9.69 0.75 14 17.06 16.45 1.04 15 5.67 5.38 1.05 16 15.83 14.91 1.06 17 7.20 6.80 1.06 Acoustics 2021, 3 283 Acoustics 2021, 3 FOR PEER REVIEW   15  Figure 6. Spectral ratios determined (for vertical‐left and horizontal‐right components) for the following pairs: (a) M of 22  Figure 6. Spectral ratios determined (for vertical-left and horizontal-right components) for the following pairs: (a) M of November 2014, 19:14 and EGF of 22 November 2014, 20:30; (b) M of 19 January 2015, 23:53 and EGF of 20 December,  22 November 2014, 19:14 and EGF of 22 November 2014, 20:30; (b) M of 19 January 2015, 23:53 and EGF of 20 December, 02:52. 02:52.  Table 5. Final source parameters for main events determined using the SR technique. Using equations from (1) to (7), we determined the seismic moment, source radius  and stress drop for the main and EGF events (Tables 5 and 6). The source radius estimated  Seismic Source Radius (m) Stress Drop (MPa) Event Mw P S P S from corner frequency Moment (Nm)  is an average between f (Hz)  the estimations f (Hz) using Pf‐wave and Sf‐wave  c c c c P S corner frequencies. Since relation (2) assumes a ratio of about 1.5 for fc /fc  and for our data  M1 1.76  10 5.4 1236 914 41.0 101.0 P S P fc  ≈ fc , the radius computed 14  from fc  is systematically greater than the radius computed  M2 1.14  10 2.7 234 166 3.5 11.0 from fc  roughly by a factor of 1.5 (see Tables 5 and 6).  M3 2.39  10 3.4 622 397 4.4 16.7 M4 3.3 402 256 5.0 19.4    7.43  10 M5 7.55  10 3.1 417 294 4.6 13.0 M6 4.28  10 3.2 366 241 3.8 13.4   Acoustics 2021, 3 284 Table 6. Final source parameters for EGFs determined using the SR technique. Source Radius (m) Stress Drop (MPa) Date Origin Time Seismic Mw P S P S DDMMYYYY hh:mm:ss Moment (Nm) from f from f c c 22112014 19:27:39 1.80  10 2.1 144 88 2.6 11.6 22112014 20:24:47 5.92  10 2.6 215 168 2.6 5.5 22112014 20:30:56 2.7 234 166 3.5 11.0 1.14  10 23112014 01:14:39 4.63  10 2.4 194 163 2.8 4.7 23112014 02:21:05 3.11  10 2.4 184 143 2.2 4.7 23112014 04:01:58 2.5 213 147 1.9 5.8 4.23 10 23112014 05:27:58 3.93  10 2.3 189 132 2.5 7.5 23112014 23:22:49 2.14  10 2.3 144 136 3.1 3.7 24112014 00:45:06 2.48  10 2.3 194 120 1.5 6.3 25112014 01:52:25 2.21  10 2.8 303 193 3.5 13.5 07122014 21:04:05 3.4 622 397 4.4 16.7 2.39 10 14122014 17:24:47 1.30  10 2.7 236 213 4.3 5.9 14122014 18:24:34 4.83  10 2.5 266 134 1.1 8.8 18012015 22:41:40 2.0 115 80 2.5 7.4 8.71  10 19012015 23:53:07 7.55  10 3.1 417 294 4.6 13.0 20012015 02:52:46 5.12  10 2.2 137 98 8.7 24.5 01072015 04:34:24 6.24 10 2.8 249 176 1.8 5.0 3.3. Empirical Green’s Function Analysis For the same pairs of events considered in the SR method, we applied in parallel the method of deconvolution with EGF to compute for M the STFs, rise time (t1/2), source duration (t), corner frequency (fc) and source radius (r) using Equations (2) and (8). To remove any biases, we computed the relative source time function for a given station by summing the functions resulting from deconvolution from all three components of that station. The obtained values are listed in Table 7 and examples of STFs resulting from the deconvolution method are displayed in Figure 7. The source radius and stress drop values are equivalent to the same values inferred by the SR method. The only exception is the case of the mainshock where the differences are ~30% smaller for source radius and ~40% larger for stress drop. Table 7. Final source parameters for main events determined using the EGF technique. Source Radius (m) Stress Drop (MPa) Seismic Moment t (s) f (Hz) Event Mw c t t (Nm) M1 1.76  10 5.4 0.5 1.27 666 260.7 M2 1.14  10 2.7 0.16 3.98 213 5.2 M3 2.39  10 3.4 0.26 2.45 346 25.2 M4 3.3 0.18 3.54 240 23.6 7.43  10 M5 7.55  10 3.1 0.23 2.77 306 11.5 M6 4.28  10 3.2 0.22 2.90 293 7.4 Acoustics 2021, 3 FOR PEER REVIEW   17  Acoustics 2021, 3 M4  7.43 × 10   3.3  0.18  3.54  240  23.6  285 M5  7.55 × 10   3.1  0.23  2.77  306  11.5  M6  4.28 × 10   3.2  0.22  2.90  293  7.4  Figure 7. Examples of source time functions computed for the sequence occurred in the Mărăsesti area: (a) deconvolution of Figure 7. Examples of source time functions computed for the sequence occurred in the Mărășești area: (a) deconvolution  , , of M of 22 November 2014, 19:14 with EGF of 22 November 2014, 20:30 at Pogoanele (PGOR) and (b) Amara (AMRR)  M of 22 November 2014, 19:14 with EGF of 22 November 2014, 20:30 at Pogoanele (PGOR) and (b) Amara (AMRR) stations. stations. (c) deconvolution of M of 19 January 2015, 23:53 with EGF of 20 January 2015, 02:52 at all stations (left side) and  (c) deconvolution of M of 19 January 2015, 23:53 with EGF of 20 January 2015, 02:52 at all stations (left side) and the average the average of these pulses (right side). The dashed lines represent the standard error.  of these pulses (right side). The dashed lines represent the standard error. 3.4. Scaling Relationships  3.4. Scaling Relationships The source parameters determined in the present analysis and their scaling relation‐ The source parameters determined in the present analysis and their scaling relation- ships are investigated as indicators of geotectonic peculiarities of the Carpathians fore‐ ships are investigated as indicators of geotectonic peculiarities of the Carpathians foredeep deep region as in previous papers [16,24,25,45,46,64]. Up to now, there has been no sys‐ region as in previous papers [16,24,25,45,46,64]. Up to now, there has been no systematic tematic investigation of the source parameters for the earthquakes that occurred close to  investigation of the source parameters for the earthquakes that occurred close to Mărăsesti , , Mărășești city. The scaling of M0 with ML is displayed in Figure 8. The data are well con‐ city. The scaling of M with M is displayed in Figure 8. The data are well constrained 0 L strained (correlation coefficient of 0.97) by the regression line:  (correlation coefficient of 0.97) by the regression line: (10) 𝑙𝑜𝑔 𝑀 0.97 0.06 𝑀 11.11 0.19   log M = (0.97  0.06) M + (11.11  0.19) (10) 0 L R = 0.97, σ = 0.25.  The scaling shows a similar trend with those obtained for other areas. Thus, the slope  R = 0.97, s = 0.25. value of 0.97 is similar to the values of ~1.1, previously obtained by other studies [65–67].  The scaling shows a similar trend with those obtained for other areas. Thus, the slope The deviation highlighted by the mainshock (ML = 5.7) can be explained by a possible  value of 0.97 is similar to the values of ~1.1, previously obtained by other studies [65–67]. underestimation  of  ML  for  large  earthquakes  since  the  value  of  M0  determined  in  this  The deviation highlighted by the mainshock (M = 5.7) can be explained by a possible underestimation of M for large earthquakes since the value of M determined in this study L 0 is similar to those computed by the international seismological centers (GFZ, EMSC, CSM, NEIC). A second possible explanation for this deviation is given by an overestimation of seismic moments of small earthquakes (M < 4). Several studies achieved in different L Acoustics 2021, 3 FOR PEER REVIEW   18  Acoustics 2021, 3 FOR PEER REVIEW   18  Acoustics 2021, 3 286 study is similar to those computed by the international seismological centers (GFZ, EMSC,  CSM, NEIC). A second possible explanation for this deviation is given by an overestima‐ study is similar to those computed by the international seismological centers (GFZ, EMSC,  regions [68,69] pointed out, for these magnitudes, seismic moments with values smaller by tion of seismic moments of small earthquakes (ML < 4). Several studies achieved in differ‐ CSM, NEIC). A second possible explanation for this deviation is given by an overestima‐ up to an order of magnitude. ent  regions  [68,69]  pointed  out,  for  these  magnitudes,  seismic  moments  with  values  tion of seismic moments of small earthquakes (ML < 4). Several studies achieved in differ‐ Scaling of the seismic moment with source radius is shown in Figure 9 for both P and smaller by up to an order of magnitude.  ent  regions  [68,69]  pointed  out,  for  these  magnitudes,  seismic  moments  with  values  S waves and is approximated by the regression lines: Scaling of the seismic moment with source radius is shown in Figure 9 for both P and  smaller by up to an order of magnitude.  S waves and is approximated by the regression lines:  Scaling of the seismic moment with source radius is shown in Figure 9 for both P and  P waves log M = (3.85  0.26) log r + (4.83  0.642) (11) S waves and is approximated by the regression lines:  (11) 𝑃 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.85 0.26 𝑙𝑜𝑔 𝑟 4.83 0.642   R = 0.96, s = 0.27. 𝑃 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.85 0.26 𝑙𝑜𝑔 𝑟 4.83 0.642   (11) R = 0.96, σ = 0.27.  S waves log M = (3.89  0.24) log r + (5.35  0.565) (12) R = 0.96, σ = 0.27.  (12) 𝑆 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.89 0.24 𝑙𝑜𝑔 𝑟 5.35 0.565   (12) R = 0.96, s = 0.26. 𝑆 𝑎𝑣𝑒𝑤𝑠 𝑙𝑜𝑔 𝑀 3.89 0.24 𝑙𝑜𝑔 𝑟 5.35 0.565   R = 0.96, σ = 0.26.  R = 0.96, σ = 0.26.  Figure 8. Scaling of the seismic moment with local magnitude.  Figure 8. Scaling of the seismic moment with local magnitude. Figure 8. Scaling of the seismic moment with local magnitude.  Figure 9. Scaling of the seismic moment with source radius for P (blue) and S (purple) waves. The  dashed lines are the fitting lines, while solid lines are the fitting lines imposing the theoretical  Figure 9. Scaling of the seismic moment with source radius for P (blue) and S (purple) waves. The  Figure 9. Scaling of the seismic moment with source radius for P (blue) and S (purple) waves. The slope (3).  dashed lines are the fitting lines, while solid lines are the fitting lines imposing the theoretical  dashed lines are the fitting lines, while solid lines are the fitting lines imposing the theoretical slope (3). slope (3).    Acoustics 2021, 3 287 Acoustics 2021, 3 FOR PEER REVIEW   19  The slope of the regression line is higher than the theoretical value for scaling the The slope of the regression line is higher than the theoretical value for scaling the  seismic source with a homogeneous rupture process (compare dashed with solid lines in seism Figur ic eso9urc ). e Since  within a hom all these ogeneous cases rup theture same  pro relative cess (compa deconvolution re dashed with techniques  solid line wer s in e   applied, Figure 9). we Sin assume ce in althat l these the case devis athe tion sam from e relative a theor dec etical onvolution scaling (slope  techniques ~ 4 instead  wereof applied, 3) is a   consequence we assume th ofatthe  theunder  deviaestimation tion from aof the the oreti corner cal sca frequency ling (slope for ~the  4 in smaller stead of earthquakes.  3) is a conse‐ quence of the underestimation of the corner frequency for the smaller earthquakes.   P waves logDs = (0.28  0.05) log M (3.46  0.67) (13) P 0 𝑃 𝑣𝑎𝑒𝑠𝑤 𝑙𝑜𝑔 ∆𝜎 0.28 0.05 𝑙𝑜𝑔 𝑀 3.46 0.67   (13) R = 0.80, s = 0.20. R = 0.80, σ = 0.20.  S waves logDs = (0.28  0.04) log M (2.99  0.63) (14) 𝑆 𝑣𝑎𝑒𝑠𝑤 𝑙𝑜𝑔 ∆𝜎 S 0.28 0.04 𝑙𝑜𝑔 𝑀 0 2.99 0.63   (14) R = 0.82, σ = 0.19.  R = 0.82, s = 0.19. The scaling of stress drop with the seismic moment (Figure 10) indicates an increas‐ The scaling of stress drop with the seismic moment (Figure 10) indicates an increasing ing value stress drop over the entire magnitude range. The stress drop values are in the  value stress drop over the entire magnitude range. The stress drop values are in the range range 1–41 MPa for P waves and 4–101 MPa for S waves. However, the increasing trend  1–41 MPa for P waves and 4–101 MPa for S waves. However, the increasing trend of the of the distribution may be apparent as the decisive element is given by the comparatively  distribution may be apparent as the decisive element is given by the comparatively higher higher value of the stress drop in the case of the mainshock of 22 November 2014 (for the  value of the stress drop in the case of the mainshock of 22 November 2014 (for the rest rest of the events, if we remove the largest event, a constant stress drop scaling seems to  of the events, if we remove the largest event, a constant stress drop scaling seems to be rbe easonable).  reasonable The ). The statistics  statistic ars e are too too small  small for for larger  larger earthquakes  earthquakes to have  to have an eloquent an eloque image nt im‐ age of scaling. In Figure 10 we can notice both for P and S‐wave determinations an in‐ of scaling. In Figure 10 we can notice both for P and S-wave determinations an increasing trcreasing end of str tress enddr of op stres with s dr theop seismic  with th moment e seismiwhich c moment may wh suggest ich may a higher  sugge slip st arather  higher than  slip  arath higher er than fault a higher dimension  fault [ 19 di].mension [19].  Figure 10. Scaling of stress drop with the seismic moment for P (blue) and S (purple) waves. Figure 10. Scaling of stress drop with the seismic moment for P (blue) and S (purple) waves.        Acoustics 2021, 3 288 Acoustics 2021, 3 FOR PEER REVIEW   20  3.5. Frequency–Magnitude Distribution 3.5. Frequency–Magnitude Distribution  The analysis of the frequency–magnitude distribution at the scale of the entire region The analysis of the frequency–magnitude distribution at the scale of the entire region  (0.75  0.75 deg.) outlines a typical distribution with the b slope close to 1 for ‘normal’ time (0.75 × 0.75 deg.) outlines a typical distribution with the b slope close to 1 for ‘normal’ time  intervals (see Figure 11a,d). intervals (see Figure 11a,d).  Figure 11. Frequency–magnitude distribution computed for four‐time intervals: (a) 2005–2011; (b) 2011–2014; (c) 100 days  Figure 11. Frequency–magnitude distribution computed for four-time intervals: (a) 2005–2011; (b) 2011–2014; (c) 100 days since 22 November 2014; (d) 2015–2020.  since 22 November 2014; (d) 2015–2020. We performed the analysis for four times intervals, two of them situated outside the  We performed the analysis for four times intervals, two of them situated outside perturbed activity before the mainshock of 22 November 2014 (2005–2011), one for a three  the perturbed year intervalactivity  before the befor  mainesh the ock mainshock occurrence (2of 01122 –20November 14), one for 10 2014 0 day(2005–2011), s starting with one for a the mainshock occurrence and the last one for a five year interval after the sequence (2015– three year interval before the mainshock occurrence (2011–2014), one for 100 days starting 2020). To compute the b value, we use the maximum likelihood method [53]. The magni‐ with the mainshock occurrence and the last one for a five year interval after the sequence tude of completeness (Mc) of 2.1 and the b slope of 1.14 characterize the first interval  (2015–2020). To compute the b value, we use the maximum likelihood method [53]. The magnitude of completeness (Mc) of 2.1 and the b slope of 1.14 characterize the first interval (2005–2011). Mc keeps the value 2.1 for the next interval, before the sequence triggering (2011–2014), while the b slope has a significant increase to 1.4. During the post-sequence anomalous interval, the b value comes back to ‘normal’, close to 1 value (b = 0.99), while the Mc parameter drops down to 1.2. During the last interval (2015–2020) the seismic- ity regime largely preserves the characteristics observed during the aftershock activity (Mc = 1.3, b = 1.04) due to the expansion of the RSN (Figure 12). Acoustics 2021, 3 FOR PEER REVIEW   21  (2005–2011). Mc keeps the value 2.1 for the next interval, before the sequence triggering  (2011–2014), while the b slope has a significant increase to 1.4. During the post‐sequence  anomalous interval, the b value comes back to ‘normal’, close to 1 value (b = 0.99), while  Acoustics 2021, 3 289 the Mc parameter drops down to 1.2. During the last interval (2015–2020) the seismicity  regime largely preserves the characteristics observed during the aftershock activity (Mc =  1.3, b = 1.04) due to the expansion of the RSN (Figure 12).  Figure 12. The number of seismic stations distributed function of the time in the study region and  Figure 12. The number of seismic stations distributed function of the time in the study region and the adjacent areas.  the adjacent areas. 4. Discussion  4. Discussion We discuss in the following the results of the present work taking into account the  We discuss in the following the results of the present work taking into account the tectonic and geophysical characteristics of the study region as well as comparison with  tectonic and geophysical characteristics of the study region as well as comparison with the the results of previous studies.  results of previous studies. The space–time and frequency–magnitude analyses on the crustal seismic events that  The space–time and frequency–magnitude analyses on the crustal seismic events that occurred within the study region in 2014–2015, during and after the seismic sequence of  occurred within the study region in 2014–2015, during and after the seismic sequence of Mărășești, show that the seismic activity is distributed in several clusters of earthquakes  Mărăsesti, show that the seismic activity is distributed in several clusters of earthquakes , , (Figure 2). The mainshock (ML = 5.7), located close to the Moho boundary (Figure 13),  (Figure 2). The mainshock (M = 5.7), located close to the Moho boundary (Figure 13), triggered a significant increase in seismic activity, not only in the source neighborhood  triggered a significant increase in seismic activity, not only in the source neighborhood but but spread over a larger area, as compared with its source dimension. It was also unusual  spread over a larger area, as compared with its source dimension. It was also unusual that that this intensification consisted of a relatively large number of small‐magnitude earth‐ Acoustics 2021, 3 FOR PEER REVIEW   22  this intensification consisted of a relatively large number of small-magnitude earthquakes quakes spanning a long‐time interval.  spanning a long-time interval. Figure 13. Cross‐sections (modified after [70,71]) through the Eastern Carpathian Orogen, crossing (a) the Mărășești area,  Figure 13. Cross-sections (modified after [70,71]) through the Eastern Carpathian Orogen, crossing (a) the Mărăsesti , , (b) the Râmnicu Sărat region and (c) parallel with the Eastern Carpathian Orogen (d) 3D view of the selected profiles  area, (b) the Râmnicu Sărat region and (c) parallel with the Eastern Carpathian Orogen (d) 3D view of the selected overlayed on the geological map (see Figure 1 for cross‐sections locations). Moho depth is taken from [72]. The hypocenters  profiles overlayed on the geological map (see Figure 1 for cross-sections locations). Moho depth is taken from [72]. The distribution of the 2014 Mărășești seismic sequence is given by blue dots with the mainshock colored in red. The abbrevi‐ hypocenters distribution of the 2014 Mărăsesti seismic sequence is given by blue dots with the mainshock colored in red. The , , ations are as follows: BF‐Bistrita Fault, PCF‐ Peceneaga‐Camena Fault, NDO‐ North Dobrogea Orogen, TF‐Trotuș Fault,  abbreviations are as follows: BF-Bistrita Fault, PCF- Peceneaga-Camena Fault, NDO- North Dobrogea Orogen, TF-Trotus VF‐Vaslui Fault.  Fault, VF-Vaslui Fault. The mainshock occurrence first generated an aftershock activity located in the epi‐ central area, near Mărășești city. Then an intensification of seismic activity was observed  in the Râmnicu Sărat area (~50 km south of Mărășești, see Figures 1 and 2). Apart from  the small size seismic activity triggered there, a moderate earthquake occurred in the same  area (12 January 2015, 06:08 GMT, ML = 4.2). An increase in seismic activity has also been  emphasized towards the north (~30km relative to Mărășești), close to Adjud city (Figure  2). Similar to the previous case, but delayed in time, a moderate size earthquake occurred  close to Adjud city (29 June 2015, 22:20 GMT, ML = 4.0). Intensification of the seismic ac‐ tivity was pointed out to the East and South‐East of Mărășești, but here only small‐mag‐ nitude events were recorded. Overall, the epicenters distribution within the study region  follows a direction parallel to the Eastern Carpathian Bend zone, as was also shown by  previous studies [2,7,24,25,73]. However, if the seismicity is investigated at a smaller scale  (for specific areas) the shape of the epicenters’ distribution may change (Figure 5).   The depth distribution (Figures 2 and 13) emphasizes higher depths for the events  that occurred close to the Mărășești city with most of them located within the lower crust,  while the ones located in the adjacent regions were placed in the middle or upper crust  (Figures 2, 3 and 13). The depth estimations of these events are rather confident, even for  the low size events, since all the locations were manually performed and the seismic sta‐ tion coverage is good (Figure 1) as a consequence of a continuous improvement of the  seismic network in the study area [74] (Figure 12). The depth distribution gives us valua‐ ble indications about the depth of the faults. The mainshock of the sequence is located in  the Focșani Basin, at ~5 km, west of the Peceneaga‐Camena Fault, a deep crustal fault with  a Moho offset of ~5 km [75]. It separates the Moesian Platform from the buried part of the  North Dobrogea orogen since Neogene [76]. In our analysis, the epicenters distribution  seems to follow the characteristics described by [70,77] which showed, based on reflection  seismic lines, that in the eastern side of the Focșani Basin the deformations occur along  numerous normal faults in a wide area with clear topographic expression (see also [1]).  The frequency–magnitude distribution for the earthquakes of the Mărășești sequence  (Figure 11c) shows a magnitude of completeness of ML = 1.2 and a slope of the distribution  (0.99) similar to the normal value (b ~ 1.0) [78]. This is a good indication that these earth‐ quakes are of tectonic origin. On the other hand, we notice important variations of b value    Acoustics 2021, 3 290 The mainshock occurrence first generated an aftershock activity located in the epicen- tral area, near Mărăsesti city. Then an intensification of seismic activity was observed in , , the Râmnicu Sărat area (~50 km south of Mărăsesti, see Figures 1 and 2). Apart from the , , small size seismic activity triggered there, a moderate earthquake occurred in the same area (12 January 2015, 06:08 GMT, M = 4.2). An increase in seismic activity has also been emphasized towards the north (~30km relative to Mărăsesti), close to Adjud city (Figure 2). , , Similar to the previous case, but delayed in time, a moderate size earthquake occurred close to Adjud city (29 June 2015, 22:20 GMT, M = 4.0). Intensification of the seismic activity was pointed out to the East and South-East of Mărăsesti, but here only small-magnitude , , events were recorded. Overall, the epicenters distribution within the study region follows a direction parallel to the Eastern Carpathian Bend zone, as was also shown by previous studies [2,7,24,25,73]. However, if the seismicity is investigated at a smaller scale (for specific areas) the shape of the epicenters’ distribution may change (Figure 5). The depth distribution (Figures 2 and 13) emphasizes higher depths for the events that occurred close to the Mărăsesti city with most of them located within the lower crust, , , while the ones located in the adjacent regions were placed in the middle or upper crust (Figures 2, 3 and 13). The depth estimations of these events are rather confident, even for the low size events, since all the locations were manually performed and the seismic station coverage is good (Figure 1) as a consequence of a continuous improvement of the seismic network in the study area [74] (Figure 12). The depth distribution gives us valuable indications about the depth of the faults. The mainshock of the sequence is located in the Focsani Basin, at ~5 km, west of the Peceneaga-Camena Fault, a deep crustal fault with a Moho offset of ~5 km [75]. It separates the Moesian Platform from the buried part of the North Dobrogea orogen since Neogene [76]. In our analysis, the epicenters distribution seems to follow the characteristics described by [70,77] which showed, based on reflection seismic lines, that in the eastern side of the Focsani Basin the deformations occur along numerous normal faults in a wide area with clear topographic expression (see also [1]). The frequency–magnitude distribution for the earthquakes of the Mărăsesti sequence , , (Figure 11c) shows a magnitude of completeness of M = 1.2 and a slope of the distribu- tion (0.99) similar to the normal value (b ~ 1.0) [78]. This is a good indication that these earthquakes are of tectonic origin. On the other hand, we notice important variations of b value in time, for the first period (2005–2011), the b value is about 1.14, increasing up to ~1.4 in the second period (2011–2014), and returning close to the normal (b ~ 1.04) for the last period (2015–2020). The changes of b value could be caused by several factors such as geological conditions, degree of heterogeneity of cracked medium, and strain and stress variation in the region [79,80]. The higher b value before the sequence could be explained by a relative blocking of the stress on moderate-size resisting areas (asperities) and intensi- fication instead of small-size earthquakes, taking into account the high heterogeneity of the region [1,81]. The decrease of Mc after 2014 is directly related to the seismic network improvements to monitor seismicity in the region (Figure 12). Our results for the fault plane solutions reveal three new mechanisms determined with rather good accuracy for events 14, 18 and 19 (see Table 2). The overall mechanisms highlight a wide variety for earthquakes associated or triggered by the 2014 Mărăsesti , , seismic sequence, although most of these events occurred in the Focsani Basin, a thick Neogene foredeep sediment layer as thick as 13 km located near the Carpathian bend zone [70]. Although the region is relatively narrow, this variety of fault plane solutions is not uncommon for the SE Carpathians foredeep area as shown by other studies [7,31,32,54,55] due to the geological complexity of this region, pointed out on one hand by the collision of three major units with different geometries and characteristics, all representing cratonic continental platforms [82,83] and on the other hand by the significant subsidence, although it is still uncertain whether these faults influenced the lateral variations in the thin-skinned thrusting kinematics [83]. We noticed particular characteristics for each group of events. For the moderate-size earthquakes that occurred near Mărăsesti city, normal fault plane solutions are prevalent , , Acoustics 2021, 3 291 with the nodal planes oriented in the NW–SE direction, corresponding to the PCF direc- tion. The small size earthquakes that occurred in the same region and in the same depth range have in general the same type of faults, highlighting acceptable deviations of the focal planes, probably caused by a reduced amount of data used in their determination. Conversely, earthquakes that occurred in the upper depth segments (but still at the level of the lower or middle crust) tend to change their fault plane solutions towards a normal left lateral mechanism, which may involve activation of secondary faults of the same system (PCF). Given that the epicenters are distributed parallel with the advancing orogen, a flexural rupture of the lower plate with partial reactivation of inherited normal faults might be an interpretation for the earthquakes located further south of the PCF (Figure 13b). The normal faulting highlighted by our results for this area was also emphasized by the results of previous studies [6,77]. In addition, [1] pointed out a large number of normal faults with offsets in the order of tens of meters at the base Quaternary level which belong to the system of the PCF. The moderate size earthquakes generated near Adjud city (~30 km northern part of Mărăsesti city) were located in the upper crust (Figures 1, 2 and 13), show- ing a tendency to change the mechanism to strike-slip. At least one of the nodal planes is oriented in the same direction (NW–SE) with PCF, which may indicate an activation of a secondary faults system located towards its northern side and at the upper crust level dominated by horizontal movements. The faults pattern observed and described above can be explained by the differences in localization deformation transferred from the advancing Carpathian Orogen to the plate. These differences are generated by the sharp changing rheology and mechanical behavior along inherited lower plate blocks and their obliquity to the advancing orogen [1], such as the case of the major border faults TF and PCF separating the Scythian Platform/NDO from Moesian Platform (Figure 13). In the case of the TF, which is almost perpendicular to the advancing orogen, the inherited structures are not inverted, but instead form a new left lateral strike-slip system (with associated normal and reverse faults) located above the contact between strong Scythian vs weak Moesian Platform. The geometry of the PCF, orientated roughly at a 45 angle from the advancing orogen, makes it susceptible for inversion during orogenic loading and flexure, with the normal faults located at the contact between NDO and Moesian Platform. The earthquakes from the Râmnicu Sărat region (~50 km south relative to Mărăsesti) , , were also located in the upper crust, with a predominant reverse mechanism, with the nodal planes oriented in the NE–SW direction, parallel to the Eastern Carpathians’ bending. This type of mechanism was observed in a large number of earthquakes that occurred in the past in the same area [6,25,84], the existence of a possible fault along the Eastern Carpathians’ bending being a debated topic [85]. These NE–SW change in fault type pattern can be explained by the ongoing compres- sional deformation of the SE Carpathians fold and thrust belt which is responsible for both low angles thrusting on the leading edge orogen fault (Pericarpathian thrust Fault, see Figures 1 and 13) and high angle reverse faults, probably related to the inversion of previous Mesozoic normal faults as suggested by [71]; however, the exact geometry of the faults is not resolvable with the current available refraction seismic data (see also [76]). It is worth noticing that for the event that occurred in 12012015, 06:08 (GMT), M = 4.2, our results indicate a reverse focal mechanism, with the nodal planes NE–SW oriented, which contradicts the normal fault plane solution determined by [32]. We assume that the fault mechanism determined by the above study is flawed due to the event location, which in their study is reported as being located ~40 km towards the north, and at a greater depth (with ~20 km) as compared to the location provided by the ROMPLUS catalogue. Previous results have shown that source parameters determined using SR and EGF techniques have a better accuracy since the determination uncertainties decrease by averag- ing the data [23,35,45]. Our data set consists on average of five EGFs for each main event, allowing an uncertainty decrease of up to 20%. The results obtained by using two different techniques (SR and EGF deconvolution) highlight rather close values with an average Acoustics 2021, 3 292 difference of ~30% (for the stress drop) which indicates an adequate degree of accuracy. The largest event in our dataset has an average stress drop of 134 MPa. Similar high-stress drop values were observed in other regions for normal-faulting earthquakes [86] or for the deeper location of the events [87]. At the same time, we noticed an increasing trend of stress drop with seismic moment. According to our analysis, the STF has a simple pulse-like form for all the studied events. However, the apparent pulse-like form for the source of the largest event as obtained by EGF deconvolution is not so well constrained as for the other smaller main events. This could be attributed to some complexity of the rupture process in the source, visible in the waveform recordings of the mainshock. If we assume a multi-shock release of seismic energy for this event, the resulting stress drop is a dynamic value rather than static, which explains the unusually high-stress drop values obtained for the mainshock (Tables 5 and 7). Source parameter scaling relationships fit well with the results previously obtained for the Carpathian foredeep region [24,25,45,64]. The deviations observed for the seismic moment–source radius scaling (slope ~3.9 instead of 3) and seismic moment–stress drop (deviation from a constant stress drop scaling) from the theoretical values could be due to the limitation of the empirical Green’s function and spectral ratios techniques to constrain the corner frequency for the weakest earthquakes and the anomalous high-stress drop value of the mainshock. On the other hand, the high attenuation effects within the upper crust might act as a filter, removing the high frequencies content (f > 10 Hz) and limiting the corner frequencies resolution [28] affecting especially small earthquakes and leading to an underestimation of their stress drop. It is also likely that the usage of these methods for the small earthquakes to lead to a saturation of the corner frequency due to the significant increase of the recorded noise at high frequencies. Because of this, we can expect that our analysis is no longer able to discern the higher corner frequencies and, for this reason, the source dimension becomes overestimated and, correspondingly, the stress drops underestimated. We need more high-quality data for different size earthquakes to validate or not the scaling deviations obtained in the present work. 5. Conclusions The seismic sequence triggered on 22 November 2014 close to Mărăsesti city offered , , the possibility of evaluating the source parameters and the focal mechanism solution for the strongest crustal event (M = 5.7) ever recorded in the area since the instrumental recordings are available. The focal mechanism solution of the mainshock, obtained from local, regional or global data, indicates normal faulting with the compression axis almost vertical and expansion axis almost horizontal, in agreement with the seismotectonic of the eastern and southern flanks of the Focsani Basin. Both nodal planes are oriented NW–SE, in the direction of the PCF. Going toward the south, in the Râmnicu Sărat area, reverse faulting is also obtained, which is characteristic of this area as highlighted in previous investigations. Applying SR and EGF methods the source parameters for 15 earthquakes of the sequence of November 22, 2014 (2.0  M  5.7) and for five earthquakes produced in the first seven months of 2015 (1.8  M 4.2) were determined. The source parameter values determined through SR and EGF methods are close, indicating an increasing trend of stress drop with seismic moment, induced especially by the high value of the mainshock. This seems to explain the not very well constrained pulse-like shape obtained by EGF deconvolution for the mainshock which could indicate a multi-shock release of seismic energy for this event. Investigation of the frequency–magnitude distribution at regional scale outlines a ‘nor- mal’ regime with b-slope value around 1, if we are situated sufficiently far from the perturbed activity related to the occurrence of the strong crustal earthquake of 22 November 2014, and a possible anomaly (b = 1.4) before the occurrence of this event. A behavior with pre-shock increase of b from about 1 to 1.5 and sudden decrease during the aftershock activity and Acoustics 2021, 3 293 further on is typical for acoustic experiment tests and for critical systems in which the population of cracks concentrates after the critical phase along preferential paths. Author Contributions: Conceptualization, A.O.P. and F.B.; methodology, E.P.; software, M.R.; val- idation, A.O.P., F.B., M.R. and I.M.; formal analysis, A.O.P.; investigation, A.O.P.; resources, M.R., A.O.P.,.; data curation, A.O.P.; writing—original draft preparation, A.O.P.; writing—review and editing, A.O.P., F.B., M.R., I.M.; visualization, F.B. and I.M.; supervision, M.R.; project administration, M.R.; funding acquisition, M.R., A.O.P., F.B. All authors have read and agreed to the published version of the manuscript. Funding: The research was partially supported by the “NUCLEU” programs (CREATOR and MULTIRISC) of the Romanian Ministry of Research and Innovation through the projects PN16350307 and PN19080102 and by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI) through the projects PN-III-P2-2.1-PED-2019-1693, 480 PED/2020 (PHENOMENAL) and PN-III-P4-ID-PCE- 2020-1361, 119 PCE/2021 (AFROS). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: We would like to thank two anonymous reviewers for their constructive com- ments which improved the original version of this manuscript. Several figures were made using GMT software. Conflicts of Interest: The authors declare no conflict of interest. References 1. Matenco, L.; Bertotti, G.; Leever, K.; Cloetingh, S.; Schmid, S.; Tărăpoancă, M.; Dinuc, C. Large-scale deformation in a locked collisional boundary: Interplay between subsidence and uplift, intraplate stress, and inherited lithospheric structure in the late stage of the SE Carpathians evolution. Tectonics 2007, 26, TC4011. [CrossRef] 2. Bălă, A.; Toma-Danila, D.; Radulian, M. 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Journal

AcousticsMultidisciplinary Digital Publishing Institute

Published: Apr 19, 2021

Keywords: earthquake sequence; source parameters; seismic source scaling; SE Carpathians foredeep

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