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Article Local Site Effects Investigation in Durres City (Albania) Using Ambient Noise, after the 26 November 2019 (M6.4) Destructive Earthquake 1, 2 2 1 3 4 Nikos Theodoulidis *, Edmond Dushi , Llambro Duni , Ioannis Grendas , Areti Panou , Ardit Hajrullai , 2 2 Neki Kuka and Rexhep Koci Institute of Engineering Seismology and Earthquake Engineering, 55535 Thessaloniki, Greece Department of Seismology, Institute of Geosciences, Polytechnic University Tirana, 1000 Tirana, Albania Department of Geophysics, Aristotle University Thessaloniki, 54124 Thessaloniki, Greece GeoSeis-IT Consulting, London N14 6JF, UK * Correspondence: firstname.lastname@example.org Abstract: Site characterization of metropolitan areas, especially after an earthquake, is of paramount importance for interpretation of spatial damage distribution and taking measures that assure real- istic design actions to strengthen existing constructions and create new ones. Such a case is the city of Durres, Albania, that was hit by the disastrous earthquake of 26 November 2019 (M6.4). Signifi- cant differences in structural damage were observed throughout the city, despite its uniform epi- central distance (approximately 15 km); this could be either due to varying vulnerability of the af- fected constructions and/or to spatial variation of strong ground motion in the city, resulting from local site effects; the latter factor was investigated in this study. This was achieved by taking single station ambient noise measurements throughout the city, at approximately 80 sites. Ambient noise Citation: Theodoulidis, N.; Dushi, measurements are favorable, as acquiring ambient noise data is an easy and effective noninvasive E.; Duni, L.; Grendas, I.; Panou, A.; approach within urban environments. Measurements were processed using the widely applied Hajrullai, A.; Kuka, N.; Koci, R. Horizontal-to-Vertical Spectral Ratio (HVSR) method, following the SESAME project (2004) guide- Local Site Effects Investigation in lines. Their fundamental and dominant frequencies, fo and fd, respectively, were calculated and re- Durres City (Albania) Using lated to the iso-depth contours of the investigated area, as well as their corresponding amplitudes, Ambient Noise, after the 26 Ao, and Ad. These experimental parameters and the HVSR curves were used to group all examined November 2019 (M6.4) Destructive sites into classes with similar properties. This clustering provided a zonation map with four catego- Earthquake. Appl. Sci. 2022, 12, 11309. https://doi.org/10.3390/ ries consisting of similar shapes and amplitudes, applicable to the city of Durres. This map can be app122211309 utilized as a first level zonation of local site effects for the city. In addition, dynamic properties of soil profiles in selected sites were investigated and tested using 1D synthetic ambient noise data, Academic Editor: Igal M. Shohet based on the Hisada (1994, 1995) simulation method, and compared to experimental HVSRs in prox- Received: 31 August 2022 imity to the selected sites. A comparison of the proposed four categories zonation map to the ob- Accepted: 4 November 2022 served damage of the 26 November 2019, mainshock is attempted and evaluated. The four catego- Published: 8 November 2022 ries zonation map with similar expected local site effects proposed in this study can be used as a Publisher’s Note: MDPI stays neu- first level seismic microzonation of Durres. Undoubtedly, corrections for 2D/3D effects on ground tral with regard to jurisdictional shaking must be applied to sites lying in the edges of the Durres basin. claims in published maps and institu- tional affiliations. Keywords: earthquake ground motion; local site effects; ambient noise HVSR; seismic microzonation Copyright: © 2022 by the authors. Li- 1. Introduction censee MDPI, Basel, Switzerland. After the disastrous earthquake of Durres, Albania, on 26 November 2019 (M6.4) This article is an open access article characterizing the city in terms of soil properties related to seismic site response was distributed under the terms and con- deemed to be of paramount importance. Despite the recent methodological and techno- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- logical developments, determining the seismic ground motion response of various geo- ativecommons.org/licenses/by/4.0/). logic formations with traditional methods still proves to be a difficult and costly task [1,2]. Appl. Sci. 2022, 12, 11309. https://doi.org/10.3390/app122211309 www.mdpi.com/journal/applsci Appl. Sci. 2022, 12, 11309 2 of 27 Large-scale urban areas that need to be densely covered, or areas with moderate to low seismicity are especially difficult, due to the time period required to compile a statistically significant number of seismic records in order to produce results using traditional meth- ods (e.g., Standard Spectral Ratio; ). In the last three decades, ambient noise recordings have been extensively employed and have proven to be a useful tool in determining local site effects. Ambient noise is defined as the constant vibration of the Earth’s surface, generated either by human activ- ities (e.g., machinery, traffic, pedestrians) in high frequency ranges (f > ~1 Hz) or by natu- ral phenomena (e.g., wind, variations in atmospheric pressure, sea waves) in low fre- quency ranges (f < ~1 Hz). It is often assumed that ambient noise recordings are dominated by Rayleigh and Love waves at more than one wavelength from the source . The most common technique, routinely applied today, relies on the calculation of the Horizontal-to-Vertical Spectral Ratio (HVSR) of ambient seismic noise, typically recorded at a location by a three-component velocity sensor. According to , this single-station ambient noise approach was initially developed in Japan by , based on the work by  for characterizing site response under seismic loading, and was later popularized and dis- seminated by [8,9]. In the early 2000s, the application and further development of the method exhibited mobility and broader use in Europe (e.g., [10,11]) and later in Canada (;) and South America . In general, HVSR analysis is currently applied world- wide to estimate the site fundamental frequency, and in certain circumstances site ampli- fication factors, as it is observed in weak ground motion of earthquake excitation (linear soil behavior), though this factor is still debatable among the community (among others; [1,15]). In fact, the appropriate density of site characterization in the built environments pro- posed in this study is achievable by easy to perform, low cost, non-invasive geophysical methods. Such methods are based on ambient noise from both anthropogenic and natural sources. A series of investigations have been carried out over the past three decades worldwide, aimed at deriving quantitative information on site amplification through non- invasive techniques, based principally on surface wave interpretations of ambient noise measurements . In urban areas affected by strong earthquakes, microzonation studies are usually per- formed. As a first step, ambient noise measurements are applied, and using the HVSR method. some dynamic properties of surficial geologic layers are estimated, while com- parison with earthquake damage distribution is also attempted (among others [16–19]). In summary, HVSR can provide useful information on surface geologic layers, such as their fundamental frequency (fo) and its corresponding amplitude (Ao). If used in joint inversion with dispersion curves (DC), it can constrain the shear wave velocity profile of the site under investigation. It is a low-cost and easy to apply method, constituting a first basic step in microzonation study. After the disastrous earthquake of 26 November 2019, investigating the underlying geologic formation’s role in seismic response was deemed necessary. Basic soil dynamic properties of surficial geologic strata (e.g., fundamental frequency, amplification, shear wave velocity) must be estimated in order to be implemented in respective seismic re- sponse analyses. For this purpose, in the present work, ambient noise measurements at 80 sites were performed in the urban area of Durres, and the HVSR method was applied to estimate their fundamental and dominant frequencies, as well as their corresponding am- plitudes. In addition, spatial distribution of these parameters in the city is presented and compared alongside other available geological or/and geophysical characteristics of Durres complex 3D basin. The experimental HVSRs for selected sites are compared with HVSRs from synthetic ambient noise data based on available 1D profiles, thus assessing their reliability. In addition, classification of all experimental HVSRs using their entire curve is attempted, and four discrete classes are proposed for the city of Durres. The cor- relation of the proposed four zones/classes with the observed damage is discussed and Appl. Sci. 2022, 12, 11309 3 of 27 these zones are proposed as a satisfactory basis for any future microzonation and/or seis- mic risk analyses of the Durres built environment. 2. Data and Methods Used 2.1. Geographic, Demographic and Territorial Features of the Study Area The study area predominantly covers the urban area of Durres. The city lies along the Adriatic coast of western Albania and is one of the oldest cities in the country (known as “Dyrrachium”), with a history of over 2500 years . From a physical-geographical standpoint, Durres is situated in the western Low- lands, on a flat alluvial plain between the Erzeni and the Ishmi rivers, and the surrounding hills. Its highest peaks are found in the Rodon–Erzeni Hills (at 272 m) and in the Durresi Hills (at 178 m). Most of the territory is a plain shaped by the reclaim of the Durresi swamp in the last century. The area of Durres covers only 1% (332 km ) of the Albanian territory, defined by a north-south longitudinal extension of about 36 km and a width of 19 km . Durres is one of Albania’s most significant urban and economic centers, representing one of the most important nodes of national and international transport, crossed by many im- portant national and international road axes and maritime links. As the second largest metropolitan area in Albania, it has a population of approximately 202,000 . According to the Institute of Statistics (INSTAT) census, up to 2011, the highest per- centage of constructions in the city of Durres consisted of 1–2 story buildings . It is noted that during the period of 1991–2011, the number of buildings with over 6 floors has increased by 79%, and currently constitute about 3.5% of the total number of constructions . Building typologies in Durres comprise of masonry buildings, predominantly resi- dential structures up to 1993, consisting of unreinforced load bearing masonry structures up to 5 stories, and confined masonry structures of 3–6 stories; prefabricated concrete panel buildings built between 1970–1990, of 4 and 6 stories; and concrete-frame buildings with in-fill masonry walls. The development of reinforced-concrete frame systems and infill masonry walls began after 1993, rising from 2–3 stories up to more than 10 stories . 2.2. Geological Setting and Seismotectonic Aspect From a geological point of view, the Durres Bay, and the weathered hills of Tortonian age, lay on poor, thick and unconsolidated Quaternary sediments [24–28]. The corre- sponding surface geology (Figure 1a) show that the western part of the hilly chain is com- posed of Miocene molassic formations comprising clay, siltstone, sandstone, and gypsum, while on the central and eastern parts a composition of Pliocene sandstones and conglom- erates is found . The Durres basin is filled with Pliocene and Quaternary deposits. The Pliocene deposits constitute the basement of the overlying Quaternary ones, including sandstones and conglomerates found in the central and eastern flanks of the Durres hills. In the Durres basin, the Quaternary deposits include alluvial, lagoon, marshy and marine deposits, reaching an overall thickness of nearly 130 m [26–28,30]. From a tectonics point of view, Durres and its surroundings belong to the Peri-Adri- atic Foredeep. It constitutes a northern segment along the southern convergent margin of the Eurasia Plate, where the Albanian orogenic front is thrusting over the Adria micro- plate, partly over the Apulian platform and the southern Adriatic basin [25,26,31]. Conse- quently, it is strongly influenced by the Adria-Eurasia continental collision [27,28,31–33]. As a result, the Durres region is affected by a compressional stress regime, acting along a series of thrust and back-thrust faults (Figure 1b). The back-thrust faults have been active since the Pliocene-Quaternary period, namely the Durres and Preza ones, while thrusts along the Durres and Tirana syncline are traced to earlier periods [27,31,33,34]. Active faults of Durres and the surrounding regions (Figure 1b) have contributed to both historical and present seismic activity [25,26,31,34,35]. This notion is reinforced by the latest strong earthquakes that struck Durres, namely the September 21 (M5.8) and Appl. Sci. 2022, 12, 11309 4 of 27 November 26 (M6.4), 2019. Both events showed the same type of reverse-faulting kine- matics (Figure 2), based on their focal mechanism (FM) solutions, supporting the idea of a blind reverse causative fault, NW–SE striking (140°–155°) and ENE dipping (65°–72°), rooted at the basal thrust front [32,36–39]. The results also agree with the aforementioned regional tectonics (Figure 1b), showing significant consistency between observed data and the Durres thrust fault, whose ramp dip-angle tends to decrease at the hypocenter depth of these strong earthquake [32,37,38]. Appl. Sci. 2022, 12, 11309 5 of 27 Figure 1. (a): Geological map of Durres (section from the Geological Map of Albania 1:200,000, from ); the profile trace line (A—B), oriented WSW—ENS, crosses the main geological units of Durres, through Porto Romano Bay; (b): Schematic geologic section, along the A—B profile, trending NE, from the Adriatic Sea to Kruja-Tirana region, . The red star indicates the location the of 26 No- vember 2019 mainshock’s hypocenter (M6.4) . Figure 2. Seismic sequence of September 2019–June 2020, corresponding focal mechanism solutions (FM) of the larges shocks and the active faults influencing Durres region. 2.3. Historical and Recent Seismicity Durres and the surrounding region were hit by strong earthquakes (M > 6.0), in the past . The ancient city of Durres (Dyrrachium) has been nearly destroyed by several strong historical earthquakes in 58 BC, 334 AD, 346 AD, 506 AD, 521, 1273 and 1870 (see Table 1). The historical abandonment of Durres was a consequence of a number of devas- tating earthquakes with epicentral intensity IMAX = VIII–IX degree (MSK-64), in 334, 346 Appl. Sci. 2022, 12, 11309 6 of 27 and 506 AD (Figure 3), ; in 1273, with a magnitude M6.7 ([41,42]) and in 1926, with a magnitude M6.3, [40–43]. Table 1. Historical and instrumental era earthquakes (M > 6.0), 58BC-2019, in the Durres region (M implying moment magnitude and with * being the equivalent moment magnitude . Date Latitude Longitude Depth Mag. Location yyyy/mm/dd N-S E-W km M 58 BC 41.2 19.3 33 6.5 * Durres 334 41.3 19.5 33 6.2 * Durres 346 41.3 19.3 33 6.5 * Durres 506 41.3 19.5 33 6.2 * Durres 521 41.3 19.5 33 6.0 * Durres 1273 41.2 19.3 33 6.7 * Durres 1617 19.700 41.500 15 6.2 * Kruja 1852 19.500 41.600 15 6.2 * Rodoni Cape 1860 19.70 41.300 15 6.2 * Ndroqi 1870 19.446 41.314 15 6.5 * Durres 1926 19.50 41.300 11 6.3 Durres 1934 19.60 41.250 12 5.7 Ndroqi 1970 19.60 41.15 55 5.6 Vrapi 1975 19.30 41.50 15 5.4 Rodoni Cape 1979 42.16 18.87 16 6.9 Montenegro 1988 41.22 19.81 5 5.7 Tirana 2019, Sept. 21 19.503 41.476 23.4 5.6 Durres 2019, Nov. 26 19.469 41.421 20.5 6.4 Durres The most significant seismic events in the region (Table 1 and Figure 3), that have also impacted Durres, are the 1617 Kruja earthquake (M6.2), with an epicentral Intensity IMAX = VIII (MSK-64); the 26 August 1852 Rodoni Cape earthquake (M6.2), with an epicen- tral Intensity IMAX = VIII (MSK-64); the May 16, 1860 Ndroqi (Beshiri Bridge) earthquake (M6.2), with epicentral Intensity IMAX = VIII (MSK-64); the 4 February 1934 Ndroqi earth- quake (M5.7); the 19 August 1970 Vrapi earthquake (M5.6), with an epicentral Intensity IMAX = VII degree (MSK-64); the 16 September 1975 Rodoni Cape earthquake (M5.4); the 15 April 1979 (M6.9) Montenegro coastal area earthquake; and the 9 January 1988 Tirana earthquake (M5.9), with an epicenter Intensity IMAX = VII-VIII degree (MSK-64). The most recent strong earthquakes, having a significant socio-economic impact for Durres and its surrounding regions, are the 21 September 2019 (M5.6), with an epicentral Intensity IMAX = VII-VIII degree (MSK-64) and the earthquake of 26 November 2019 (M6.4), with epicen- ter Intensity IMAX = VIII-IX degree (MSK-64), ([40–42,45]). (Figure 3 and Table 1). Appl. Sci. 2022, 12, 11309 7 of 27 Figure 3. Historical and recent earthquakes affecting Durres and the surrounding region seismicity. The 16 November 2019 (M6.4) mainshock was located 16 km north of Durres city, with its epicenter on the Adriatic coast of Albania (Figure 2), about 33 km northwest of Tirana, the capital of Albania ([32,37–39]) It was subsequently followed by intense after- shock activity. On 21 September 2019, the same area was hit by an earthquake of moderate magnitude (M5.6), 12 km north of Durres. A posteriori it is believed to be the main fore- shock of the November 26, 2019, earthquake. An absence of seismic activity until 25 No- vember was observed. Immediate foreshock activity started on 25 November 2019, at 03:22 am (UTC), including eight earthquakes with magnitudes between 2.0–4.6 (ML). The mainshock, followed nearly one hour after the 01:47 am (UTC) foreshock (ML4.6), located approximately 4 km north of its hypocenter . The seismic sequence of 26 November 2019 is apparently characterized by a high number of aftershocks [45,46]. Until 30 June 2020, approximately 2000 earthquakes occurred (Figure 2), located by the National Seis- mic Network of Albania (ASN). The effects of the ground shaking of the 26 November earthquake in the city of Durres were very significant in terms of building damage. Moderately to heavily damaged build- ings were observed in two broad zones. One including its central-west part, elongated in a north-south direction, and a second in its coastal area in south-to-south-eastern direction ([23,47], as well as personal communication with local structural engineers and personal autopsy during the experiment, Figure 4a). The unique accelerometer station (DURR) was installed within the most highly af- fected part of the city. It showed a horizontal PGA~0.2 g and a broad response acceleration spectrum . More specifically, the ground shaking at the DURR accelerometer station exhibited high spectral acceleration values (>0.3 g) for a wide frequency range (f = 1/Pe- riod), 0.8 Hz < f < 5 Hz (Figure 4b). That is, amplification of strong ground motion was intense in this frequency range. Appl. Sci. 2022, 12, 11309 8 of 27 (a) (b) Figure 4. (a) Building damage distribution in Durres (not an exhaustive list) after the mainshock of 26 November 2019 (modified from ); (b) The acceleration response spectrum at the DURR accel- erometer station from the 26 November 2019 mainshock. Influence of site effects on seismic hazard and on seismic vulnerability of buildings has been documented in many cases worldwide (among others: [49–52]). Seismic ground motion is amplified according to the geophysical and geotechnical properties of the site under investigation, highlighting the need to know these properties. 2.4. Geotechnical and Geophysical Data in the Durres Region The mainshock struck a modern urban environment, the metropolitan area of Durres, and caused extended damage downtown as well as in several villages in the epicentral area. The recent layers of surface geology (Figures 1, 4 and 5) are related to the highest expected seismic intensities and/or PGA values [34,53–55], as well as with problems con- cerning soil instabilities, such as induced liquefaction on poor sandy deposits of the Durres lagoon and the Durres Bay area . The Quaternary deposits in the Durres basin Appl. Sci. 2022, 12, 11309 9 of 27 include alluvial, lagoon, marshy and marine deposits with a total thickness of around 130 m . The marshy Holocene deposits, comprised of clay, clayey loam, sand, and organic material, have played an important role in forming strong ground motion and distribution of the earthquake-induced building damage in Durres . A large part of the city is founded on these marshy deposits, which reach a maximum thickness at the central part of the swamp. The geological properties and related lithologies of this area are also re- vealed by its name, “marshes.” The latter represents an area where expected seismic haz- ard can be drastically increased due to local site conditions, shallow underground water level, active faults surrounding the city and underground topography of basement rocks [30,57]. From the expected strong ground motion, the area is classified as the most hazardous among the Albanian coastal zone based on the seismic microzoning studies (Figure 5), through spectral characteristics, seismic intensities, and expected dynamic soil instabili- ties . Based on past seismic microzonation, the expected MSK macroseismic intensity varies between VII to IX throughout the city, exhibiting an intensity increment of more than 2 units (Figure 5a). In addition, an equal depth contour map for the city of Durres (Figure 5b), shows a 3D bedrock morphology, implying a 3D basin model. The basin is built on recent Holocene marshy deposits, clays, sands, and peat, with a part of it on Pli- ocene clays (Figure 6), with liquefaction potential of the broader residential area . The maximum thickness of sediment layers reaches up to 130 m in the central part of the basin (Figure 5). Figure 5. (a) Microzoning map for the city of Durres (b) iso-depth contours of Durres down to geo- logic rock formations (modified from ). The aforementioned characteristics are also based on bore-hole data in an East-West [IV-IV] cross-section close to the port (Figure 6) and their physical-mechanical parameters given in Table 2. The position of several boreholes, especially the deepest one, BHA, with a maximum depth of about 120 m, is shown. This cross section was developed in the framework of the Durres seismic microzonation project  and has been enriched with geophysical MASW measurements in selected points. One of these points refers to the accelerometer station of Durres (DURR), where the MASW method was performed to es- timate the shear wave velocity profile . The measured point is located ~70 m west of the DURR accelerometric station (white arrow in Figure 7). The IV-IV cross section passes Appl. Sci. 2022, 12, 11309 10 of 27 through the location of the accelerometric station of Durres (DURR). Values of P and S- wave velocity and NSPT are given up to an 80 m depth profile (Figure 8). Figure 6. Geologic cross-section IV—IV . 3 3 Table 2. Description of physical-mechanical parameters: ϒ (T/m )—specific weight; Δ (T/m )—vol- ume weight; δ (T/m3)—skeleton volume weight; W %—humidity; n %—porosity; ϕ—porosity co- efficient; Φ (°)—internal friction angle; C (kg/cm )—cohesion; μ (m/24 h)—permeability. Granulometry Lithology Marshy-lagoon 0–3 m sub sand with 4.0 24 72 2.64 1.84 1.31 38 39 0.8 24 0.05 - peat Powdery water- 3–15 m 3.0 15 82 2.66 1.86 1.48 35 48 0.89 26 0.05 1.0 saturated sand 15–28 m Sub-clay 17 60 23 2.68 1.89 1.5 35 48 0.92 14 0.1 0.05 28–130 m Clay 30 60 10 2.72 1.9 1.6 38 49 0.95 14 0.25 - Clayey-sand >130 m - - - 2.75 2.08 1.9 11.13 40 0.68 - - - basement rock Thickness (m) Clay Grit Sand ϒ (T/m ) Δ (T/m ) δ (T/m3) W % n % Φ ( ) C (kg/cm ) μ (m/24 h) Appl. Sci. 2022, 12, 11309 11 of 27 Figure 7. Location of the cross section IV-IV, 6.0 km long (top part); The position of DURR station (red triangle) and several boreholes (SH-xx) are also presented (bottom part). The white arrow in- dicates the MASW-Vs measurement profile-P39 carried out close to the DURR accelerometric sta- tion. Appl. Sci. 2022, 12, 11309 12 of 27 Figure 8. East-west cross section and lithology (upper); Vs profile of the DURR station for the upper 30 m (lower). 2.5. Ambient Noise Data For the single station ambient noise measurements two CityShark-II dataloggers (24- bits resolution) coupled with Lenartz-3D 5 sec seismometers were used. In the framework of the present work, 80 single-site measurements were performed in the city of Durres (Figure 9, red dots) during the period 18–21 February 2020. Most of the measurements were performed with a seismometer-to-ground coupling on soil conditions to avoid any high frequency contamination from an artificial surface layer (e.g., cement, asphalt, etc.). A typical ambient noise measurement installation is presented in Figure 9. All Appl. Sci. 2022, 12, 11309 13 of 27 measurements were taken during the daytime, avoiding the impact of near source anthro- pogenic noise as much as possible. The duration of each measurement was 30 min per site. The mesh of investigated sites was denser in the western part of the city. This choice was based on the fact that observed structural damage from the Nov. 26 mainshock, was greater at the central-to-western part of the city as well as on the steeper basement mor- phology of the western part, as it is evident in Figure 6. The minimum and maximum distances between the measurement points were 175 m and 750 m, respectively. Urbanization of the Durres city has been evolved on a soft sediment basin that is elongated in a north-south direction. Soft soil deposits of the Durres basin deepen from its edges to the center, reaching a depth of ~130 m (Figure 6), indirectly implying low fundamental frequencies (fo < 1 Hz) for most of the investigated sites. Figure 9. Measurement points within the Durres city (top); An example of measurement site (bot- tom). 2.6. Methods Used There are different approaches to site characterization in urban environments. Each one has its pros and cons as well as various cost levels. There are two main categories of methods for in situ site characterization; the active and passive (non-invasive) ones. The active ones (e.g., cross-hole, down-hole, CPT, etc.) usually affect the environment and thus Appl. Sci. 2022, 12, 11309 14 of 27 prove challenging to implement within cities. In contrast, non-invasive methods are based on the measurement of environmental noise, and do not affect the built environment, while also being much more cost-effective to implement. Among the non-invasive meth- ods based on ambient noise recordings, the one based on a single station measurement per site was implemented in this study. In addition, a 1D ambient noise simulation tech- nique was used to validate the proposed soil profiles up to the bedrock for the city of Durres. (a) Data Processing and Analyses of Single Station HVSR In this study, compiled ambient noise data was processed based on the SESAME pro- ject guidelines , and more specifically the criteria for reliability of results proposed in the Table of its 2.1 section. Their spatial distribution was investigated to divide the city in various categories of similar HVSR curves, that is, zones of similar dynamic properties (fo, Ao). In addition, for certain sites where an estimated Vs-depth profile was available (see Section 3.3). 1D synthetic ambient noise recordings were generated and their HVSRs were calculated. Comparison between experimental and synthetic HVSRs showed the degree of reliability of the latter, while any deviation from the actual HVSRs either indicated pos- sible attenuation factor (Q) issues and/or 2D/3D imprints due to basin effects. The data set of 80 measurements was processed in a uniform way, using the Geopsy software package (http://www.geopsy.org, accessed on 5 November 2022). Each record- ing of 30 min ambient noise was automatically divided into 50 s windows, using the fol- lowing set of parameters  : Short Time Average (STA) = 1 s, Long Time Average (LTA) = 30 s, min[STAamplitude/LTAamplitude] = 0.20, max[STAamplitude/LTAamplitude] = 2.5 A Fourier transform was applied to each one of the selected windows, with 5% ta- pering, and smoothed using the approach of , with the b coefficient value set to 40. The mean horizontal spectra for each 50 sec time windows were derived by taking the geometric average, and then calculating the corresponding Horizontal-to-Vertical (H/V) spectral ratio. An example of the H/V processing using Geopsy is shown in Figure 10. Certain cri- teria for selecting H/V peaks as fundamental or higher mode frequency (ideal and/or clear peaks) were taken into account based on the SESAME guidelines (http://sesame.ge- opsy.org/SES_Reports.htm, accessed on 5 November 2022). For each site the mean ± one standard deviation (H/V) spectral ratios were calculated. After the initial processing, 76 out of 80 measurements were considered of good quality and further processed in this study. These 76 HVSR curves are provided as a Supplementary Materials of this study, in ascii format. Appl. Sci. 2022, 12, 11309 15 of 27 Figure 10. Snapshot example of the H/V processing using Geopsy. (b) Theoretical 1D simulation of ambient noise In modeling 1D synthetic ambient noise recordings for sites with heterogeneous sub- surface structure, such as Durres, we used a two-step numerical code that has been devel- oped in the European SESAME project. In the first step, the noise sources are represented by surface or shallow subsurface point forces randomly distributed in space, direction (vertical or horizontal), amplitude and time . The source time function is either a delta- like signal (impulsive sources) or a pseudo-monochromatic signal (“machinery” sources, harmonic carriers with the Gaussian envelope). In the second step, computation of the associated wavefield is performed with different patterns in accordance with the 1D soil structure. For 1D horizontally layered media, Green’s functions are computed by using the wavenumber method proposed by [63,64], schematically presented in Figure 11. Appl. Sci. 2022, 12, 11309 16 of 27 Figure 11. The multi-layered half space model considered by . Point sources are located at the Z axis, and receivers at (r, θ, z), with the displacement components of the Green’s functions Ur, Uθ, Uz (modified from ). 3. Results 3.1. Experimental HVSR Results After the 76 HVSRs were calculated, each fundamental frequency, fo, was selected along with corresponding amplitude Ao. The following four categories are proposed in terms of fundamental frequency and shown in Figure 12 (left): Category 1: 0.20 Hz < fo < 0.60 Hz Category 2: 0.61 Hz < fo < 0.98 Hz Category 3: 0.99 Hz < fo < 1.70 Hz Category 4: 1.89 Hz < fo < 20 Hz Correspondingly, four categories are proposed in terms of amplitude and shown in Figure 12 (right): Category 1: Ao < 2.0 Category 2: 2.01 < Ao < 2.99 Category 3: 3.00 < Ao < 3.99 Category 4: 4.00 < Ao < 5.60 Appl. Sci. 2022, 12, 11309 17 of 27 Figure 12. Distribution of the fundamental frequency, fo, in the city of Durres (left); Distribution of the corresponding to the fundamental frequency, Ao, amplitude (right). Bedrock depth iso-contours by . In Figure 12, the bedrock depth iso-contours are also given. For Category 1 (0.20 Hz < fo < 0.60 Hz), the fundamental frequency, fo, dominates the central part of the basin with the deepest recent deposits. Category 2 (0.99 Hz < fo < 1.70 Hz) appears at the western part of the basin, while at the eastern part there is a broader transition from Category 2 to Category 3 (0.99 Hz < fo < 1.70 Hz), ending with Category 4 (1.89 Hz < fo < 20 Hz). The amplitude, Ao, reaches high values mainly in the western part of the basin with a maximum of ~6, as well as in the central part, with 3.0 < Ao < 4.0. A broad zone with lower amplitudes, 2.0 < Ao < 3.0, is evident in the eastern part of the basin. In Figure 13 the dominant frequency, fd, of the examined sites—that is, the frequency where the largest amplitude of the HVSR appears—is presented. Similarly, four categories as in the case of fo can be distinguished. The amplitude of the dominant frequency, Ad, also gives a similar pattern to that of the fundamental frequency, Ao. Figure 13. Distribution of the dominant frequency, fd, in the city of Durres (left); distribution of the corresponding to the dominant frequency, Ad amplitude (right). Bedrock depth iso-contours by . 3.2. Clustering of the HVSRs and Their Spatial Distribution in Durres In Seismic Zonation maps spatial grouping of certain geophysical or/and geotech- nical parameters (e.g., VS30, fo, A0, etc.) is usually attempted either in urban environment or even in wider regional scale [65,66]. Appl. Sci. 2022, 12, 11309 18 of 27 To cluster spatially the 76 HVSRs, the entirety of their curve (in frequency and am- plitude) was considered. The best HVSR grouping has been achieved in four categories, as shown in Figure 14. For Category 1 (0.20 Hz < fo < 0.60 Hz) an average fo~0.35 Hz with average Ao~3, is observed. For Category 2 (0.61 Hz < fo < 0.98 Hz) an average fo~0.9 Hz with average Ao~3.2 while for Category 3 (0.99 Hz < fo < 1.70 Hz) fo~1.2 Hz with Ao~2.5, and for Category 4 (1.89 Hz < fo < 20 Hz) fo~3 Hz with Ao~2.0 (almost flat), are observed. Figure 14. Four categories of HVSRs for the city of Durres (average red line ± one standard deviation blue lines). Two out of seventy-six HVSR curves exhibited a double peak (Figure 15 left), while four out of seventy-six were affected by industrial noise (Figure 15 right) and were not included in the clustering procedure. Figure 15. HVSRs of two sites exhibited double peaks (left); HVSRs of four sites affected by indus- trial noise (right). Nevertheless, fundamental frequency with depth-to-bedrock distribution shows a satisfactory correlation with large dispersion (Figure 16). Appl. Sci. 2022, 12, 11309 19 of 27 In Figure 17, all HVSRs clustered in four categories are presented for the city of Durres. It is evident that Category 1 (0.20 Hz < fo < 0.60 Hz) corresponds to the deepest part of the basin while Category 2 (0.61 Hz < fo < 0.98 Hz) corresponds to both western and eastern edge of the basin. However, in the eastern part of the basin, a gradual transition from Category 2 to Category 3 (0.99 Hz < fo < 1.70 Hz) is observed. Category 4 (1.89 Hz < fo < 20 Hz) is mainly seen in the eastern hilly area. Categories 1 and 2 exhibit an average amplitude slightly higher than that of 3, Category 3 higher than 2 and Category 4 up to that of 2. Figure 16. Fundamental frequency (fo) in relation to the depth-to-bedrock based on the 76 HVSRs and the underlying bedrock of the Durres basin. In red circles flat HVSRs are depicted. Figure 17. Spatial distribution of the four HVSR site categories in the city of Durres, together with iso-depth contours. Bedrock depth iso-contours by . 3.3. Synthetics HVSR in Selected Sites Appl. Sci. 2022, 12, 11309 20 of 27 Synthetic data are used to test models in comparison with observations. In geophys- ics, 1D or 2D/3D subsoil profiles may be available after field investigation, including un- certainties according to the implemented technique. Validation testing of these models or even more their refinement can be performed after they are compared with actual obser- vations [67,68]. Such an approach is presented in this section. The noise sources were assumed to be randomly distributed around each station in a 1 km radius and lying at depths ranging from 0 to 10 m. The locations for which synthetic recordings of ambient noise were generated are shown in Figure 18. The minimum dis- tance between synthetic sources was 3 m, and the maximum was 1 km. The soil profiles of each site were deduced from the output of the Durres seismic microzonation study  and are given into Table 3. In the absence of any information about quality factors Qp and Qs, we assumed Qp = 2Qs = Vs/5 [69,70]. Synthetic spectral ratio HVSRs were calculated using Geopsy, following the recom- mendations of the SESAME guidelines. In Figure 19 synthetic HVSRs (black dashed line) are plotted together with the corresponding observed ones (black solid line) using the closest experimental ambient noise measurement site. Table 3. Information on the 8 site profiles (Figure 18) , used in the 1D synthetic ambient noise recordings. 2West 1West P38 DURR H (m) Vs (m/s) Vp (m/s) H(m) Vs (m/s) Vp (m/s) H(m) Vs (m/s) Vp (m/s) H(m) Vs (m/s) Vp (m/s) 20 190 1500 10 80 400 10 150 1450 10 120 1400 10 210 1550 5 120 1400 20 235 1550 16.7 201 1500 20 233 1580 10 201 1500 30 250 1570 23.3 233 1580 0 700 2000 55 233 1580 80 265 1600 40 242 1600 0 700 2000 0 700 2000 0 700 2000 BHA SH17 BH15 P40 H (m) Vs (m/s) Vp (m/s) H(m) Vs (m/s) Vp (m/s) H(m) Vs (m/s) Vp (m/s) H(m) Vs (m/s) Vp (m/s) 10 120 1400 40 120 1400 12 120 1400 10 140 1450 20 201 1500 10 201 1500 13 201 1500 20 240 1550 130 238 1580 55 233 1580 30 233 1580 20 300 1710 0 700 2000 0 700 2000 0 700 2000 0 700 2000 Appl. Sci. 2022, 12, 11309 21 of 27 Figure 18. West−East geologic cross section developed in the framework of the Durres seismic mi- crozonation project and the sites (black arrows & triangles) where synthetic ambient noise record- ings were generated. Bedrock depth iso-contours by . Comparisons between synthetic HVSRs (Hisada 1D) and observed HVSRs for the examined eight sites in Figure 19 show that the fundamental frequency is generally similar for all sites, though the amplitude of synthetics is consistently higher, up to double around the fundamental frequency as in the case of P40. In two cases (1West and BHA) a double peak appears in the synthetics, which contrasts the actual data. The shape of the synthetic HVSR is different from the observed one in the cases of 1West, P38, DUR, BHA, and SH17. The underlying structure at these stations is laterally heterogeneous, indicating possible 2D/3D effects. For the sites 2West, BH15 and P40 a lower amplitude of the actual HVSRs is observed, most probably due to attenuation of surface layers (low Q-values of the up- permost layered structure). To assess this assumption, the site P40 was chosen, and the Q- values of the s layers overlying the bedrock were decreased 20-fold, thus increasing the attenuation potential. The results are presented on the bottom of Figure 19 (site P40_test) and a very good agreement is observed between synthetic and actual HVSRs. This is in agreement with the findings of , who showed the importance of Q-values of the sur- face geologic layers for generating a smooth bump in HVSRs and decreasing their ampli- tude around the fundamental frequency of the investigated site. Appl. Sci. 2022, 12, 11309 22 of 27 Appl. Sci. 2022, 12, 11309 23 of 27 Figure 19. Synthetic (dashed line) versus observed (solid line) HVSRs for the eight selected sites on the West-East cross section of Figure 18. 4. Discussion and Conclusions In this study, 80 single station ambient noise measurements were performed in the city of Durres within the period of 18–21 February 2020, following the disastrous earth- quake of 26 November 2019, that hit the city and inflicted heavy damages to the built environment. Seventy-six (76) out of eighty (80) measurements were processed for the purpose of this study. The unique strong motion recording in the city’s center (DURR accelerometric sta- tion) showed PGA~0.20 g and high spectral values (>0.3 g) in a broad range of periods (0.2 s ≤ T ≤ 2.0 s), implying existence of a low fundamental frequency (down to fo~0.5 Hz). From the geologic and bedrock iso-depth contour maps, it is evident that the city has been developed on a basin of soft geologic layers, with an elongated north-south direction, reaching a thickness of 130 m in its center. Surface geologic layers down-town exhibit low shear-wave velocity, with VS30~200 m/s. Data processing and analyses based on the ambient noise Horizontal-to-Vertical spectral ratio (HVSR) method showed that an extended part of the Durres city correlates to a low fundamental frequency, fo < 1.0 Hz. This is in satisfactory agreement with the characteristics of the strong motion recording in the city’s center, thus validating the find- ings of the applied methodology. This part is mainly in the center-west side of the city, while fundamental frequency decreases smoothly towards its eastern part (the eastern edge of the basin. The vast majority of examined sites (95%) exhibited amplitude of fun- damental and dominant frequencies greater than 2, reaching a value of ~6. Based on the consensus that HVSR peak amplitude may represent a lower threshold of actual ground amplification than that estimated by the Standard Spectral Ratio method, one would ex- pect equal and/or higher amplification due to basin excitation by the seismic wavefield. A second, higher mode frequency peak, predominantly found in the western edge of the basin, may either indicate a thinner surface geologic layer of high velocity contrast inter- face with a deeper one, or a more complex site response behavior due to edge effects of the basin. Such an observation needs further investigation using theoretical 2D/3D mod- eling. A satisfactory correlation between fundamental frequency and thickness of the soft sediments is observed, though with large dispersion. This may be due either to errors of the bedrock depth estimation or to possible existence of additional superficial strata (e.g., soft colluvium, man-made deposits) which were not included in the subsoil profile. Targeted geological, geophysical and geotechnical data collection is needed to shed light on this issue and refine our understanding with respect to seisxmic risk of the city of Durres. The four site class categories proposed in this study indirectly indicate spatial varia- tion of amplification, in linear soil behavior. Categories 1 and 2 include zones with higher Appl. Sci. 2022, 12, 11309 24 of 27 average HVSR amplitude (>3.0) and are mainly extended in the central and western part of the city. It is remarkable that moderate to heavy earthquake damage was concentrated in the central and western part of the city (Figure 4a), potentially indicating a correlation of damage to site effects. Consequently, this categorization may form the backbone of any detailed and updated microzonation study of the city of Durres. For each category, an average fundamental frequency (±1 standard deviation) can be safely assigned along with its average amplitude. The average HVSR amplitude could be converted to actual S-waves horizontal spectral amplification using appropriate correction factors of the vertical am- plification (VACFs) proposed either for other regions of the world ([72,73]) or even better based on Albanian data. In summary, the ambient noise analysis implemented in this study for the city of Durres can be considered efficient and cost effective and confirms its ability to provide reliable results for seismic microzonation purposes in extended metro- politan areas. However, some assumptions made in this work (e.g., 1D consideration of subsoil profile, linear soil behavior) may constitute limitations of the research carried out and consequently additional effort is needed to improve our findings. Finally, the 1D ambient noise synthetics HVSRs, in comparison to actual HVSRs at the same sites, can effectively validate 1D geophysical profiles for use as seismic response input to any earthquake excitation scenario. In the case of Durres, the 1D approximation seems to be satisfactory. However, seismic site response corrections due to 2D/3D effects and the attenuation factor (Q) of the upper geologic layers must be carefully investigated to realistically assess future ground shaking in Durres. Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app122211309/s1. Author Contributions: Conceptualization, N.T., E.D., L.D. and N.K.; Data curation, E.D., L.D., I.G., A.P. and A.H.; Investigation, N.T., E.D., L.D., I.G., A.H., N.K. and R.K.; Methodology, N.T., L.D.; Software, A.P.; Supervision, N.T.; Writing – original draft, N.T. and E. D. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable Data Availability Statement: Data used in this study are available upon request. 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Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Nov 8, 2022
Keywords: earthquake ground motion; local site effects; ambient noise HVSR; seismic microzonation
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