Evaluation of Key PSHA Assumptions—Case-Study for Romania

geosciences

Review Evaluation of Key PSHA Assumptions—Case-Study for Romania

Florin Pavel

Department of Reinforced Concrete Structures, Technical University of Civil Engineering Bucharest, 020396 Bucharest, Romania; ﬂ[email protected]

Abstract: This case-study focuses on the analysis of several key assumptions necessary for the probabilistic seismic hazard assessment of Romania in the light of a future seismic hazard zonation of the territory. Among the aspects analyzed in this study are the appropriateness of the Poisson assumption which is tested on the earthquake catalogs of several seismic sources (crustal and the Vrancea intermediate-depth source), the azimuthal dependence of ground motion amplitudes from Vrancea intermediate-depth earthquakes and possible ground motion amplifications due to basin effects. The analyses performed in this study show that the Poisson distribution is able to model the observed earthquake frequency occurrence for the larger magnitude seismic events both for crustal and intermediate-depth seismic sources. Similar ground motion attenuation patterns irrespective of the azimuth with respect to the Vrancea intermediate-depth seismic source were observed only in the case of the 30 May 1990 earthquake, while in the case of the seismic events of 30 August 1986 and 31 May 1990 significant azimuthal ground-motion attenuation differences were observed. No significant differences in terms of ground motion amplitudes were observed at three seismic stations in Iasi area during the Vrancea intermediate-depth earthquakes of 30 May 1990 and 31 May 1990 possibly due to the limited elevation difference. Finally, significant long-period spectral amplifications were observed on the ground motions recorded at several sites from intramountainous depressions in Romania. Citation: Pavel, F. Evaluation of Key PSHA Assumptions—Case-Study for Keywords: poisson assumption; azimuthal attenuation; basin effects; ground motion amplification; Romania. Geosciences 2021, 11, 70. seismic hazard; Vrancea seismic source https://doi.org/10.3390/ geosciences11020070

Academic Editors: Enrico Priolo and Jesus Martinez-Frias 1. Introduction Received: 4 January 2021 Romania can be considered as one of the countries with the highest seismic hazard Accepted: 3 February 2021 levels in Europe, mainly due to the highly active Vrancea intermediate-depth seismic Published: 7 February 2021 source. This seismic source which is capable of generating two-three large magnitude earthquakes in each century affects mainly the southern and eastern part of Romania, as Publisher’s Note: MDPI stays neutral well as the territories of several neighboring countries, such as Moldova, Bulgaria and with regard to jurisdictional claims in Ukraine. The seismic hazard at national level is dominated for about two thirds of the published maps and institutional affil- territory by this intermediate-depth seismic source. The last earthquake with a moment iations. magnitude MW ≥ 6.0 occurred in October 2004, while the last large magnitude event was the 30 May 1990 event with a moment magnitude MW ≥ 6.9. The Vrancea earthquake of 4 March 1977 is the most damaging seismic event of the XXth century both in terms of human casualties and economic losses. The long-period spectral amplifications observed Copyright: © 2021 by the author. on the single free-field ground motion recorded in the eastern part of Bucharest and which Licensee MDPI, Basel, Switzerland. represents one of the peculiar characteristics of this earthquake were discussed in the This article is an open access article literature soon after the event [1]. distributed under the terms and An in-depth characterization of this peculiar seismic source from the seismological conditions of the Creative Commons point of view can be found in the studies of Fillerup et al. [2], Koulakov et al. [3], Ismail- Attribution (CC BY) license (https:// Zadeh et al. [4], Bokelmann and Rodler [5] and Petrescu et al. [6]. The most recent study of creativecommons.org/licenses/by/ Petrescu et al. [6] concludes based on the observed seismicity and stress regime patterns 4.0/).

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that a weak coupling (or an initiation of decoupling) between the sinking slab and the overriding crust currently still exists. The subduction process beneath the Carpathian Mountains is estimated to have stopped about 10 million years ago [7]. From the engineering point of view, the analysis of the spectral characteristics of the ground motions recorded during Vrancea intermediate-depth earthquakes which have occurred in the period 1977–2013 (10 seismic events) can be found in the study of Pavel et al. [8]. More recently, three additional intermediate-depth moderate seismic events (MW ≈ 5.5) occurred in September 2016, December 2016 and October 2018. From the point of view of seismic hazard assessment, the study of [9] shows that the mean peak ground acceleration with 10% exceedance probability in 50 years (mean return period of 475 years) can be as high as 0.5 g in the regions close to the Vrancea intermediate-depth seismic source and about 0.35 g in the capital city of Bucharest. In another seismic hazard study [10], the peak ground acceleration on rock conditions with 10% exceedance probability in 50 years has much smaller values, not exceeding 0.35 g in Romania. However, it was observed that signiﬁcant differences in terms of the seismic hazard assumptions used in the two above-mentioned studies [9] and [10] exist. This case-study focuses on several key aspect related to the probabilistic seismic hazard assessment performed recently within the BIGSEES (Bridging the gap between seismology and earthquake engineering) research project [9]. These aspects were not discussed previously in the seismic hazard studies performed neither within the BIGSEES project [9], nor within the SHARE (Seismic hazard harmonization in Europe) project [10] and are important in order to derive meaningful results for the future seismic zonation of Romania. The main aspects related to the probabilistic seismic hazard assessment analyzed in this study are: (i) the applicability of the Poisson assumption on the earthquake catalogs of several seismic sources (two crustal seismic sources affecting mainly the western and central part of Romania and the Vrancea intermediate-depth), (ii) the azimuth-dependent ground motion attenuation, as well as (iii) the possible ground motion ampliﬁcations due to basin effects in the case of Iasi and other sites in Romania (including some sites situated in the intramountainous depressions near the Carpathian Mountains).

2. Evaluation of the Poisson’s Assumption The seismicity of Romania comprises a mix of 13 crustal seismic sources situated in Romania and in some neighboring countries, as well and the Vrancea intermediate-depth seismic source which is located within the Carpathian Mountains [11]. The positions of all the seismic sources affecting the Romanian territory was deﬁned within the BIGSEES research project by the National Institute of Earth Physics (NIEP) in Romania [9]. The Poisson assumption is a very important aspect in the probabilistic seismic hazard assessment (PSHA) [12,13]. Ordaz and Arroyo [14] note that that if the earthquake occurrence process in time is Poissonian, then the occurrence process of earthquakes with intensities larger than a pre-scribed value is also Poissonian. In the case of non-Poisson pro- cesses, Ordaz and Arroyo [14] have derived general expressions to compute the probability distribution, the expected value, and the variance of the number of earthquakes per year. The earthquake catalogs used in the probabilistic seismic hazard assessment performed in the study of Pavel et al. [9] were declustered using the algorithm proposed by Gardner and Knopoff [15]. The minimum magnitude for all the earthquake catalogs of crustal seismic sources was taken as Mmin = 4.5, while in the case of the Vrancea intermediate-depth seismic source, a higher Mmin = 4.9 was employed. In this study, the assessment of the suitability of the Poisson assumption is performed for the earthquake catalogs corresponding to three seismic sources: the Vrancea intermediate-depth seismic source, as well as the Banat and Fagaras-Campulung crustal seismic sources which are based on the ROMPLUS earthquake catalog developed by the National Institute of Earth Physics (NIEP) [16]. The characteristics of the three earthquake catalogs (no. of seismic events, starting year of the catalog, as well as the minimum and the maximum magnitude of the dataset) are given in Table1. Geosciences 2021, 11, x FOR PEER REVIEW 3 of 17

Geosciences 2021, 11, 70 3 of 17 (no. of seismic events, starting year of the catalog, as well as the minimum and the maxi‐ mum magnitude of the dataset) are given in Table 1.

Table 1.1. CharacteristicsCharacteristics ofof thethe earthquakeearthquake catalogscatalogs forfor thethe threethree considered considered seismic seismic sources. sources.

No. of Seismic Starting Minimum MagniMinimum‐ MaximumMaximum Magni‐ No. of Seismic SeismicSeismic Source Source Starting Year Magnitude Magnitude EventsEvents Year tude Mmin tude Mmax Banat 1843 86 3.0 Mmin 5.6M max Fagaras‐BanatCampulung 1872 1843 69 863.0 3.06.5 5.6 Fagaras-Campulung 1872 69 3.0 6.5 Vrancea intermediate‐ Vrancea 18951895 809 8094.0 4.07.7 7.7 intermediate-depthdepth

A comparison between the observed and the theoretical (Poisson) earthquake fre‐ A comparison between the observed and the theoretical (Poisson) earthquake frequency occurrence for the three analyzed seismic sources and for four magnitude thresh‐ quency occurrence for the three analyzed seismic sources and for four magnitude thresholds olds are shown in Figures 1–3. are shown in Figures1–3.

(a) (b)

Figure 1. Comparison between the observed and the theoretical earthquake frequency occurrence for earthquakes occurring

in the Vrancea intermediate-depth seismic source with: (a)MW ≥ 4.5; (b)MW ≥ 5.0; (c)MW ≥ 5.5; (d)MW ≥ 6.0.

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Figure 1. Comparison between the observed and the theoretical earthquake frequency occurrence for earthquakes occur‐ ring in the Vrancea intermediate‐depth seismic source with: (a) MW ≥ 4.5; (b) MW ≥ 5.0; (c) MW ≥ 5.5; (d) MW ≥ 6.0.

(a) (b)

FigureFigure 2. 2. ComparisonComparison between between the the observed observed and and the the theoretical theoretical earthquake earthquake frequency frequency occurrence occurrence for for earthquakes earthquakes occurring occur‐ ring in the Banat seismic source with: (a) MW ≥ 3.5; (b) MW ≥ 4.0; (c) MW ≥ 4.5; (d) MW ≥ 5.0. in the Banat seismic source with: (a)MW ≥ 3.5; (b)MW ≥ 4.0; (c)MW ≥ 4.5; (d)MW ≥ 5.0.

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(a) (b)

FigureFigure 3.3. ComparisonComparison between between the the observed observed and and the the theoretical theoretical earthquake earthquake frequency frequency occurrence occurrence for for earthquakes earthquakes occurring occur‐ ring in the Fagaras‐Campulung seismic source with: (a) MW ≥ 3.5; (b) MW ≥ 4.0; (c) MW ≥ 4.5; (d) MW ≥ 5.0. in the Fagaras-Campulung seismic source with: (a)MW ≥ 3.5; (b)MW ≥ 4.0; (c)MW ≥ 4.5; (d)MW ≥ 5.0.

ItIt can be noticed noticed from from all all the the plots plots that that the the Poisson Poisson distribution distribution is able is able to model to model the theobserved observed earthquake earthquake frequency frequency occurrence occurrence for the for larger the larger magnitude magnitude seismic seismic events events in the indataset. the dataset. The same The observation same observation was made was madein other in otherstudies studies with the with same the sametopic topic(e.g., (e.g.,[17,18]). [17 ,A18 Chi]).‐ Asquare Chi-square test is performed test is performed for each for earthquake each earthquake catalog in catalog order into orderevaluate to evaluatethe threshold the threshold magnitude magnitude at which at the which Poisson the Poissonassumption assumption in no‐longer in no-longer rejected. rejected. The re‐ Thesulting resulting threshold threshold magnitudes magnitudes are as arefollows: as follows:

• MMWW = 6.0 6.0 for for the the Vrancea Vrancea intermediate intermediate-depth‐depth seismic source;  MW = 5.2 for the Banat crustal seismic source;  MW = 5.3 for the Fagaras‐Campulung crustal seismic source.

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Geosciences 2021, 11, x FOR PEER REVIEW 6 of 17 • MW = 5.2 for the Banat crustal seismic source; • MW = 5.3 for the Fagaras-Campulung crustal seismic source. In order to evaluate the impact of the Poisson assumption, a seismic hazard disag- In order to evaluate the impact of the Poisson assumption, a seismic hazard disaggre‐ gregation [19] is performed for several cities affected by earthquakes originating in the gation [19] is performed for several cities affected by earthquakes originating in the three three analyzed seismic sources, namely Bucharest, Focsani and Iasi which are under the analyzed seismic sources, namely Bucharest, Focsani and Iasi which are under the influ‐ influence of the Vrancea intermediate-depth seismic source, Sibiu which is affected by ence of the Vrancea intermediate‐depth seismic source, Sibiu which is affected by the Fa‐ the Fagaras-Campulung seismic source and Timisoara which is directly influenced by the garas‐Campulung seismic source and Timisoara which is directly influenced by the Banat Banat seismic source. The positions of the five sites and the contours of the three analyzed seismic source. The positions of the five sites and the contours of the three analyzed seis‐ seismic sources are illustrated in Figure4. mic sources are illustrated in Figure 4.

FigureFigure 4. 4.Positions Positions of of the the ﬁve five analyzedanalyzed sites and and contours contours of of the the three three seismic seismic sources. sources.

TheThe contribution contribution of each magnitude magnitude range range on on the the seismic seismic hazard hazard level level for for the the peak peak groundground accelerationacceleration (PGA) (PGA) with with a mean a mean return return period period of 475 of years 475 years(probability (probability of exceed of‐ ex- ceedanceance of 10% of 10% in 50 in years) 50 years) is shown is shown in Table in Table 2 based2 based on the on results the results from [9]. from [9].

Table 2. Seismic hazard disaggregation (peak ground acceleration with a mean return period of Table 2. Seismic hazard disaggregation (peak ground acceleration with a mean return period of 475 years) as a function of the earthquake magnitude for five cities in Romania [9] 475 years) as a function of the earthquake magnitude for ﬁve cities in Romania [9]. Contribution of Magnitude Range (%) Seismic Source City Contribution of Magnitude Range (%) Seismic Source <5.0 City 5.0–6.0 6.0–7.0 >7.0 Banat Timisoara 11.31 47.05 <5.0 5.0–6.041.64 6.0–7.00.00 >7.0 Fagaras‐Campulung SibiuBanat 12.63 Timisoara39.46 11.3146.45 47.05 41.641.46 0.00 Fagaras-CampulungBucharest 0.00 Sibiu0.83 12.63 39.467.87 46.4591.30 1.46 Bucharest 0.00 0.83 7.87 91.30 Vrancea intermediate‐depth FocsaniVrancea 0.00 0.01 3.68 96.31 Focsani 0.00 0.01 3.68 96.31 intermediate-depthIasi 0.00 0.84 7.11 92.05 Iasi 0.00 0.84 7.11 92.05 It can be observed from Table 2 that the contribution to the seismic hazard (e.g., peak groundIt can acceleration be observed with from a mean Table return2 that period the contribution of 475 years) to of the the seismicearthquake hazard magnitudes (e.g., peak groundfor which acceleration the Poisson with assumption a mean return can be period rejected of 475is relatively years) of small the earthquake for the two magnitudescities af‐ forfected which by the the crustal Poisson seismic assumption sources and can can be rejectedbe totally is disregarded relatively smallfor the for sites the under two the cities

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affected by the crustal seismic sources and can be totally disregarded for the sites under the inﬂuence of the Vrancea intermediate-depth seismic source. Consequently, it can be concluded that the impact of disregarding the Poisson assumption on the probabilistic seismic hazard results is rather limited. Finally, it has to be noted that the procedure for performing aftershock probabilistic seismic hazard assessment conditional on the main- shock occurrence can be found in the paper of Yeo and Cornell [20], while the procedure of sequence-based probabilistic seismic hazard analysis is given in [21].

3. Assessment of the Azimuthal Dependency of Ground Motion Amplitudes Pavel et al. [22] have performed an analysis of the possible directional effects observed during four moderate and large Vrancea intermediate-depth seismic events from 1986, 1990 and 2004 using the ground motion measures proposed in [23,24]. The study has shown, based on the available ground motion recordings, a visible directional pattern for IJMA (Japan Meteorological Agency seismic intensity) [25], while in the case of RotD100 [24] the directional patterns (angles corresponding to RotD100) were less visible. Moreover, it has to be highlighted the fact that a significant number of ground motion prediction equations developed for the Vrancea intermediate depth seismic source employ azimuth-dependent coefficients [26–30]. On the other hand, the ground motion models of Vacareanu et al. [31,32] had proposed different attenuation coefficients only for the backarc vs. forearc regions (both regions are defined with respect to the Carpathian Mountains). The same observation regarding the different attenuation coefficients only for the backarc vs. forearc regions instead of azimuth-dependent coefficients was made in the study of [33]. The seismic hazard performed in SHARE project [10] employed a Vrancea intermediate- depth seismic source of more than 50,000 km2 in order to capture an azimuth-dependent attenuation of ground motions from earthquakes originating in this seismic source. In this study, the analysis of the azimuthal dependency of the ground motion amplitudes is performed considering the recordings from three Vrancea intermediate-depth seismic events from 30 August 1986 (moment magnitude MW = 7.1 and focal depth h = 131 km), 30 May 1990 (MW = 6.9, h = 91 km) and 31 May 1990 (MW = 6.4, h = 87 km). The assessment is performed in order to check: (i) the influence of the epicentre position on the ground motion attenuation and (ii) the influence of the strike on the ground motion attenuation. The strike of the analyzed seismic events is taken from [34] and [35]. The 30 August 1986 and the 30 May 1990 events have a strike of about 230◦, while the 31 May 1990 earthquake has a strike of 308◦ (almost perpendicular fault rupture plane as compared with the other two analyzed events). The parameters of the three considered earthquakes are given in Table3[34,35].

Table 3. Parameters of the focal mechanisms for the three considered earthquakes [34,35].

Moment Focal Depth Date Strike (◦) Dip (◦) Rake (◦) Magnitude MW (km) 30 August 1986 7.1 131 227 65 104 30 May 1990 6.9 91 232 58 89 31 May 1990 6.4 87 308 71 97

Figure5 shows the distribution of the seismic station used in this study, as well as the position of the three earthquakes from 1986 and 1990. The soil conditions for each seismic station can be evaluated using the study of Pavel et al. [36]. The comparisons are performed considering the same recording stations for two pairs of seismic events (1986 vs. 1990(1) and 1990(1) vs. 1990(2)) in order to remove the effect of the site conditions and to have, thus, meaningful results. The positions of the seismic stations are deﬁned in Figure5 as a function of the position relative to the epicenters of the Vrancea intermediate-depth earthquakes (a total of three zones). The notation of the seismic stations from Figure5 is similar with the one from [33]. The data for the seismic stations situated in zone 3 as Geosciences 2021, 11, 70 8 of 17

Geosciences 2021, 11, x FOR PEER REVIEW 8 of 17 Geosciences 2021, 11, x FOR PEER REVIEWdenoted in [33] are not used in this study due to the very limited number of ground8 motions of 17

recorded during the three analyzed events in this study.

Figure 5. Positions of the recording seismic stations and epicenters of the Vrancea intermediate‐depth earthquakes of FigureFigure 5. 5.Positions Positions of of the the recording recording seismic seismic stationsstations and epicenters of of the the Vrancea Vrancea intermediate intermediate-depth‐depth earthquakes earthquakes of of August 1986 and May 1990. The color code is as follows: orange—zone 1 seismic stations, green—zone 2 seismic stations AugustAugust 1986 1986 and and May May 1990. 1990. The The color color code code is is as as follows:follows: orange—zone 1 1 seismic seismic stations, stations, green—zone green—zone 2 seismic 2 seismic stations stations and yellow—zone 4 seismic stations. andand yellow—zone yellow—zone 4 seismic4 seismic stations. stations. Figures 6–8 show the attenuation with the hypocentral distance of the peak ground FiguresFigures6 6–8–8 show show the the attenuationattenuation with the the hypocentral hypocentral distance distance of of the the peak peak ground ground accelerations and of the spectral accelerations at T = 0.s s and T = 1.0 s for the seismic accelerations and of the spectral accelerations at T = 0.s s and T = 1.0 s for the seismic accelerationsstations in the and three of zones the spectral which have accelerations recorded both at T =the 0.s 30 s August and T =1986 1.0 and s for the the 30 seismicMay stations in the three zones which have recorded both the 30 August 1986 and the 30 May stations1990 Vrancea in the three intermediate zones which‐depth have earthquakes recorded and both the the corresponding 30 August 1986 logarithmic and the 30 fit May 1990 Vrancea intermediate‐depth earthquakes and the corresponding logarithmic fit 1990curves, Vrancea as well. intermediate-depth earthquakes and the corresponding logarithmic ﬁt curves, ascurves, well. as well.

(a) (b) (a) (b) Figure 6. Comparison between the hypocentral distance attenuation curves for peak ground acceleration for: (a) the 1986 Figure 6. Comparison between the hypocentral distance attenuation curves for peak ground acceleration for: (a) the 1986 FigureVrancea 6. Comparison earthquake; between (b) the 1990(1) the hypocentral Vrancea earthquake. distance attenuation Dashed lines curves show forthe peakcorresponding ground acceleration logarithmic for:fit curves. (a) the 1986 Vrancea earthquake; (b) the 1990(1) Vrancea earthquake. Dashed lines show the corresponding logarithmic fit curves. Vrancea earthquake; (b) the 1990(1) Vrancea earthquake. Dashed lines show the corresponding logarithmic ﬁt curves.

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(a) (b) (a) (b) FigureFigure 7. 7. ComparisonComparison between between the the hypocentral hypocentral distance distance attenuation attenuation curves curves for for the the spectral spectral acceleration acceleration at at 0.2 0.2 s s for: for: (a) (a) the the 1986Figure1986 Vrancea Vrancea 7. Comparison earthquake; earthquake; between (b) (b) the the the 1990(1) 1990(1) hypocentral Vrancea Vrancea distance earthquake. earthquake. attenuation With With dashed dashed curves lines linesfor the are are spectral shown shown theacceleration the corresponding corresponding at 0.2 slogarithmic logarithmic for: (a) the fit curves. 1986ﬁt curves. Vrancea earthquake; (b) the 1990(1) Vrancea earthquake. With dashed lines are shown the corresponding logarithmic fit curves.

(a) (b) Figure 8. Comparison between(a) the hypocentral distance attenuation curves for the spectral (accelerationb) at 1.0 s for: (a) the 1986 Vrancea earthquake; (b) the 1990(1) Vrancea earthquake. With dashed lines are shown the corresponding logarithmic FigureFigure 8. 8. ComparisonComparison between between the the hypocentral hypocentral distance distance attenuation attenuation curves curves for for the the spectral spectral acceleration acceleration at at 1.0 1.0 s s for: for: ( (aa)) the the fit curves. 19861986 Vrancea Vrancea earthquake; earthquake; ( (bb)) the the 1990(1) 1990(1) Vrancea Vrancea earthquake. earthquake. With With dashed dashed lines lines are are shown shown the the corresponding corresponding logarithmic logarithmic fit curves. ﬁt curves. Figures 9–11 show the attenuation with the hypocentral distance of the peak ground accelerationsFiguresFigures 9–119 and–11 showof the the spectral attenuation accelerations with the at hypocentral T = 0.2 s and distance T = 1.0 of s the for peak the groundseismic stationsaccelerationsaccelerations in the and three and of of zones the the spectral spectralwhich have accelerations accelerations recorded bothat at T T =the = 0.2 0.2 30 s sMay and and 1990T T = = 1.0and 1.0 s sthe for for 31 the the May seismic seismic 1990 Vranceastationsstations inintermediate in the the three three zones zones‐depth which which earthquakes have have recorded recorded and the both both corresponding the the 30 30 May May 1990 1990logarithmic and and the the fit31 31 curves,May May 1990 1990 as well.Vrancea The intermediate number of ground‐depth motionearthquakes recordings and the per corresponding event ranges fromlogarithmic 6 to 14. fit The curves, ground as well. The number of ground motion recordings per event ranges from 6 to 14. The ground

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Vrancea intermediate-depth earthquakes and the corresponding logarithmic fit curves, as well. The number of ground motion recordings per event ranges from 6 to 14. The ground motionmotion parameter parameter used used in in the analysis is GMRotD50 (Median orientation orientation-independent‐independent g geometricgeometric mean mean using using period period-dependent‐dependent rotation angles) [23]. [23]. This This parameter parameter was chosen chosen becausebecause it it is is a a commonly commonly used used parameter parameter for for modern modern ground ground motion motion prediction prediction equations equations (GMPEs)(GMPEs) [37–41]. [37–41]. The The analysis analysis shows shows somewhat somewhat similar similar trends trends in in the the attenuation for for all threethree zones zones and and for for all all three three spectral spectral periods periods only only in in the the case case of of 30 30 May May 1990 1990 earthquake. earthquake. However,However, in in the the case case of of the the 31 31 May May 1990 1990 event event and and especially especially in the in case the caseof 30 of August 30 August 1986 earthquake,1986 earthquake, the differences the differences in attenuation in attenuation among among the three the three zones zones are aresignificant. significant. The The fit betweenfit between the theobserved observed data data and and the proposed the proposed model model is extremely is extremely poor poorin many in many cases, cases, thus thus the attenuation process is much more complex and it requires additional parameters the attenuation process is much more complex and it requires additional parameters in in order to better explain the recorded data. One of the reasons for the data scattering order to better explain the recorded data. One of the reasons for the data scattering can be can be related to the influence of the focal mechanism, as well as the complex geologic related to the influence of the focal mechanism, as well as the complex geologic structures structures from Zone 2 which are encountered by the seismic waves (Focsani Depression from Zone 2 which are encountered by the seismic waves (Focsani Depression and the and the North-Dobrogea orogenic belt). In addition, the number of available ground North‐Dobrogea orogenic belt). In addition, the number of available ground motion re‐ motion recordings is relatively limited, thus the data fit cannot be accurately estimated. cordings is relatively limited, thus the data fit cannot be accurately estimated. The rupture The rupture propagation direction can also play a role, but in this case, too, more data are propagation direction can also play a role, but in this case, too, more data are necessary in necessary in order to validate the hypothesis. The largest event for which ground motion order to validate the hypothesis. The largest event for which ground motion recordings recordings are available and which has a rupture propagating on a perpendicular direction are available and which has a rupture propagating on a perpendicular direction to the to the Carpathian Mountains is the 31 May 1990 event. Carpathian Mountains is the 31 May 1990 event.

(a) (b)

Figure 9. ComparisonComparison between between the the hypocentral hypocentral distance distance attenuation attenuation curves curves for peak for peak ground ground acceleration acceleration for: (a )for: the ( 1990(1)a) the 1990(1)Vrancea Vrancea earthquake; earthquake; (b) the 1990(2)(b) the 1990(2) Vrancea Vrancea earthquake. earthquake. With dashed With dashed lines are lines shown are shown the corresponding the corresponding logarithmic logarith ﬁt‐ mic fit curves. curves.

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(a) (b) (a) (b) FigureFigure 10. Comparison between thethe hypocentralhypocentral distance distance attenuation attenuation curves curves for for the the spectral spectral acceleration acceleration at 0.2at 0.2 s for: s for: (a) the(a) the1990(1)Figure 1990(1) Vrancea10. ComparisonVrancea earthquake; earthquake; between (b) the ( bthe) 1990(2) the hypocentral 1990(2) Vrancea Vrancea distance earthquake. earthquake. attenuation With dashedWith curves dashed lines for are thelines shown spectral are shown the acceleration corresponding the corresponding at 0.2 logarithmic s for: log (a‐) arithmic fit curves. ﬁtthe curves. 1990(1) Vrancea earthquake; (b) the 1990(2) Vrancea earthquake. With dashed lines are shown the corresponding log‐ arithmic fit curves.

(a) (b) Figure 11. Comparison between(a) the hypocentral distance attenuation curves for the spectral(b acceleration) at 1.0 s for: (a) the 1990(1) Vrancea earthquake; (b) the 1990(2) Vrancea earthquake. With dashed lines are shown the corresponding log‐ Figure 11.11. ComparisonComparison betweenbetween the the hypocentral hypocentral distance distance attenuation attenuation curves curves for for the the spectral spectral acceleration acceleration at 1.0 at s1.0 for: s for: (a) the(a) arithmicthe 1990(1) fit curves.Vrancea earthquake; (b) the 1990(2) Vrancea earthquake. With dashed lines are shown the corresponding log‐ 1990(1) Vrancea earthquake; (b) the 1990(2) Vrancea earthquake. With dashed lines are shown the corresponding logarithmic arithmic fit curves. ﬁt curves. Subsequently, a discussion regarding the seismic damage observed during the last two significant Vrancea intermediate‐depth seismic events, namely the 10 November 1940 Subsequently, a discussion regarding the seismic damage observed during the last earthquake (MW = 7.7, h ≈ 150 km) and 4 March 1977 (MW = 7.4, h = 94 km) is also shown two significant signiﬁcant Vrancea intermediate-depthintermediate‐depth seismicseismic events,events, namely thethe 10 November 1940 inearthquake this section (M basedW = 7.7, on h recently ≈ 150 km) revealed and 4 Marchdata [42,43]. 1977 (M InW the = 7.4, above h =‐ mentioned94 km) is also two shown stud‐ earthquake (MW = 7.7, h ≈ 150 km) and 4 March 1977 (MW = 7.4, h = 94 km) is also shown ies,in this the sectiongeographic based distribution on recently of revealed the damage data observed [42,43]. In throughout the above ‐Romanianmentioned during two stud the‐ ies, the geographic distribution of the damage observed throughout Romanian during the

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in this section based on recently revealed data [42,43]. In the above-mentioned two studies, the geographic distribution of the damage observed throughout Romanian during the two largest intermediate-depth Vrancea seismic events from the XXth century is discussed. The main observations from the two studies are summarized below: • The damage for the 1940 event which had an epicenter in the region of the May 1990 events was concentrated mainly towards the North-East and in the regions close to the epicenter. Thus, a slower attenuation towards Zones 1 and 2 (as deﬁned in Figure5 ) can be deduced from the collected damage data; • The damage for the 1977 event which had an epicenter in the region of the August 1986 event was concentrated mainly towards the South-West and in the regions close to the epicenter. In this case, a slower attenuation towards Zone 4 can be inferred from the damage observations; • Some of the regions which were affected in both the 1940 and 1977 earthquakes are situated near rivers, in soft soil conditions. In some of these areas, liquefaction phenomena were observed during both the 1940 and the 1977 earthquakes [44]. Thus, from the analyses performed in this study, it can be concluded that some azimuthal ground motion attenuation differences can occur in the case of individual seismic events, but no pattern has been observed. Moreover, since the probabilistic seismic hazard assessment accounts for all possible magnitude and epicenter positions (which are equally likely within the seismic source), the impact of azimuthal ground motion attenuation differences should be considered as very limited.

4. Analysis of Ground Motion Amplifications Due to Basin Effects In this section of the paper, an analysis is made regarding the amplification of ground motion amplitudes due to possible basin effects, based on ground motion recordings from past Vrancea intermediate-depth earthquakes and based on observed damage during significant earthquakes. The GMRotD50 [23] response spectra of the ground motions recorded at three seismic stations in Iasi during the Vrancea intermediate-depth earthquake of 30 May 1990 and 31 May 1990 are analyzed subsequently. The distance between the three analyzed seismic stations, as well as the corresponding elevations, are shown in Figure 12. The first two seismic stations are situated in the river valley, with the second station having the smallest elevation, while the third one is situated on the slope. The elevation difference between the first and the second seismic station is 27 m, while the elevation difference between the second and the third one is 57 m. The elevation and the estimated average shear wave velocity in the upper 30 m of soil deposits Vs, 30 [45] for each seismic station is given in Table4.

Table 4. Elevations and estimated soil Vs, 30 for the three seismic stations in Iasi [45].

Seismic Station Elevation Estimated Vs, 30 (m/s) 1 67 320 2 40 320 3 124 350

The GMRotD50 [23] response spectra for the three seismic stations observed during the two seismic events of May 1990 are illustrated in Figure 13. Geosciences 2021, 11, 70 13 of 17 Geosciences 2021, 11, x FOR PEER REVIEW 13 of 17

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Figure 12. Positions of the three seismic stations in Iasi, as well asas thethe distancedistance betweenbetween them.them. Figure 12. Positions of the three seismic stations in Iasi, as well as the distance between them.

(a) (b) Figure 13. Comparison between(a) the GMRotD50 response spectra of the ground motions recorded(b) at two seismic stations in Iasi area during: (a) the 1990(1) Vrancea earthquake; (b) the 1990(2) Vrancea earthquake. Figure 13. Comparison betweenbetween thethe GMRotD50 GMRotD50 response response spectra spectra of of the the ground ground motions motions recorded recorded at at two two seismic seismic stations stations in in Iasi area during: (a) the 1990(1) Vrancea earthquake; (b) the 1990(2) Vrancea earthquake. Iasi area during: (a) the 1990(1) Vrancea earthquake; (b) the 1990(2) Vrancea earthquake. It can be observed from Figure 13 that the three GMRotD50 [23] response spectra exhibitIt can significant be observed similarities from in Figure terms 13 of that amplitudes, thethe threethree thus GMRotD50GMRotD50 denoting [[23] similar23] responseresponse local soil spectraspectra con‐ exhibitditions. significant signiﬁcantHowever, similaritiessome similarities differences in in terms terms between of ofamplitudes, amplitudes, the spectral thus thusamplitudes denoting denoting similar for similar the local three local soil seismic con soil‐ ditions.conditions.stations However,can be However, observed some some differences at periods differences ofbetween about between 0.3–0.4the spectral the s, especially spectral amplitudes amplitudes between for the seismic three for the stationsseismic three stationsseismic1 and 3 stationsoncan one be observedhand can be and observed at seismic periods at station periodsof about 2 on of 0.3–0.4 the about other s, 0.3–0.4 especially hand. s, especiallyThe between largest between seismic spectral stations seismic ampli‐ 1stationstudes and 3occur on 1 and one at 3 seismichand on one and station hand seismic and 2 in seismicstation the case 2 station onof thethe 2 30other on May the hand. other1990 The earthquake hand. largest The largestspectral and at spectral seismic ampli‐ tudesstation occur 3 in theat seismic case of thestation latter 2 inevent the ofcase 31 ofMay the 1990. 30 May 1990 earthquake and at seismic station 3 in the case of the latter event of 31 May 1990.

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amplitudes occur at seismic station 2 in the case of the 30 May 1990 earthquake and at seismicThe station studies 3 in of the Riga case et of al. the [46,47] latter provide event of values 31 May of 1990. the aggravation factors (ratio be‐ tweenThe the studies 2D spectral of Riga acceleration et al. [46,47 and] provide the 1D valuesspectral of acceleration) the aggravation as a function factors (ratio of the betweenwidth of the the 2D basin, spectral angle acceleration of the slope, and thickness the 1D spectral of the sediments, acceleration) and as the a function shear wave of thevelocity width of of the the sediments. basin, angle Thus, of the significant slope, thickness differences of the of sediments, the ground and motion the shear amplitudes wave velocityshould ofbe the expected sediments. between Thus, the significant valley and differences the slope area. of the However, ground motion such differences amplitudes are shouldnot noticeable be expected in the between case of theIasi, valley even andthough the the slope position area. However, of the seismic such stations differences is differ are ‐ notent. noticeable This situation in the can case be of due Iasi, to even the thoughlimited thedifference position in of elevation the seismic between stations the is different.three seis‐ Thismic situation stations and can further be due data to the from limited other difference seismic stations in elevation are necessary between in the order three to seismic evaluate stationsthis phenomenon. and further data from other seismic stations are necessary in order to evaluate this phenomenon.Another city which was affected by the Vrancea 1977 earthquake is Craiova, situated at moreAnother than city 200 which km from was the affected epicenter by the [43]. Vrancea In this 1977 case, earthquake the damage is Craiova, was also situated concen‐ attrated more thanin the 200 river km fromvalley the area epicenter situated [43 on]. In recent this case, sedimentary the damage layers. was alsoUnfortunately, concentrated no infree the‐field river ground valley area motion situated was recorded on recent in sedimentary Craiova during layers. the Unfortunately, August 1986 and no free-field May 1990 groundearthquakes motion in was order recorded to bring in Craiovasome new during relevant the Augustinformation 1986 andon this May topic. 1990 The earthquakes only seis‐ inmic order station to bring in Craiova some new was relevant placed informationin the underground on this topic. of a high The‐ onlyrise reinforced seismic station concrete in Craiova was placed in the underground of a high-rise reinforced concrete building. building. Other examples of possible ground motion amplifications due to basin effects are the Other examples of possible ground motion amplifications due to basin effects are the intramountainous depressions situated in the areas affected by Vrancea intermediate-depth intramountainous depressions situated in the areas affected by Vrancea intermediate‐ earthquakes. Two such examples of sites situated in such depressions are Joseni and Turnu- depth earthquakes. Two such examples of sites situated in such depressions are Joseni Rosu. The normalized GMRotD50 [23] response spectra observed at the two sites during and Turnu‐Rosu. The normalized GMRotD50 [23] response spectra observed at the two the Vrancea earthquake of November 2014 are illustrated in Figure 14. The long-period sites during the Vrancea earthquake of November 2014 are illustrated in Figure 14. The spectral amplifications observed at both sites are clearly visible in Figure 14. Thus, it has long‐period spectral amplifications observed at both sites are clearly visible in Figure 14. to be highlighted the fact that the design response spectra from the current Romanian Thus, it has to be highlighted the fact that the design response spectra from the current seismic design code P100-1/2013 [48] appears to underestimate the long-period spectral Romanian seismic design code P100‐1/2013 [48] appears to underestimate the long‐period amplifications which might occur at such sites during future large magnitude Vrancea intermediate-depthspectral amplifications earthquakes. which might occur at such sites during future large magnitude Vrancea intermediate‐depth earthquakes.

FigureFigure 14. 14.Comparison Comparison of of the the normalized normalized GMRotD50 GMRotD50 responseresponse spectraspectra (SA/PGA)(SA/PGA) for for the the ground ground motionsmotions recorded recorded at at Joseni Joseni and and Turnu Turnu Rosu Rosu during during the the Vrancea Vrancea earthquake earthquake of of November November 2014. 2014.

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5. Conclusions This case-study is focused on the analysis of several key assumptions necessary for the probabilistic seismic hazard assessment of Romania in the light of a future seismic zonation of Romania. The most important findings of the study can be summarized as follows: • The Poisson distribution is able to model the observed earthquake frequency occurrence for the larger magnitude seismic events in the dataset. The threshold magnitudes below which the Poisson assumption can be rejected are MW = 5.2–5.3 for the Ba- nat and Fagaras-Campulung crustal seismic sources and MW = 6.0 for the Vrancea intermediate-depth seismic source; • Similar attenuation trends in the attenuation were observed for all three zones and for all three spectral periods only in the case of 30 May 1990 earthquake. In the case of the 31 May 1990 event and especially in the case of 30 August 1986 earthquake, the differences in attenuation among the three zones are significant. However, it has to be highlighted the fact that the observations made in this study are based on a rather limited ground motion dataset and thus further analyses are needed in order to validate the findings in this study. Moreover, the fit between the observed data and the proposed model is extremely poor in many cases, thus the attenuation process is much more complex and it requires additional parameters in order to better explain the recorded data; • The damage for the 1940 event which had an epicenter in the region of the May 1990 events was concentrated mainly towards the North-East and in the regions close to the epicenter. In the case of the 1977 event which had an epicenter in the region of the August 1986 event, the damage was concentrated mainly towards the South-West and in the regions close to the epicenter; • No significant differences in terms of ground motion amplitudes were observed at three seismic stations in Iasi area during the Vrancea intermediate-depth earthquakes of 30 May 1990 and 31 May 1990 even though two seismic stations were situated in the valley and the other one on the slope, possibly due to the small elevation difference between them. • Long-period spectral amplifications were observed at seismic stations situated in the intramountainous depressions (e.g., Joseni and Turnu-Rosu). A re-estimation of the design spectral shapes for such sites should be performed in the future version of the Romanian seismic design code, as the current version of the code [48] seems to underestimate these amplifications.

Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The author declares no conﬂict of interest.

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