International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania

PROBABILISTIC SEISMIC HAZARD ASSESSMENT FOR ROMANIA CONSIDERING INTERMEDIATE-DEPTH (VRANCEA)

AND SHALLOW (CRUSTAL) SEISMICITY

Vladimir Sokolov1, Friedemann Wenzel1, Rakesh Mohindra2, Bogdan Grecu3, and Mircea Radulian3

ABSTRACT

The earthquake risk on Romania is one of the highest in Europe, and seismic hazard for almost half of the territory of Romania is determined by the Vrancea seismic region, which is situated beneath the southern Carpathian Arc. The region is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume at depth from 70 km to 160 km. Besides the Vrancea area, several zones of shallow seismicity located within and outside territory of Romania are considered as seismically dangerous. We present results of probabilistic seismic hazard analysis, which implemented “logic tree” approach based on recent information on earthquake ground motion characteristics and which considers both intermediate-depth and crustal seismicity. Seismic hazard in terms of macroseismic intensities, peak ground acceleration, and response spectra was evaluated for various return eriods. For the Vrancea area, the region-dependent attenuation relationships were used (Sokolov et al., 2007). These attenuation relationships are based on Fourier Amplitude Spectrum (FAS) source scaling and attenuation models, and generalized site amplification functions. For the crustal events, due to lacking of strong motion data, the attenuation relationships developed for Europe by Ambraseys et al. (1996) are used.

INTRODUCTION

The earthquake risk on Romania is one of the highest in Europe, and seismic hazard for almost half of the territory of Romania is determined by the Vrancea seismic region, which is situated beneath the southern Carpathian Arc. The region is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume at depth from 70 km to 160 km (e.g, Wenzel et al., 1999). Besides the Vrancea area, several zones of shallow seismicity located within and outside territory of Romania are considered as seismically dangerous.

The information relating to expected seismic effect and expressed in terms of earthquake ground motion parameters is necessary for design of buildings and structures in earthquake prone regions, seismic risk estimation and management, and insurance business. The specification of engineering (or design) ground motion parameters, such as seismic intensity, peak amplitudes of ground motion, response spectra and ground motion time histories, is the goal of Seismic Hazard Analysis (SHA). It involves the quantitative estimation of ground-shaking hazard at a particular site taking into account characteristics of potentially dangerous earthquakes around the site. The relation between deterministic and probabilistic approaches for SHA is a subject of much controversy (e.g. Bommer, 2002; Krinitzsky, 2003; McGuire, 2002). The decision what approach should be applied depends on the final goal – how and where do we expect to use the result. Seismic hazard mapping, development of design codes, and financial planning of earthquake losses requires probabilistic assessment.

1 Geophysical Institute, University of Karlsruhe, Karlsruhe, Germany

2 RMSI Pvt. Limited, Noida, India

3 National Institute for Earth Physics, Bucharest - Magurele, Romania

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Several studies have been carried out to evaluate the seismic hazard in Romania using the probabilistic approach. Among the most recent ones, which consider only the Vrancea seismicity, we should mention the following works, namely: Lungu et al. (1995, 1999), Mantyniemi et al. (2003); Sokolov et al. (2004a). The crustal and intermediate-depth seismicity were jointly considered by Musson (2000), Ardeleanu et al. (2005), and Mohindra et al. (2007). Moldovan et al., (2004) estimated seismic hazard from crustal events for central Romania in terms of macroseismic intensity.

Description of development of the codes for earthquake resistance of buildings and structures in Romania during last 60 years, as well as the current standards for seismic zonation, and design provisions, was given by Lungu et al. (2006). The present seismic code of Romania (P100-1/2004) considers the earthquake hazards for return period (mean recurrence interval) of 100 years (40 % probability of being exceeded in 50-year exposure time). Therefore, as it has been noted by Lungu et al. (2006), the assessment of seismic hazard should be revised in accordance to recent requirements. As recommended in EUROCODE 8, two variants should be considered for ordinary constructions, namely: a probability of exceedance of 10% in 50 years (recurrence period of 475 years), and a probability of exceedance of 10% in 10 years (recurrence period of 95 years).

Recently logic trees become a popular tool in seismic hazard studies. The logic trees technique is used for incorporating the epistemic uncertainty, or uncertainty reflecting the incomplete knowledge of the nature of seismic motion, into the hazard calculations. There are opposite opinions regarding the possibility of using the logic trees in PSHA (e.g., Bommer et al., 2005; Krinitzsky, 2003). Keeping in mind that handing uncertainties is a key element of seismic hazard assessment, we applied in this study PSHA with elements of logic trees approach.

In this article we describe results of PSHA performed for the Romanian territory considering the intermediate-depth and crustal seismicity and using the most recent knowledge on seismic hazard in Romania. When possible, we used various available models of seismicity and ground motion attenuation, as alternative variants, which will be described below. The PSHA was performed in terms of peak ground acceleration (PGA), response spectrum amplitudes (PSA) and seismic intensity (MSK or MM) for various return periods or probabilities of being exceeded during specified exposure time.

THE METHOD

We apply the same technique that was used in our recent works and the detailed description of the technique may be found in the following sources (Chernov, 1987; Sokolov et al., 2004a). The method is based on Cornell’s approach for probabilistic seismic hazard assessment (Cornell, 1968), and assumes that earthquake occurrence is a stationary random process. It incorporates the effect of all potential sources of earthquake and the activity rate assigned to them. However, our approach and computational scheme differ somewhat from the classical one. Each potential earthquake is considered as an individual event, instead of the use of the probability density functions for magnitude and distance in a seismic source zone. At the first step of the computational procedure, a log-normal distribution of the ground motion parameter’s values is assumed for a given magnitude and source-to-site distance. The probability that the ground motion parameter will not exceed a certain value is calculated for this single event (N = 1) with the given magnitude M, depth H, and source-to-site distance R. At the next steps, the technique considers the distribution of earthquake sources of the given magnitude and source-to-site distance through the depth and earthquake occurrence. Finally, the influence of all potential earthquakes of the given magnitude range located at various distances is taken into account.

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The aleatory variability that is related to apparent randomness of ground motion parameters during a single earthquake is considered introducing the probability distribution function for the parameter at the first stage. The scheme of seismic hazard evaluation considers all possible earthquakes that may affect the point of calculation. A system of grid points (elementary segments) with 0.1°×0.1° spacing is used. Possible earthquakes occur within a volume below the segment, and the epicenters of earthquakes within a grid are supposed to be located at the center point of the segment. Every elementary segment is characterized by the following parameters: (1) minimum (Mmin) and maximum (Mmax) magnitudes of possible earthquakes which will

occur within the segment; (2) probability distribution of hypocentral depths for earthquake

sources of Mmin ≤ M ≤ Mmax; and (3) rates of earthquake recurrence per unit time.

INPUT DATA

Seismicity and seismic sources A primary component of the probabilistic seismic hazard model is the earthquake catalogue. A composite and updated catalogue, which covers earthquake epicenters located within a range of 200 km from Romanian boundary, has been compiled for the present study using catalogues from three different sources. The first catalogue that we used is the Romplus catalogue compiled by Oncescu et al. (1999) for Romania territory. The authors claim that

the catalogue is complete between 1411-1800 for MW ≥ 7.0, between 1801-1900 for MW ≥

6.5, between 1901-1935 for MW ≥ 5.5, between 1936-1977 for MW ≥ 4.5, between 1978-1997

(2003) for MW ≥ 3.0. However, the magnitude estimates before about 1800 are affected by large errors. The earthquake catalogues of Romania were originally compiled by Radu (Gutenberg-Richter magnitude) and Constantinescu and Marza (MS magnitude). The ROMPLUS catalogue includes both the Radu's catalogue (Radu, 1979, 1991) and the Constantinescu and Marza (1980) catalogue. The only difference is the magnitude converted everywhere to moment magnitude in ROMPLUS and the update. The second catalogue is the “Earthquake Catalogue of Central and Southeastern Europe”. The Europe catalogue is compiled jointly by The Federal Institute for Geosciences and Natural Resources (BGR) and the Institute of Physics of the Earth, Moscow for European countries (Poland, Czech Republic, Slovakia, Hungary, former Yugoslavia, Albania, Romania, Bulgaria, and the European part of Turkey). This catalogue covers data for the period from 342 BC to 1990 AD including the previously published epicenters and parameters of earthquakes. The third catalogue is the USGS PDE catalogue containing earthquake data for the Romanian region for the period 1973 to 2006 with the magnitude format Mb, MS and ML. This data downloaded in expanded file format from the link http://neic.usgs.gov/neis/epic/epic_rect.html. While compiling the homogeneous and composite catalogue, earthquakes are selected from various catalogues with respect to time and space of occurrence. The Romplus catalogue is more authentic and reliable source, therefore the earthquake events lying within the territory of Romania are taken from the Romplus catalogue. The events located outside Romania were taken from BGR Europe catalogue (Fig. 1). This catalogue contains earthquakes up to December 1990 and events from 1991 onwards were taken from USGS PDE catalogue. A uniform magnitude scale of moment magnitude (Mw) consistent with ground motion computation is used in model. The Romplus catalogue referred to moment magnitude, which had been already calculated from other magnitude types. Magnitudes of BGR and PDE catalogues were converted to moment magnitude using regressions and empirical formulas of Johnston (1996).

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It is essential also to clean the catalogue by removing duplicate events and accessory shocks. Simple software was used for the purpose based on standard algorithm describing dependence of the accessory events on the time and space window of main shocks. These magnitude-dependent time (number of days) and space (distance from main shock location) functions were taken as that used by Musson (2000).

Figure 1. Distribution of earthquake epicenters and selected source zones: red lines – the Vrancea zone (VD); black lines – zones of shallow (crustal) earthquakes (model Z1, see text). Description of characteristics of the zones is given in Table 1. Seismic sources are geographical areas that have experienced seismic activity in the past and serve as potential sources of earthquakes in the future. Seismic sources are delineated based on seimotectonic features and homogeneity of seismic activity. For each seismic source, it is assumed that past earthquake activity is a reliable predictor of future activity. The seismicity of the Vrancea area is characterized by three peculiar features: (1) strong earthquakes occur at intermediate depth in a very narrow source volume; (2) the seismogenic zone is situated beneath continental crust, at the SE corner of the highly arcuate Carpatian arc; (3) no evidence for active ongoing subduction is found today. A summation of recent studies devoted to the phenomenon of the Vrancea seismicity may be found in a monograph edited by Wenzel et al. (1999) and a PAGEOPH special volume (2000, vol. 157). Besides the Vrancea intermediate-depth earthquakes, the influence of crustal sources should be taken into consideration. Description of the crustal sources on the Romanian territory and neighboring regions was presented, for example, by Bala et al. (2003); Musson (2000); Pantea (1994); Radulian et al. (2000). In our calculations we used two models of crustal seismic zones, first of which (Z1) is based on Radulian et al. (2000) zonation (Fig. 1)

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and the second one (Z2) was used by Musson (2000). Only zones with maximum magnitude more than 5.0 were considered. Table 1 summarizes characteristics of seismic zones. When describing the rate of

earthquake occurrence using standard relation bmaN +=lg , instead of cumulative

magnitude-recurrence model which determines number N of events with a magnitude m larger than M, an alternative model is used in our scheme and N is defined as the number of

events with a magnitude m = M ± ∆M (∆M = 0.25). The cumulative models taken from correspondent literature were adjusted to satisfy our requirements. Also, the Gutenberg-Richter magnitudes used by Lungu et al. (1999) were recalculated to moment magnitudes. Table 1. Characteristics of the earthquake sources collected from available literature. The recurrence rate is normalized to a 1-year time interval.

Recurrence model* Zone (labeling in Fig.1)

a b Mmax Reference

3.15 -0.66 8.0 Lungu et al. (1999) (R1)

4.41 -0.92 7.5 Musson (2000) (R2) Vrancea (VD)

4.62 -0.87 8.0 Sokolov et al. (2004a) (R3)

Shallow (crustal) sources

Model Z1

Crisana-Maramures (CM) 1.27 -0.604 6.5

Barlad-Moldavia (MD) 0.997 -0.602 6.0

Banat (BA) 1.545 -0.578 6.0

Danubian (DA) 1.29 -0.602 6.0

Fagaras-Campulung (FC) 0.185 -0.339 6.5

Vrancea crustal (VC) 3.58 -1.07 6.0

Predobrian (PD) 0.974 -0.596 6.0

Intramoesian fault (IM) 1.4 -0.6 7.0

Bulgaria (BU) 0.59 -0.38 7.0

Recurrence rates were calculated using composite

catalogue and zonation proposed by Radulian et al.

(2000)

Model Z2

BANA 3.43 -0.968 6.0

EPDM 2.91 -0.953 6.5

ESPM 3.31 -0.954 7.0

IMOF 1.66 -0.632 7.0

SECM 1.969 -0.772 6.5

TRVR 2.11 -0.672 6.0

Musson (2000)

Obviously, we can not assume that parameters of seismicity sharply change at boreders of the zones. Therefore a smoothing procedure is applied. Earthquake recurrence parameters (number of earthquakes of different magnitude per unit time and unit volume) were assigned to the elementary segments as follows: the central (axial) segments of the zone are characterized by the maximum number of earthquakes, and gradually decreased numbers are assigned to the segments, which are located toward the edge of the zone.

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Attenuation relations The Vrancea earthquakes Analysis of the macroseismic and instrumental data from the intermediate-depth Vrancea earthquakes revealed several peculiarities of earthquake effects (e.g., Ivan et al., 1998; Mandrescu and Radulian, 1999; Moldovan et al., 2000; Radulian et al., 2006). They may be summarized as follows:

� The earthquakes affect very large areas with a predominant NE-SW orientation; � The local and regional geological conditions can control the amplitudes of earthquake

ground motion to a larger degree than magnitude or distance. The strong ground motion parameters exhibit a large variability depending on site location.

The Vrancea earthquakes should be considered as a very specific case of subcrustal events. As it has been shown recently (e.g. Sokolov et al., 2005), the earthquakes are characterized by magnitude-dependent nature of stress drop, the values of which increase from 30 bars for M 3.0-3.5 up to 200-250 bars for M 5.0-5.5, and 800-1000 bars for M 7.5-7.7. The peculiar features of the Romanian seismicity and distribution of ground motion parameters do not allow performing a proper selection of appropriate ground-motion model developed for the other regions.

The relationships between seismic intensity and earthquake characteristics in the Vrancea zone are represented by empirical azimuth-dependent attenuation equations (e.g., Ivan et al., 1998; Marza, 1996; Enescu and Enescu, 2007). These equations relate decrease of maximum intensity I0 with hypocentral distance D. The relations between earthquake magnitude and maximum intensity obtained by different authors are summarized by Marza (1996). However, all these relations do not take into account the earthquake depth. Moreover, the maximum intensity can not be called epicentral intensity, because the instrumental epicenters almost for all Vrancea earthquakes are not located within the area of maximum effect (e.g. Mandrescu and Radulian, 1999). The difference may reach several tens or even one hundred kilometers, which makes it difficult to apply these maximum-intensity-dependent relations in seismic hazard assessment.

Lungu et al. (1997) evaluated attenuation of maximum peak ground acceleration (PGAM) using records of three large Vrancea earthquakes, namely: March 4, 1977 (MW = 7.4); August 30, 1986; May 30, 1990, in the following form

ε++++= hcRcMccPGAM 4321 lnln (1)

where M is magnitude; R is hypocentral distance; h is the focal depth; c1, c2, c3, c4, are the

data dependent coefficients; ε is the parameter describing the data variation. Four attenuation models were evaluated using different data sets, which represent sectors (areas) located to three directions from the Vrancea zone. The data sets are the following: all data; the Bucharest (azimuth 1800-2700, South-West area in our study) sector; the Cernavoda (azimuth 900-1800, South-East area) sector; the Moldova (azimuth < 900, North area) sector. The relationships given in Lungu et al. (1997) use so-called Gutenberg-Richter (GR) magnitude. The conversion between GR magnitude M and moment magnitude MW are given as MW =1.09M - 0.36. Musson (2000) presented the Lungu’s equations that use surface wave magnitude MS. Table 2 lists the coefficients (Eq. 1) as given by Lungu et al. (1997).

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Table 2. Coefficients of the attenuation function for Vrancea subcrustal earthquakes

Coefficients Complete data set

Bucharest sector

Cernavoda Moldova

Lungu et al. (1997) based on Gutenberg-Richter magnitude

C1 5.128 7.249 8.886 4.601 C2 1.063 0.904 0.905 0.929 C3 -1.297 -1.492 -1.786 -1.030 C4 -0.009 -0.0081 -0.0096 -0.008

ε (ln PGA) 0.449 0.358 0.349 0.465

The PGA attenuation relations for the Vrancea earthquakes were developed also by Moldovan et al. (2000), as a single azimuth-independent relationship, and Stamatovska (2002), which relations cover only the southern and south-western directions. Based on the PGA relationships proposed by Lungu et al. (1997), we made an attempt to develop azimuth/region-dependent relations for macroseismic intensity using available relationships between macroseismic intensity and PGA for Vrancea earthquakes. The relationships were recently proposed by Enescu (1997) and Bonjer et al. (2001) as follows

181.02712.0log max += Ia V < I < IX after Enescu (1997) (3a)

14.029.0log max += Ia after Bonjer et al. (2001) (3b)

where maxa is the maximum value from two horizontal components. Enescu (1997) obtained

his relationship using 38 records of large Vrancea earthquakes (1986 and 1990) and observed intensities. The instrumental intensity values evaluated from using technique proposed by Sokolov (2002) were used by Bonjer et al. (2001).

Figure 2. Scheme of characteristic regions (numbers), for which the region-dependent attenuation relations for Vrancea earthquakes were developed (Sokolov et al., 2007; Sokolov and Wenzel, 2007).

An indirect approach is used by Sokolov et al. (2007) (see also Sokolov and Wenzel, 2007) for developing of regional azimuth-dependent attenuation relations for Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), Response Spectra Amplitudes (RSA), and seismic intensity (MSK or MMI scale) for the Vrancea intermediate depth earthquakes (SE-Carpathians) and territory of Romania.

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The relations have been developed on the basis of Fourier Amplitude Spectrum (FAS) source scaling and attenuation models (Sokolov et al., 2005), and generalized site amplification functions (Sokolov et al. 2004b). The territory of Romania was divided into 8 characteristic regions (Fig. 2) bearing in mind both general geological and geomorphological conditions and azimuthal direction from the Vrancea zone. The attenuation relationships were developed separately for each of the region. The PGA, PSV, and RSA attenuation relationships were calculated using stochastic technique (Boore, 2003) from the site-dependent spectra. The seismic intensity attenuation models were evaluated using the recently developed relations between intensity and FAS (Chernov and Sokolov, 1999; Sokolov, 2002). The regression analysis of the modeled data was performed to obtain the azimuth-dependent attenuation equations, in which the ground motion parameter is described as function of magnitude, depth and epicentral distance. The empirical data (macroseismic data and strong-motion records) were used for calibration of the relationships. The shallow earthquakes

For crustal events, due to lacking of strong motion data, the attenuation relationships developed for Europe by Ambraseys et al. (1996) are used. The PGA relationship is given as

ε++−+= )ln()ln( 2

0

2

321 hdCMCCPGAS

(4)

where C1= -1.39; C2=0.266; C3=0.922; d is the shortest distance to the surface projection of

the fault rupture (epicentral distance); h0 = 3.5; ε is the standard deviation of PGA (0.25). Similar equation is used for amplitudes of response spectra with frequency-dependent

coefficients C1, C2, C3, and ε. The intensity attenuation relations for crustal earthquakes in the Romania were analysed by Pantea (1994) using data from 18 events (M 4 – 7), which occurred during 1978-1969. The epicentral intensity varied from V to IX-X degrees (MSK scale). The zone-dependent equations are given in the following form

DcDbaII −−+= lnlnln 0 (5)

where I is intensity at the site located at the hypocentral distance D; I0 is the epicentral intensity; b is the coefficient of geometric spreading; c is the coefficient of absorbtion; the coefficients a, b, and c are azimuth-dependent. Zsiros (1996) and Pantea and Moldovan (2000) presented intensity attenuations as

)()/(log0 hDbhDaII −−−= (6)

where D is the hypocentral distance; h is the characteristic depth; a and b are the zone-dependent coefficients. The simple relation between epicentral intensity and magnitude based on the data provided by Pantea (1994) for earthquake depth less than 30 km may be expressed as I0 = 1.34 M ± 0.7. Radu (1974) presented the following I0 (M) relations, namely: I0 = 1.52 M - 1.23 for shallow earthquake zones located in the central and eastern parts of Romania; I0 = 1.67 M - 0.52 for zones located in the northern and western parts. Intensity attenuation relation for crustal earthquakes in the northern Bulgaria was taken from Glavcheva (1990) in the following form

62.3log0.323.1 10 +−= DMI (7)

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Composition of logic tree The objective of the logic tree is handling of incomplete knowledge and of variability of interpretation of available data. The related uncertainty is accounted in SHA through consideration of alternative models that are weighted in the analysis according to their probability of being correct. The number of the alternative models is selected bearing in mind the following general principles (e.g., Bommer et al., 2005), namely: the options should encompass the complete range of physical possibilities that particular parameter could be expected to take; to avoid using models (1) with very small differences between the options that they carry and (2) the extreme and impossible models.

Figure 3. Logic trees of input parameters showing accepted models and assigned weights. Fig. 3 shows the logic tree components that we used in this study. We selected the following alternative models for Vrancea zone in our calculations. First, three models of recurrence rate were considered (see Table 1). Second, two models of earthquake depth distribution were used for Vrancea intermediate-depth seismicity, namely: (1) uniform distribution through depth; (2) distribution based on catalogue. Third, two variants of attenuation relations were used, namely: (1) the attenuation models developed by Sokolov et al. (2007) (models PGA_V1, and MSK_V1); (2) the attenuation relations proposed by Lungu et al. (1995, 1997) including relationships for macroseismic intensity obtained using Eqs. 3ab (models PGA_V2 and MSK_V2). Only one model of attenuation relation for response spectra (RSA_V1) was used due to absence of the alternatives.

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For the crustal seismicity we selected two models of earthquake sources (see Table 1) and five variants of source depth. Only one model of attenuation relation for peak ground acceleration (PGA_S1) and response spectra (RSA_S1) was taken from Ambraseys et al. (1996) due to absence of the alternative relationships. These models do not consider variation of earthquake depth. Macroseismic intensity is represented by two models, which allow the depth variation. The first one (MSK_C1) contains three attenuation relations proposed by Pantea and Moldovan (2000) (zones FC, VC, MD, PD, and IM), Zciros (2000) (zones BA, CM, and DA), and Glavcheva (1990) (zone BU). The second model (MSK_2C) was constructed based on four region-dependent models (see Pantea, 1994, for details) for crustal seismic zones located within Romania and one model for Bulgaria (Glavcheva, 1990).

RESULTS

In this work assessment of seismic hazard was performed for points with 0.25°×0.25° spacing. The attenuation relationships provided by Lungu et al. (1997) and Sokolov et al. (2007) are azimuth- or region-dependent functions, therefore a linear interpolation was used when calculating ground motion for intermediate directions (Lungu et al., 1997) and areas located near the borders of the characteristic regions (see Fig. 2, Sokolov et al., 2007).

Figure 4. Results of PSHA for intermediate-depth seismicity. Left plots display intermediate results obtained using two models of attenuation relations (see text).

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Fig. 4 shows, as the examples, results of PSHA for intermediate-depth seismicity. Note that the attenuation relationships proposed by Lungu et al. (1997) can not be applied for north-eastern part of Romania, located to three directions from the Vrancea zone. Only the territories located at azimuths from 00 to 2700 from the Vrancea area are covered by the relationships. We suppose that the relations can not be applied also for mountain areas.

Figure 5. Results of PSHA for crustal seismicity. Upper two schemes display intermediate results obtained using two models of seismic source zonation (see Table 1 and text) Examples of the PSHA results for crustal seismicity are shown in Fig. 5. The influence of the shallow earthquakes for return period T=100 years can be considered as negligible (intensity less than VI MSK, PGA less than 100 cm/s2) almost for the whole territory of Romania. However, when taking the larger values of probability of being exceeded (return period 475 years), some areas may be affected by a considerable (more than VI-VII MSK, PGA up to 200-250 cm/s2) level of shaking caused by zones of Intramoesian fault, Fagaras-Campulung, Crisana-Maramures, and Banat. The results are in good agreement with those obtained recently by Moldovan et al. (2004).

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Figure 6. Probabilistic seismic hazard maps obtained after joint analysis of intermediate-depth and crustal seismicity. Gray symbols show location of particular points for which the Uniform Hazard Response Spectra are plotted in Fig. 7. Combined influence of intermediate-depth and crustal seismicity is shown at Fig. 6. The assessments were obtained without consideration of local site conditions. The site-dependent estimations may be performed, as first approximation, by introducing amplitude-dependent soil amplification factors to modify the ground motion parameters obtained after application of generalized attenuation relationships. The design ground motion parameters (intensity and PGA) for Focsani basin and Bucharest, taken as the examples, are as follows: (1) return period 100 years, Focsani – VIII MSK and 250 cm/s2 - 290 cm/s2; Bucharest – VII-VIII MSK and 150 cm/s2 - 280 cm/s2; (1) return period 475 years, Focsani – IX MSK and 450 cm/s2 - 500 cm/s2; Bucharest – VIII MSK and 300 cm/s2 - 350 cm/s2; Figure 7 shows 5%-damped Uniform Hazard Response Spectra (UHRS) calculated for particular points, location of which are given at Fig. 6. The normalized spectra were obtained as UHRS amplitudes divided by correspondent PGA-hazard values. The area of Moesian platform is characterized by relatively high amplitudes at frequencies lower than 1.0-0.5 Hz and the phenomenon comes into particular prominence with the increase of return period. We have to note, however, that for Moesian platform the UHRS amplitudes may be underestimated at vibration periods more than 1.0 sec, because we did not take into account in this work some peculiarities of site response during large earthquakes within this range of periods (Mandrescu et al., 2007).

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ACKNOWLEDGMENTS The authors would like to express their gratitude to J. Miksat who designed the colour PSHA schemes. This study was carried out in the Collaborative Research Center (CRC) 461 “Strong Earthquakes: a Challenge for Geosciences and Civil Engineering”, which is funded by the Deutsche Forschungsgemeinschaft (German Research Foundation) and supported by the state of Baden-Württemberg and the University of Karlsruhe.

Figure 7. Uniform Hazard Response Spectra and Normalized Spectra calculated for particular points (see Fig. 6). Black dots denote periods for which the spectral amplitudes were estimated.

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