PROBABILISTIC SEISMIC HAZARD ASSESSMENT IN THE BLACK SEA AREA

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1 PROBABILISTIC SEISMIC HAZARD ASSESSMENT IN THE BLACK SEA AREA I.A. MOLDOVAN, M. DIACONESCU, R. PARTHENIU, A.P. CONSTANTIN, E. POPESCU, D. TOMA-DANILA National Institute for Earth Physics, 12 Calugareni Street, Magurele, Ilfov, Romania Received August 18, 2016 Abstract. The paper has as final goal the probabilistic assessment of seismic hazard in the Black Sea area as input for the tsunami hazard evaluation. Maximum and most expected magnitudes and their recurrence periods have been computed for all defined seismogenic sources from the marine area, and hazard curves have been plotted. Key words: Seismogenic zones, probabilistic seismic hazard assessment, Black Sea. 1. INTRODUCTION The Black Sea region is known to be an area of active tectonics and moderate to high seismicity, that very rare triggers tsunami waves. Black Sea Basin represents a back arc basin opened in the early Cretaceous- Early Paleogene subduction of the Neotethys below the Balcanides-Pontides volcanic arc and is surrounded by a system of Alpine orogenic chains, such as: Balkanides-Pontides, Caucasus-Crimea system and North Dobrogea and Strandja- Sakarya zones [1]. Deep seismic reflection studies demonstrate the existences of two extensional sub-basins, one to the West, called Western Black Sea Basin, was opened in Early Cretaceous, and another to the East, called Eastern Black Sea Basin, which was opened in Eocene; these basins are separated by the continental uplifted block called Mid-Black Sea Ridge (or Andrusov Ridge) [1]. The largest earthquake recorded in the Black Sea is the one from March 31, 1901, M w = 7.2, and depth of 15 km, occurred near Balchik, Bulgaria and triggered a 3 4 m tsunami waves, that hit the Bulgarian and Romania coast [2, 3]. Romanian Journal of Physics 62, 809 (2017)

2 Article no. 809 I.A. Moldovan et al. 2 From the seismotectonical point of view the earthquakes which are responsible for tsunami are those associated with thrust faults (subduction zones), normal and inverse faults and less strike slip faults (only if the oblique-slip and deep slip components are predominant), with magnitude higher than 6.5 (even the USGS cited tsunami at 5.1 magnitude) and depth, a shallow one, less than 20 km depth. In order to delimit the seismic sources from Black Sea and to discrimate among them the tsunamigenic ones, the following elements have to be taken into account: depth of the earthquakes foci, that allow separation of two major categories: deeper than 40 km depth and crustal, normal, (less than 40 km deep); development of the earthquakes epicenters in the orogen zone or in zones with active tectonics (fault systems); establishment of the areas of active faults along which the earthquakes epicenters are aligned; the absence of a recent or actual tectonic activity; the epicenters recorded in these tectonically stable zones are considered as the result of a diffuse, accidental seismicity. The studies on active tectonics have clearly shown the position of the seismic sources (connected to well define active fault) which do not interfere and do not result in alternatives of other seismotectonic model constructions. According to the distribution map of earthquakes and as well as to the map of the areas with active tectonics, ten seismic sources were established [4, 5, 6]. In the present paper the maximum possible magnitude of each seismic source was obtained through three aproaches: (i) using Gutenberg Richter s [7] a and b parameters; (ii) using Cornell [8] statistical distributions and (iii) using extreme values Gumbel I [9] to model the seismogenic process for all the earthquake sources from the Black Sea region. The advantage of the statistical methods is the possibility to compute all the quantities used in probabilistic hazard assessment, including recurrence times for different magnitudes. Another important issue of the paper was to estimate the seismic hazard for the Black Sea seismogenic sources using a probabilistic approach. 2. SEISMIC ZONATION The seismic zonation of the Black Sea Area was obtained using the distribution map of earthquakes and the map of the zones with active tectonics. We took into consideration various past seismic zonation studies carried out in the framework of different projects (SHARE project MARINEGEOHAZARD project DARING project and ASTARTE RO project

3 3 Probabilistic seismic hazard assessment in the Black Sea area Article no. 809 BIGSEES project infp.infp.ro/bigsees/default.htm,). The seismic source configuration in Fig. 1 is a synthesis of all the previous approaches. The present configuration of the potential seismic sources contains fifteen crustal and one intermediate-depth seismic sources [4, 5]: Vrancea intermediatedepth (VRI), Vrancea normal (VN), Barlad Depression (BD), Intramoesian Fault (IMF), North Dobrogea (PD), North Dobrogea Black Sea (BS1), Central Dobrogea (BS2), Shabla (BS3), Istanbul (BS4), North Anatolian Fault (BS5), Georgia (BS6), Novorossjsk (BS7), Crimeea (BS8), West Black Sea (BS9) and Mid Black Sea (BS10). Only five sources are inland, the rest being marine seismic sources. Fig. 1 The seismic zonation of the Eastern part of Romania and the Black Sea Area, for earthquakes with M w > 3.5 [6]. In order to have the most reliable and homogeneous seismic dataset, the catalogues available at the European scale covering historical and modern instrumental seismicity until present days (ANSS Advanced National Seismic System-USA, NEIC National Earthquake Information Centre, World Data for Seismology Denver-USA, ISC International Seismological Centre-UK) and the catalog of the National Institute for Earth Physics (Romplus catalogue, updated) have been compiled. The parameters describing the seismic sources from Fig. 1 are given in Table 1.

4 Article no. 809 I.A. Moldovan et al. 4 Table 1 Black Sea seismic sources (SeS) parameters [6] SeS BS1 BS2 BS3 BS4 BS5 Coordinates h km M w Seismic activity rate SeS Coordinates BS BS BS BS BS h km M w Seismic activity rate SEISMIC HAZARD ASSESSMENT IN THE BLACK SEA AREAL USING STATISTICAL TOOLS In this chapter, statistical tools are applied for seismic hazard assessment. We have applied a common statistical techniques to derive the required parameters describing the rates at which each seismic source zone has generated earthquakes of different magnitudes in the past, which are then taken as the expected probabilities to generate future earthquakes for use in the assessment of hazard. The key parameters the activity rate, the b-value, and the maximum magnitude have been assessed for offshore seismogenic sources. The regional earthquake catalogues and the defined seismic source zone geometries have been used to derive magnitude-dependent catalogue completeness, to de-cluster aftershocks, to fix prior-distributions of maximum magnitudes and to evaluate statistical uncertainties. Seismic activity ν 0 is defined as the annual average number of earthquakes with magnitude higher than m 0 (M w ). The parameters of the Gutenberg-Richter [7] distribution (a, b) have been compiled for each source, and the b values have been mapped in Fig. 2 to emphasize the zones with low and high stress, for 115 years.

5 5 Probabilistic seismic hazard assessment in the Black Sea area Article no. 809 Using a and b values we have computed some statistical parameters of BSi zones: a) maximum possible magnitude in T 1 years (taken from the catalogue 67 years): = a/b (1) b) most probable magnitude in a return period of T R = 50 yr: T1 Mmp ( a log ) / b (2) T c) principal magnitude M that might appear annually (T R = 1): R Mp = (a log T 1 )/b. (3) In Table 2 are presented the statistical parameters obtained from Gutenberg Richter (GR) relation of each source from the Black Sea (BSi). Because BS1 and BS9 are very low risk seismic sources they will not be further analyzed. With the input data set from Tables 1 and 2, we have applied the algorithm of [8] and [10] to compute the seismic hazard parameters for BS1-S10 seismic sources: the number of events with a given magnitude per year, the annual hazard, the hazard for 50, 100, 475 and 1000 years, the return periods for different magnitudes. Using numerical computations we have also obtained the maximum possible magnitude for each zone (see columns 4 and 8 from Table 3). Fig. 2 b values for BSi sources.

6 Article no. 809 I.A. Moldovan et al. 6 Seismic Zone a Table 2 Statistical parameters obtained from GR relation b (comp) Mmp (Tr = 50y) Mp (1 year) (observed) BS BS BS BS BS BS BS BS Seismic Sources M min (M w ) Table 3 Statistical parameters obtained using [8] (M w ) comp (M w ) Seismic Sources M min (M w ) (M w ) comp BS BS BS BS BS BS BS BS BS BS Table 4 The return periods for M w = 6 computed with [8] Source BS2 BS3 BS4 BS5 BS6 BS7 BS8 Tr (years) > >10000 > We observe that the computed values from Table 3 are different from those from Table 2 and from the maximum observed magnitudes. The values from Table 2 are lower and the values from Table 3 are higher. Although, the differences do not exceed 1.0 degrees of magnitude. The return periods seem to be very large and far from those expected.

7 PSH 7 Probabilistic seismic hazard assessment in the Black Sea area Article no. 809 As an example in Table 4 are the return periods for M w = 6 for sources BS2-BS8. In Figs. 3 5 we have represented the dependence of the expected magnitude versus the return period (left panel) and the hazard curves for sources BS2 to BS8 (right pane l). 6 S1BS Tr (years) 1 T=475Years S1 BS T=100Years T=1000Years 0.4 T=50Years 0.2 T=1Year Fig. 3 Return periods for earthquakes with different magnitudes (left) and the hazard curves for different exposure periods (right) for seismic sources BS2.

8 PSH PSH PSH Article no. 809 I.A. Moldovan et al. 8 7 BS3 S2 1 S2 BS3 T=1000Years T=100Years 0.4 T=50Years T=475Years T=1Year Tr (years) BS4 S3 S3 BS4 T=1000Years T=475Years T=100Years T=50Years Tr (years) 0 T=1Year BS5 S4 1 S4 BS5 T=1000Years T=475Years T=100Years 0.4 T=50Years Tr (years) 0 T=1Year Fig. 4 Return periods for earthquakes with different magnitudes (left) and the hazard curves for different exposure periods (right) for seismic sources BS3-BS5.

9 PSH PSH 9 Probabilistic seismic hazard assessment in the Black Sea area Article no S5 BS6 1 S5 BS6 T=1000Years T=475Years T=100Years T=50Years Tr (years) S6 BS T=1Year S6 BS7 T=475Years T=1000Years T=100Years T=50Years T=1Year Tr (years) Fig. 5 Return periods for earthquakes with different magnitudes (left) and the hazard curves for different exposure periods (right) for seismic source BS6-BS8.

10 Article no. 809 I.A. Moldovan et al EXTREME VALUES GUMBEL I (GI) STATISTICAL METHOD USED FOR THE SEISMOGENIC PROCESS MODELING IN BLACK SEA The first one to recognize the close relationship between the weakest connection model and asymptotic theory of extreme values was Peirce one of the first authors of statistical models of parts. Gumbel's extreme value theory [9] implies the existence of three types of asymptotic distributions of extreme values (or cumulative distribution function) as the variable is unlimited, having lower and upper limits respectively. a Fig. 6 a) Most probable and the expected magnitude as function of the return period (Tr) for BS2 and BS3; b) Hazard curves for BS2 and BS3 (Shabla). b The extreme value theory applied to the occurrence of maximum magnitude earthquakes is based on the following hypotheses:

11 11 Probabilistic seismic hazard assessment in the Black Sea area Article no. 809 A. The occurrence of maximum magnitude earthquake in a seismic region in a certain period of time is a random, independent event. B. The behaviour of maximum magnitude earthquake in the future will be similar to that of previous years of observation. This method is mainly used when working with extreme values of statistical variables such as magnitude or maximum ground acceleration. For the maximum magnitude earthquake occurrence study were considered only the first and the third distribution. In Figs. 6 and 7 are represented the return periods for most probable and expected magnitudes (a) and the hazard curves (b) for different periods of time for BS2-BS8. Fig. 7

12 Article no. 809 I.A. Moldovan et al. 12 Fig. 7 (continued) a) Most probable and the expected magnitude as function of the return period Tr for BS5, BS7 and BS8; b) Hazard curves for BS5, BS7 and BS8. 5. CONCLUSIONS During this study we have obtained the probabilistic seismic hazard curves and the return periods of different magnitudes for the seismic sources from the Black Sea Basin, using two analyzing methods [8, 9]. With the first analyzing method [8] the return periods of different magnitudes seems to be large in comparison with the return periods obtained using the second statistical processing method [9]. As an example in Table 4 are the return periods for M w = 6 for sources BS2- BS8 for both analyzing methods. Comparing the results from Table 5 we can see the huge differences in the values of the return period. A possible explanation is given by the fact that the GI distribution is not limited in the superior part leading to a very fast growth of the magnitudes in time. Table 5 The return periods for M w = 6 for sources BS2-BS8 Source BS2 BS3 BS4 BS5 BS6 BS7 BS8 Tr (years) Cornell > >10000 > Tr (years) GI Another credible explanation of this differences is given by the earthquake catalogs for all this sources, catalogues that reveal the low earthquake potential of the sources and also the bad coverage with recordings systems of the Black Sea

13 13 Probabilistic seismic hazard assessment in the Black Sea area Article no. 809 basin leading to inconsistent catalogues. The solution for this issue is a joint seismic monitoring of the Black Sea basin, involving all countries around the sea [11]. The maximum expected magnitude obtained with both methods for the studied seismic sources are presented in Table 6 together with the maximum observed magnitude. Table 6 The return periods for maximum expected magnitudes in 1000 years for sources BS2-BS8 Source BS2 BS3 BS4 BS5 BS6 BS7 BS8 M (1000 years) Cornell [5] M (1000 years) GI [7] observed As we expected, the magnitude values for the GI method are very high in comparison with those obtained using the Cornell method and also with those observed. The explanation is the same as for the low values of return periods, i.e. the working hypothesis without upper limit for the magnitude. That s why the Gumbel III extreme values distribution [9] studies should be needed for the sources from Black Sea Basin. Unfortunately the existing catalogues does not permit this type of numerical statistical analysis. The final conclusion is that for a reliable statistical seismic analysis of the Black Sea areal is needed a common and uniform seismic monitoring for all states along the sea coast. Acknowledgements. This work was partially supported by the Partnership in Priority Areas Program PNII, under MEN-UEFISCDI, DARING Project no. 69/2014, and FP7 FP7-ENV Project number: /2013, ASTARTE/PNII, Capacity Module III Project 268/2014 and Nucleu Program PN and PN REFERENCES 1. I. Munteanu, L. Matenco, C. Dinu, S. Cloetingh, Kinematics of back-arc inversion of the Western Black Sea Basin, Tectonics, 30, TC5004 (2011). 2. A. Yalciner, E. Pelinovsky, T. Talipova, A. Kurkin, A. Kozelkov, A. Zaitsev, Tsunamis in the Black Sea: Comparison of the historical, instrumental, and numerical data, J. Geophys. Res. 109, C12023 (2004). 3. G.A. Papadopoulos, G. Diakogianni, A. Fokaefs, B. Ranguelov, Tsunami hazard in the Black Sea and the Azov Sea: a new tsunami catalogue, Nat. Hazards Earth Syst. Sci. 11, 945 (2011). 4. A.P. Constantin, R. Partheniu, I.A. Moldovan, Macroseismic intensity distribution of some recent Romanian earthquakes, Rom. Journ. Phys. 61 (5-6), 1120 (2016). 5. A. Bala, V. Raileanu, C. Dinu, M. Diaconescu, Crustal seismicity and active fault systems in Romania, Rom. Rep. Phys., 67 (3), 1176 (2015).

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