C05 Evaluation of Earthquake Hazard Parameters for the Different Regions in the Western Anatolia for Whole Time Periods

Transcription

1 C05 Evaluation of Earthquake Hazard Parameters for the Different Regions in the Western Anatolia for Whole Time Periods Y. Bayrak* (Karadeniz Technical University) & E. Bayrak (Karadeniz Technical University) SUMMARY The earthquake hazard parameters are estimated for different regions of the Western Anatolian (WA). The whole examined area is divided into 15 seismic regions based on their seismotectonic regime. The database used in this work was taken from different sources and catalogues such as TURKNET, International Seismological Centre (ISC), Incorporated Research Institutions for Seismology (IRIS) and The Scientific and Technological Research Council of Turkey (TUBITAK) for historical and instrumental periods. The technique of maximum likelihood method is applied. The procedure allows the use of either historical or instrumental data or even a combination of them. By using this method the earthquake hazard parameters, which are the maximum magnitude, Mmax, mean seismic activity rate, λ, and the b-value which is the slope of the frequency-magnitude Gutenberg-Richter relationship, are estimated. Also, probabilities (Pr) for the occurrence of earthquakes of a specified size in a each region are evaluated. According to earthquake hazards parameters computed for each region Aegean Sea, Aegean Arc and Gölhisar-Çameli, Acıgöl and Tatarlı Kumdanlı Faults and Dinar Graben have highest hazard potential.

2 Introduction The Western Anatolian (WA) is one of the most seismically active and rapidly prolongating areas of the Eastern Mediterranean region (e.g., Bozkurt 2001). Aegean Subduction Zone at the south where the African Plate subducting beneath the Anatolian Plate and N-S trending extensional zone are main tectonic features of this area. Active normal faults related to grabens and horsts and strike-slip faults are dominant in WA. The assessment of earthquake hazard parameters is important in seismically active region. Many quantitative techniques have been applied over the years to present the geographical distribution of earthquake hazard in several regions of the Earth and Turkey (e.g. Tsapanos, 2001; Bayrak et al., 2005). The best quantities considered as measures of seismicity are the maximum observed magnitude M max, the annual number N(M) or the mean return period T m of earthquakes. The knowledge of the return period is of great importance in studying and analyzing earthquake hazard and/or seismicity. Also, the estimation of earthquakes hazard involves the computation of long-term probabilities (P r ) for the occurrence of earthquakes of a specified size in a given area during a given time interval. Turkey is located in a very active seismic region, so the earthquake hazard in Turkey has therefore been widely studied using a number of different techniques and seismic quantities (e.g. Kayabalı and Akın, 2003; Bayrak et al., 2008). The purpose of the present study was to assess the level of seismic hazard for 15 regions in WA in terms of mean return period for an earthquake occurrence, the most probable maximum magnitude in a given time interval and the probability of a large earthquake. Method and Data The database used in this work was taken from different sources and catalogues such as TURKNET, International Seismological Centre (ISC), Incorporated Research Institutions for Seismology (IRIS) and The Scientific and Technological Research Council of Turkey (TUBITAK) and provided in different magnitude scales. The catalogues contain the origin time, different magnitudes scales (m b - body wave magnitude, M S -surface wave magnitude, M L -local magnitude, M D -duration magnitude, and M W -moment magnitude), epicenter and depth information of earthquakes. Turkey earthquake catalogue, obtained from the Boğaziçi University, Kandilli Observatory and Earthquake Research Institute (KOERI), starts from 1974 until The earthquakes from 1900 to 1974 come from the International Seismological Centre (ISC) and instrumental catalogue of KOERI. The historical earthquake catalogue used in this study is taken from database of GSHAP (Global Seismic Hazard Assessment Program) being compiled by Erdik et al. (1999). Bayrak et al. (2009) developed some relationships between different magnitude scales in order to prepare a homogenous earthquake catalogue from different data sets. We prepared a homogenous earthquake data catalogue for M S magnitude using these relationships. A complete understanding of the historical and instrumental seismicity, tectonics, geology, paleoseismology, and other neotectonic properties of the considered region are necessary for an ideal delineation of seismic source zones. In this study, we divided Western Abatolian (WA) into fifteen seismic zones for detailed study as seen in Figure 1. Also, these regions are listed in Table 1. The epicentral distributions of the historical and instrumental earthquakes are shown in Figure 1 on different seismic source zones in WA. A large number of earthquake occurrence models are currently available for seismic hazard estimation. Seismic hazard assessment involves the computation of long-term probabilities for the occurrence of earthquakes of a specified size in a given area during a given time interval. The parametric-historic procedure developed by Kijko (1988), Kijko and Sellevol (1989) and Kijko and Graham (1999) was applied in the present study to quantify the level of seismic hazard in and around Turkey. From a computational point of view, the parametric-historic method involves the areaspecific and site-specific parts. Three parameters, namely the maximum magnitude, M max, mean seismic activity rate, λ, and the b-value of the Gutenberg-Richter magnitude-frequency relation, are calculated for an area surrounding the site for which earthquake hazard analysis is needed. Using

3 these earthquake hazard parameters, the mean return periods, the maximum expected magnitude and the probability of earthquake occurrence for a given magnitude during a time span of 50 years are computed. Regional variations of the maximum expected magnitudes (Mmax) and the probability of the occurrence for the earthquakes with magnitude M 6.5 in the next 50 years (Pr) are shown in Figures 2. Figure 1. Earthquake epicenters, tectonics and different 15 seismic regions in the Western Anatolia. Conclusions The combuted b-values change between 0.71 and 1.15 for the different 15 regions in WA. The values smaller than 0.80 are computed in regions of 10, 11 and 12 which are related the Aegean Arc. The highest values are found in the regions 2 and 6 covering Akhisar and Kütahya Fault zones. The observed values in the graben sytems such as Gediz and Büyük Menderes are greater than The maximum expected magnitudes (Mmax) are calculated between 5.38 and The highest values are related to region 12 covering Marmaris, Köyceğiz and Fethiye Faults and Aegean Arc. The lowest value is observed in Kütahya Fault Zone. 6th Congress of Balkan Geophysical Society - Budapest, Hungary

4 The probability of the occurrence for the earthquakes with magnitude M 6.5 in the next 50 years (P r ) are highest in the regions related Aegean Sea, Aegean Arc and Gölhisar-Çameli, Acıgöl and Tatarlı Kumdanlı Faults and Dinar Graben. The probabilities are smaller than 0.60 for the regions of 4 and 9 including Gediz Graben, Dumlupınar Fault, Dozkırı, Çardak and Sandıklı Faults. Table 1. Earthquake hazard parameters computed from Kijko method for the different seismic regions in the Western Anatolia. Region Tectonics obs M max bˆ bˆ ˆ ˆ ˆM max ˆM max 1 Aliağa Fault Bergama-Zeytindağı Fault Zone Eskişehir, İnönü Dodurga Fault zones 4 Gediz Graben Simav, Gediz-Dumlupınar Faults Kütahya Fault Zone Karova-Milas, Muğla-Yatağan Faults 8 Büyük Menderes, Küçük Menderes Grabens 9 Dozkırı-Çardak, Sandıklı Faults Aegean Islands Aegean Arc Fethiye Fault Zone Burdur Fault Zone Sultandağı Fault Beyşehirgölü, Kaş Faults Figure 2. a) The maximum expected magnitudes (M max ) and b) the probability of the occurrence for the earthquakes with magnitude M 6.5 in the next 50 years (P r ) for different 15 seismic regions in the Western Anatolia.

5 Acknowledgements The authors are grateful to Karadeniz Technical University (Turkey) for partially supporting this work (project number: ). References Bayrak, Y. Yılmaztürk, A. and Öztürk, S. [2005] Relationships between fundamental seismic hazard parameters for the different source regions in Turkey, Natural Hazards, 36, Bayrak, Y. Öztürk, S. Koravos, G.Ch. Leventakis, G.A. and Tsapanos, T.M. [2008] Seismicity assessment for the different regions in and around Turkey based on instrumental data: Gumbel first asymptotic distribution and Gutenberg-Richter cumulative frequency law, Natural Hazards and Earth System Sciences, 8, Bayrak, Y. Öztürk, S. Çınar, H. Kalafat, D. Tsapanos, T.M. Koravos, G.Ch. and Leventakis, G.A. [2009] Estimating earthquake hazard parameters from instrumental data for different regions in and around Turkey, Engineering Geology, 105, Bozkurt, E. [2001] Neotectonics of Turkey- a Synthesis. Geodinamica Acta, 14, Erdik, M. Alpay, BY. Onur, T. Sesetyan, K. and Birgoren, G. [1999] Assessment of earthquake hazard in Turkey and neighboring regions. Annali di Geofisica, 42, Kayabalı, K. and Akin, M. [2003] Seismic hazard map of Turkey using the deterministic approach. Engineering Geology, 69, Kijko, A. [1988] Maximum likelihood estimation of Gutenberg-Richter b parameter for uncertain magnitudes values. Pure and Applied Geophysics, 127, Kijko, A. and Sellevoll, M.A. [1989] Estimation of earthquake hazard parameters from incomplete data files. Part I. Utilization of extreme and complete catalogs with different threshold magnitudes. Bulletin of the Seismological Society of America, 79, Kijko, A. and Graham, G. [1998] Parametric-historic procedure for probabilistic seismic hazard analysis, Part I: Estimation of maximum regional magnitude M max. Pure and Applied Geophysics, 152, Tsapanos, T.M. [2001] Evaluation of seismic hazard parameters for selected regions of the world: the maximum regional magnitude, Annali di Geofisica, 44,