Tsunami Hazard Assessment for Izmir Bay, Turkey
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TSUNAMI HAZARD ASSESSMENT FOR IZMIR BAY, TURKEY Gözde Güney DOĞAN1, Nazan YILMAZ KILIÇ2, Ahmet Cevdet YALÇINER3, Mehmet Semih YÜCEMEN3 ABSTRACT Aegean Sea experienced numerous strong earthquakes in history. Since the region is highly prone to catastrophic events, such as earthquakes or earthquake/landslide-induced tsunamis because of the nearby active fault zones and their recent activities, the awareness about tsunamis that might take place around the western coasts of Turkey should be raised. Therefore, a complete tsunami hazard analysis with high resolution data is necessary for better understanding of the effects of tsunamis on Izmir Bay region. In the first stage of this study, different possible seismic tsunami sources in northern Aegean Sea which may affect Izmir Bay are determined from the analysis of historical data of earthquakes/tsunamis together with the instrumental data of seismicity. Then, a series of high resolution simulations of different sources are performed using tsunami numerical code, NAMI DANCE, to understand the generation and propagation of possible tsunamis in Aegean Sea and assess their behavior and coastal amplifications in Izmir Bay. The potential effects of probable tsunamis such as the arrival time, maximum positive amplitudes near shoreline and flow depth in inundation zone are computed, presented and discussed. As mentioned above, besides the deterministic evaluation, the probabilistic tsunami hazard analysis (PTHA) is implemented to estimate the likelihood and severity of earthquake induced tsunami hazard for the Izmir Bay. The methodology applied in the study is similar to the well-established probabilistic seismic hazard analysis (PSHA). The output of the PTHA will be the annual probability of exceeding different levels of tsunami amplitudes at the most critical coastal locations of the Izmir Bay. Keywords: Tsunami; Izmir Bay; Probabilistic tsunami hazard analysis; Deterministic tsunami hazard analysis 1. INTRODUCTION The coasts of Aegean Sea have experienced numerous tsunamis in history due to the frequent earthquakes of varying magnitudes (Tselentis et al. 1988). Most of the eastern Mediterranean tsunamis occurred in Aegean Sea and along the surrounding coasts. According to the historical records, these events have affected the nearby coastal settlements (Ambraseys 1960, 1962; Antonopoulos 1978; Papadopoulos and Chalkis 1984; Papazachos et al. 1985; Papadopoulos 1993; Altinok and Ersoy 2000). There were two recent events happened in 2017 in Aegean Sea. One is the Lesvos Earthquake th with Mw=6.2 magnitude occurred on 12 of June, 2017 at a location between Lesvos Island (Greece) and Karaburun Peninsula (Izmir, Turkey). The other one is the Bodrum-Kos Earthquake with Mw=6.5 magnitude occurred on 20th of July, 2017 at a location between Bodrum Town (Turkey) and Kos Island (Greece). One of the important source areas of tsunamis in the Aegean region which is accelerating its seismic activity with the current events is the central Aegean Sea including Izmir Bay because the region is located between the North Anatolian fault zone in the north, and the Hellenic Arc in the south (Altinok et al. 2005). The region is tectonically active and the seismicity is high as declared by the Disaster and Emergency Management Presidency of Turkey (AFAD). Since numerous catastrophic tsunami events have occurred in the Aegean Sea basin and may continue to occur in the future, various studies were conducted on tsunamis in the Aegean Sea by Yalciner et al. (1994, 2002); 1PhD Student, Project Assistant, Middle East Technical University, Ankara, Turkey, [email protected] 2Dr., Earthquake Department, Disaster and Emergency Management Presidency, Ankara, Turkey, [email protected] 3Faculty Member, Prof. Dr., Civil Engineering Department, Middle East Technical University, Ankara, Turkey, [email protected], [email protected] Kuran and Yalciner (1993); Altinok et al. (2011); Perissoratis and Papadopoulos (1999); Howes (2002); Mitsoudis (2012) and Sorensen et al. (2012) and Onat and Yalciner (2013). Therefore, a complete tsunami hazard analysis with high resolution data is necessary for better understanding of the effects of tsunamis in Izmir Bay. Deterministic tsunami hazard assessment which includes the evaluation of numerically simulated nearshore tsunami parameters corresponding to ‘maximum-credible’ scenarios from different subduction zones is one of the procedures which have been commonly used to evaluate the tsunami hazard and risk distribution in coastal regions. It has traditionally been studied with little emphasis on the probability of the scenario events by Tinti and Armigliato (2003); Hébert et al. (2005); Paulatto et al. (2007); Lorito et al. (2008) and Shaw et al. (2008). Although such scenarios are very useful for understanding the effects of tsunamis on coastal regions and for response and evacuation planning, it is necessary to know the probability of occurrence of an event for risk mitigation and defining design specifications. Probabilistic tsunami hazard assessment (PTHA) has therefore gained attention remarkably in the recent years (Geist and Parsons 2006; Power et al. 2007; Thio et al. 2007). The probabilistic approach to the tsunami hazard assessment problem provides an analysis of the relative contributions of large and small events to the hazard. Furthermore, the identified critical scenarios based on the probabilistic hazard estimates can be studied in more detailed, higher resolution deterministic studies where local effects can be explained better. The scope of this study is the assessment of tsunami hazard in Izmir Bay, based on both deterministic and probabilistic procedures. In the first stage of the study, different possible seismic tsunami sources in northern and central Aegean Sea which may affect Izmir Bay are determined from the analysis of historical data of earthquakes/tsunamis together with the instrumental data of seismicity. Then, a series of high resolution simulations of different sources are performed using tsunami numerical code, NAMI DANCE, to understand the generation and propagation of possible tsunamis in Aegean Sea and assess their behavior and coastal amplifications in Izmir Bay. High resolution bathymetry and topography data for Izmir Bay is processed for input to tsunami numerical modeling. NAMI DANCE solves nonlinear form of shallow water equations with friction term in GPU environments. It computes maximum and minimum water surface elevations, velocities in horizontal plane, momentum fluxes and their directions, flow depths in the study domain. Besides the deterministic evaluation, the probabilistic tsunami hazard analysis (PTHA) is implemented to estimate the likelihood and severity of earthquake induced tsunami hazard for Izmir Bay. The methodology applied in the study is quite similar to the well-established probabilistic seismic hazard analysis (PSHA). The outputs of the PTHA are the annual probabilities of exceeding different levels of tsunami amplitudes at the most critical coastal locations of Izmir Bay. 2. DETERMINISTIC TSUNAMI HAZARD ANALYSIS 2.1 Bathymetric and Topographical Maps The bathymetric data for tsunami numerical modeling are obtained from General Bathymetric Chart of the Oceans (GEBCO 2016) with a spatial resolution of 30 arc seconds. The dataset acquired from available navigational charts is added to improve the bathymetric data in the shallow zone. Furthermore, the topographical and bathymetric data for the inner part of Izmir Bay (including the region from Mavisehir to Inciralti districts) are purchased from Izmir Metropolitan Municipality. Based on the selected study domains for numerical modeling, topographical and bathymetric data are eliminated and processed. A series of simulations are performed in this study within the nested domains B (large) and C(small). Domain B is bounded by the coordinates of 22.50o-27.30o E; 37.30o- 41.00o N with a grid size of 160m. Domain C is bounded by 26.4731o-27.1833o E; 38.2954o-38.6602o N with a grid size of 20m. The general view of the nested study domains is shown in Figure 1. 2 Figure 1. Nested study domains B and C used in the simulations (Scales are in meters) 2.2 Tsunami Source Selection and Related Rupture Parameters The distribution of earthquake epicenters occurred in Aegean Sea since 1900 are analyzed based on available and reliable earthquake catalogues. The epicenters are clustered as ellipses (Figure 2) representing the probable tsunami source areas related to seismic activities. Considering the historical documents and distribution of the main undersea fault zones as well as other probable tsunamigenic sea bottom deformations (Yalciner et al. 2001; Yalciner et al. 2008; Onat and Yalciner, 2013), 13 tsunami source areas are selected as responsible for the tsunamis that may affect Izmir Bay. The selected rupture parameters of each segment for the elliptical faults are given in Table 1. Figure 2. Distribution of earthquake epicenters since 1900 and main fault zones in the region Table 1. Estimated rupture parameters for each source segment. Name of Epicenter Dip Rake Strike Depth Length Width Vertical the Source (°) (°) (°) (km) (km) (km) Displacement (m) Z04 23.78E 40.83N 45 45 140 10 90 12 6 Z10-1 26.40E 40.40N 60 45 250 10 70 20 10 Z10-2 25.90E 40.25N 60 45 230 10 80 15 10 Z10-3 25.15E 39.75N 60 45 230 10 85 15 10 Z10-4 24.15E 39.60N 60 45 220 10 60 15 10 Z14-r 25.05E 38.50N 45 45 80 15 60 15 6 Z15-1 26.30E 38.85N 45 45 105 15 52 15 6 Z15-2 25.35E 38.90N 45 45 80 15 60 15 6 Z17-1 23.54E 39.42N 45 45 120 10 90 30 6 3 Name of Epicenter Dip Rake Strike Depth Length Width Vertical the Source (°) (°) (°) (km) (km) (km) Displacement (m) Z17-2 25.00E 39.40N 45 45 80 10 100 15 6 Z17-3 26.92E 38.81N 45 45 330 10 100 15 6 Z17-4 25.85E 39.53N 45 45 70 15 80 15 6 Z17-5 25.00E 39.00N 45 45 45 10 80 15 6 It is important to note that the results obtained in this study are conservative in comparison with the well-accepted tsunami source characteristics.