Tuncay TAYMAZ, Seda YOLSAL

Gökova Körfezi depremlerinin kaynak parametreleri ve Rodos-Dalaman bölgesinde tsunami riski

Gökova Körfezi Türkiye 'nin güneybatısında Ege Denizi kıyısında yer alan ve bölgedeki tektonik ve depremsellik açısından aktif graben sistemlerinden biridir. Son yıllarda özellikle 2004-2005 tarihleri arasında körfez içerisinde bir çok sayıda orta büyüklükte (Mw>5.0) deprem meydana gelmiştir. Bu çalışmada ters çözüm teknikleri ve telesismik uzaklıklarda kaydedilen P ve SH dalga şekilleri kullanılarak bölgede oluşan güncel depremlerin kaynak mekanizması parametreleri ve fay düzlemi üzerinde meydana getirdikleri kayma dağılımı ve yırtılma süreçleri modelleri elde edilmiştir. En küçük hatalı kaynak mekanizması çözümlerine göre depremler genel olarak D-B doğrultulu normal faylanma mekanizması ile sığ odak derinliklerinde meydana gelmektedirler. Kaynak mekanizması çözümlerinde çok küçük miktarlarda doğrultu atımlı faylanma bileşenleri bulunmaktadır. Telesismik cisim dalgalarının ve yakın alan istasyonların ters çözüm ile modellenmesi sonucu elde edilen kayma dağılımı modelleri ise depremlerin oldukça basit şekilli ve dalım yönünde ilerleyen dairesel kırılmalar ile meydana geldiklerini göstermektedir. Ayrıca, Hellenik Yayı 'nın doğu uzanımı üzerinde yer alan Rodos adası ve çevresinde tarihsel dönem içerisinde meydana gelen ve tsunami (depreşim) dalgalarına neden olduğu rapor edilen birçok sayıda deprem bulunmaktadır. 1481 yılında meydana gelen tarihsel depremler için sığ su dalgası teorisine dayalı yöntemlerle ve GEBCO-BODC batimetri verisi ile tsunami dalga simülasyonları yapılmıştır. Simülasyon sonuçları bu bölgede meydana gelen bir depremin daha çok episantıra yakın bölgelerde örneğin Rodos adası ve çevresinde ve Gökova Körfezi - Fethiye kıyılarında etkili tsunami dalgalarına neden olduğunu göstermektedir.

Source mechanism parameters of Gulf of Gökova earthquakes and tsunami risk in the Rodos-dalaman region

The Gulf of Gökova locates in soutwestern Turkey near the Aegean Sea and surrounded by Datça Peninsula to the south, the island of Kos to the west and Bodrum Peninsula to the north. Active deformation in the region has been observed from widespread seismicity, active normal faulting and tectonically generated topography. In recent and historical times, many destructive earthquakes have occurred in southwestern Turkey. Most of the seismic activity is concentrated along the northern branch of Gökova Fault Zone and it is controlled by E-W trending normal fault system. Intense earthquake activity occurred in Gulf of Gökova in August 2004 and January 2005. Source mechanism solutions and rupture histories for 10 earthquakes, of magnitude Mw > 5.0 and shallow focal depth (h < 20 km), which occurred in the region during the period 1986-2005, are used to investigate the geometry of faulting in the region. To obtain source mechanisms and slip distributions on the fault plane, we compared the shapes and amplitudes of long period P- and SH-waveforms recorded in the distance range of 30-90 degrees. The seismograms are formed by the combination of direct (P or SH) and reflected (pP and sP, or sS) phases from a point source embedded in a given velocity structure. Receiver structures are assumed to be homogeneous half-spaces. Seismograms were weighted according to the azimuthal distribution of stations. The solutions were also constrained by P-wave first motion polarities of near-field stations. All the distribution of P-wave first-motion polarities on the equal-area projection of the lower focal hemispheres are consistent with the minimum misfit solutions within a few degrees, with the strikes and dips of the nodal planes. We have also obtained earthquake rupture histories and slip distribution on the fault plane using teleseismic broad-band P waveforms. The rupture process were presented as a spatio temporal slip distribution on a fault plane which was divided into M x N subfaults with length dx and width dy. Then, slip-rate function on each sub fault was described by a series of triangle functions with a rise time.Generally, earthquakes initiates with an earthquake cluster activity and continue for a long time in the gulf. This kind of earthquake generation pattern can carry on by weeks or months decreasing in frequency and in magnitude. The kinematics of the deformation is controlled by normal faults with small strike slip components trending E-W, NE-SW and NW-SE directions. Earthquake source mechanism solutions indicate that normal fault mechanism with a strike-slip component have been observed on the E-W oriented graben and normal fault systems in the Gulf of Gökova and they confirm that extension is in a north - south direction that is in a good agreement with the geology and tectonic structure of the region. T-axes directions obtained from source mechanism solutions demonstrate the NW-SE direction of extension as a result of the convergence between the African plate and the Eurasian plate and the westward movement of the Anatolian block. On the other hand, all earthquakes have generally short source duration and uniform rupture propagation along the dip direction and their focal depths are generally less than 15 lan and thus we may suggest that seis-mogenic thickness in this region is about -10 km. All these slip distribution results show a uniform circular rupture propagation along the dip direction with a short source duration for earthquakes in Gulf of Gökova .On the other hand, the Gulf of Gökova has a real and major tsunami hazard to the lives and population since it is near the Eastern Mediterranean coast. In this study we have also investigated the tsunami wave propagations to obtain time histories of water surface fluctuations and water particle velocities created by historical 1481 Rhodes earthquakes (M-7.0-7.5) in the Eastern Mediterranean sea using TUNAMI-N2 and AVI-NAMI mathematical models based on the method of Okada (1985). The related parameters for the earthquake are adapted by an analogy of current plate boundaries and earthquake source mechanisms obtained by inversion of teleseismic P- and SH- waveforms. The understanding the faulting geometry, tectonic evolutions and source rupture processes along the active fault zones have significant importance on the tsunami generation. The major and well-known seismic generated tsunamis have occurred in Aegean and Mediterranean Seas and these waves affected the coastal regions since historical times.

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