Deterministic seismic hazard analysis for Amasya and surroundings

Abstract

The North Anatolian Fault (NAF) forms the northern boundary of the Anatolian plate and is characterized by a right-lateral strike slip motion. The fault extends between 26° and 40° E longitudes and forms a broad arc roughly parallel to the coast of the Black Sea following ci former suture zone. The only visible perturbations to the smooth geometry of the NAF are, at around 34-37°E longitude, two main splay faults named Ezinepazar-Sungurlu Fault and Suluova Fault with several related minor fault segments that bifurcate from the main fault line, possibly due to the convexity of the NAF geometry. These secondary fault structures show remarkable morphological expressions accompanied with elongated basin formations and significant microseismicity. The NAF, together with these splay faults, form a broad wedge-shaped shear zone called Amasya Shear Zone (ASZ) where major growing cities of central Anatolia such as Amasya, Çorum and Tokat are located.These cities, which today have total a popidation close to 500000 with significant industrial infrastructure, were subject to destructive earthquakes during both historical (such as 1579, 1794 and 1668) and instrumental (such as 1939, 1942a, 1942b, 1943 and 1996) seismic periods. Recent pa-leoseismological studies revealed the rupture history of NAF in a time span of 2000 years (for a review see Hartleb et al. 2006). However, the historical earthquake database of the study area contains un-located destructive earthquakes (such as 1579 and 1794 events) M'hich are proposed to be nucleated on the splay faults of the ASZ. This statement, reveals that there is an unevaluated seismic hazard potential of the region.In this work, a 6 step procedure is followed for a complete earthquake hazard assessment for the study area:(1) First, the faults of ASZ are mapped in detail with field studies to understand the behavior and segmentation of the major fault systems (SuFS and EzSFS).(2) A detailed catalogue of instrumental seismic activity is prepared to understand the seismicity of the region and to define the local clusters concentrated on the fault segments.(3) Information of historical earthquakes are investigated through the literature and field study in major cities to identify the possible related fault segment, this is accomplished by comparing intensity distributions based on historical data and modeled scenarios.(4) The results of recently published analysis of a local GPS campaign (Yavaşoğlu et. al., 2009) is used to determine the annual slip rate resolved on the overall shear zone to estimate the earthquake recurrence interval.(5) For each active fault segment, the maximum magnitude of a possible earthquake is calculated with the relations between the rupture length and magnitude (Wells and Coppersmith, 1994).(6)GIS based earthquake scenarios based on attenuation relations (Tüysüz, 2003) are prepared to model the geographic intensity distribution for each calculated earthquake.Historical documents related to earthquake activity within the study area permit us to study a time-span of 500 years in detail. During this period, two major earthquake swarms occurred on the NAF. The first was the single 1668 event (M:7.9) and the other includes 1939 (Mw:7.8), 1942 (Mw:7.2) and 1943 (M:7.4) earthquakes which ruptured the overall NAF in the study area. On the other hand, faults within the ASZ ruptured in 1579, 1794 and partly in 1939 events accompanied with several moderate earthquakes. The annual slip rate resolved on NAF is 20 mm and on the overall ASZ faults is 5 mm/year (Yavaşoğlu et. al., 2009). The relation between these earthquakes and the slip rates shows that there is not enough accumulated elastic strain yet to produce a destructive earthquake both on the NAF and its splays deforming the ASZ. Despite this conclusion, there is always a possibility for a moderate to major earthquake(s) due to the chaotic behavior of crustal deformation which yet cannot be measured and modeled.

Özet

Amasya ve yaktn çevresi, kuzeyde Kuzey Anadolu Fayı (KAF) ana kolu ile sınırlanan, orta ve güneyinde ise KAF'dan ayrılarak Anadolu içerisinde doğru uzanan yan kollar olan Esençay-Suluova (Es-SuF) ve Ezinepazar-Sungurlu (EzSF) Fay sistemleri tarafından denetlenen geniş bir deformas-yon alanı içerisinde yer almaktadır. Amasya Makaslama Zonu (AMZ) olarak tanımlanan bu alanın morfotektonik evrimi, aktivitesini günümüzde de sürdüren çok sayıda tektonik hat tarafından kontrol edilmiştir. İnceleme alanında yer alan aktif tektonik hatlardan en önemlisi olan KAF'nın ana kolu, son yüzyıldaki (1939, 1942 ve 1943) deprem serisi ile tamamen kırılmıştır. Hendek çalışmaları ile fay hattının son 2000 yıl içerisindeki deprem geçmişi ortaya konularak yayınlanmış ise de tarihsel kayıtlarda bulunan ancak KAF üzerinde bu çalışmalarla bulunamamış birçok yıkıcı depremin AMZ içerisinde yeralan yan kollar üzerinde gerçekleştiği öngörülmektedir. Bu fayların aktivite derecesi ile üzerlerinde gerçekleşebilecek depremlerin çevreye etkilerinin belirlenmesi bu çalışmanın amacını oluşturmaktadır. Amasya ve çevresinin deprem tehlikesinin belirlenmesine yönelik olarak: (1) arazi çalışmaları ile aktif fayların uzanımları ve özellikleri belirlenmiştir, (2) aletsel dönem içerisinde gerçekleşen çeşitli boyutlardaki depremler derlenmiştir, (3) tarihsel deprem kayıtlarına yönelik literatür derlemesi gerçekleştirilmiştir, (4) GPS kampanyalarından elde edilen bölgede biriken yıllık atım değerlen kullanılmıştır, (5) fay segmentlerinin uzunlukları ve olası depremlerin büyüklükleri hesaplanmıştır. (6) Elde edilen tüm verilerin ışığında AMZ dahilinde yer alan herbir fay segmenti üzerinde deprem senaryoları kurgulanmış ve CBS ortamında ivme azalım ilişkileri kullanılarak bu depremlerin şiddet dağılımı hesaplanmıştır.