ABSTRACT
The North Anatolian Fault Zone (NAFZ), which is the most active fault system of Turkey caused two large earthquakes (August 17, 1999; Izmit earthquake; Mw=7.4 and November 12, 1999; Duzce earthquake; Mw=7.2) in 1999. The earthquakes, part of an apparent earthquake mitigation from east to the west and beginning with the December 26, 1939 Erzincan earthquake, led not only to severe destructions in the epicentral area but also in a very large region, causing many deaths and loss of properties. An unbroken part of the NAFZ, in continuation of the Izmit earthquake rupture and expected to produce a larger earthquake is supposed just south of Istanbul. At the end of 2005, the Armutlu seismic network (ARNET) established in cooperation with Kocaeli University Earth and Space Sciences Research Center (ESSRC) and the GFZ German Research Centre for Geosciences to monitor micro earthquake activity of this unbroken northern branch, which poses earthquake hazard and middle branch, which has not produced large earthquakes for long time. Data of the seismic network is shared with other institutions within the scope of MARSITE project began in 2012. Data of the MARSITE project were used in this study.
Armutlu Peninsula has a very complicated tectonic structure. It is located in between two NAFZ branches with different slip velocities. The geothermal area located in Termal municipality is one of the tectonic features in this region. There, from time to time earthquakes have been observed related to geothermal activity, f.i. an earthquake swarm of approximately 5000 events in August 2014. The event sequence began with micro earthquakes in August 1, 2014 and continued with an earthquake of local magnitude 4.1, in August 3, 2014. Investigation of the causes of this earthquake swarm observed for the first time in the region is inevitable.
In this study, clustering of the earthquakes in the region, changes of daily number of earthquakes and depth distributions of the events have been analyzed. Besides, moment tensor solution of the largest earthquake occurred August 3, 2014, 22:22:24 (GMT) magnitude Mw=3.9, Ml=4.1 during this sequence has been determined. Focal mechanism of the earthquake obtained mostly vertical normal faulting with very small left lateral strike slip (strike:113o, dip:50o, rake:-55o) and CLVD content. The reason of the lateral displacement is thought to be stress accumulation on the Termal fault. Vertical normal displacement on the focal mechanism solution may be related to ongoing exploitation of the geothermal reservoir. Getting the focal mechanisms with normal faulting and shallowing depth distributions of the earthquakes will support this idea.
Authors: Evrim Yavuz, Deniz Çaka, Berna Tunç, T. Serkan Irmak, Heiko Woith, Simone Cesca, Birger Gottfried Lühr, Şerif Barış
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