An example of triggered earthquakes in western Turkey: 2000–2015 Afyon-Akşehir Graben earthquake sequences

Abstract

We investigate significant earthquake sequences in the observation period between 2000 and 2015 along the Afyon-Akşehir Graben system in western Anatolia. These sequences include a series of strong events with moment magnitudes (Mw) larger than 5.5, e.g. Eber (Mw 5.8), Sultandağı (Mw 6.5) and Çay (Mw 5.8) earthquakes. The mainshocks are shallow focus normal events at depths of 8–15km. Focal depths of aftershocks range from 2 to 30km. The focal mechanisms of the aftershocks are mainly normal faulting with variable strike-slip components. The geometry of focal mechanisms reveals a normal faulting regime with NNE-SSW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Afyon-Akşehir Graben stress field. The stress tensor inversion results display a predominant normal stress regime with a NW–SE oriented maximum horizontal compressive stress (SH). According to high-resolution hypocenter relocations of the Afyon-Akşehir Graben seismic sequences, six clusters are revealed. The aftershock activities in the observation period between 17 January 2000 and 26 October 2015 extend from east to west direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Sultandağı Fault segments. The western clusters are associated with a fault plane trending mainly E-W and dipping towards SE, while the eastern clusters are related to a fault plane trending NW-SE and dipping towards vertical. The best constrained focal depths are mainly confined in the crust (depth<30km) and run in the approximate depth range from 2 to 30km. Moreover, Coulomb stress analysis is also performed to estimate the stress loading for the activated region. We obtain more than 3bars for positive lobes. This result defines that the values are large enough to increase the Coulomb stress failure towards NE-SW direction.