Source parameters of the 2005-2008 Balâ-Sirapinar (central Turkey) earthquakes: Implications for the internal deformation of the Anatolian plate

Active tectonics of central Anatolia is mainly governed by the collision of the African, Arabian and Anatolian plates, which causes westward escape of Anatolia along the North and East Anatolian Fault zones, and the counterclockwise rotation of the Kirşehir block with insignificant internal deformation. The formation of the present-day tectonic processes in this region can be deduced from geophysical prospecting and seismological data. Although the seismicity in central Anatolia is distinctively lower than that in the northern and eastern parts of the Anatolian plate, small and moderate earthquakes (2.5≤M_w≤6.0) mostly occurred in the region in the past decades or so. For example, intense earthquake activity was observed in the Balâ-Afşar-Sirapinar (Ankara, central Anatolia) region in the period of 2005 to 2008 with destructive earthquakes of July 30, 2005 (M_w=5.2); December 20, 2007 (M_w=5.7) and December 26, 2007 (M_w=5.6). Therefore, these earthquakes are crucial to analyze the shallow crustal deformation in the central Anatolian block. In the present study, we obtained source parameters of 2005-2008 earthquake sequence using the regional moment tensor (RMT) inversion method. We analyzed complete broad-band waveforms recorded at near-field distances (0.45°≤δ≤3.6°). Our results reveal NW-SE directed right-lateral strike-slip faulting and NE-SW directed left-lateral strike-slip faulting mechanisms, which are clearly correlated with the conjugate fault systems in the Balâ-Afşar-Sirapinar region. However, some earthquakes also have E-W directed normal faulting components. We suggest that the major characteristics of 2005-2006 and 2007-2008 earthquake activity could have been dominantly associated with left-lateral and right-lateral strike-slip faulting mechanisms, respectively. The seismogenic depth is found to be about 8-10km. This result implies that earthquakes in the study region occurred mostly in the upper crust, which accommodates the strain by brittle deformation. Furthermore, our results are consistent with neotectonic features and available geophysical data (e.g., gravity, aeromagnetic and paleomagnetic) reported by previous studies. It may be envisioned that the joint interpretation of earthquake source parameters with those observations will shed light into the complex deformation processes for future studies.