Source characteristics and b-values of the Tuz Gölü Fault Zone in Central Anatolia, Turkey

Abstract

We investigate significant earthquake sequences in the observation period between 2000 and 2018 along the Tuz Gölü Fault Zone in Central Anatolia. These sequences include a series of strong events with moment magnitudes (Mw) >5.0, e.g. 30 July 2005 Bala-Ankara Mw 5.2, 20 December 2007 Bala-Ankara Mw 5.7 and 26 December 2007 Mw 5.6 Afşar-Ankara earthquakes. The mainshocks are shallow focus strike-slip events with minor normal component at depths of 12–15 km. Focal depths of aftershocks range from 2 to 30 km. 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 E-W and also NE-SW trending directions 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 Tuz Gölü Fault Zone stress field. The stress tensor inversion results display a predominant normal stress regime with a NNE–SSW oriented maximum horizontal compressive stress (SH). According to high-resolution hypocenter relocations of the Tuz Gölü Fault Zone seismic sequences, eight clusters are revealed. The aftershock activities in the observation period between 5 March 2000 and 2 June 2018 extend from NW to SE direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Tuz Gölü Fault Zone segments. The western clusters are associated with a fault plane trending mainly N-S and dipping towards almost vertical, while the eastern clusters are related to a fault plane trending NW and dipping towards SE direction. The best constrained focal depths are mainly confined in the crust (depth < 30 km) and range from 2 to 30 km. In this study, we compare the results of an analysis of b-values from seismicity and stress tensors of focal mechanism solutions to understand their coupling in terms of faulting and earthquake hazard implications. In particular, this comparison allows to investigate the spatiotemporal correlation between b-values and stress distributions and it is therefore able to locate fault segments that have a high potential of generating large earthquakes. According to recent seismicity, b-values range from 0.5 to 1.5 along the Tuz Gölü Fault Zone in Central Anatolia. The maximum horizontal compressive stress (SH) indicates N-S and NE-SW directions while the minimum horizontal compressive stress (Sh) displays E-W. These results represent that low stress variances are correlated with high b-values. Low b-values are predominant in areas with high stress rates, consistent with a high heterogeneity of focal mechanisms.