Source Mechanisms and Faulting Analysis of the Aftershocks in the Lake Ercek Area (Eastern Anatolia, Turkey) During the 2011 Van Event (Mw 7.1): Implications for the Regional Stress Field and Ongoing Deformation Processes

Abstract

In this study, we analysed the source mechanisms and faulting pattern of the aftershocks in the Lake Ercek area, Eastern Anatolia, during the 2011 Van event (Mw 7.1). The fault plane solutions of the aftershocks were used to derive a stress tensor acting around Lake Ercek. The estimated seismological parameters (focal mechanism solutions, stress tensor, and fault focal analyses) were integrated with field surveys and high resolution seismic reflection data so as to better understand the main faulting patterns and deformational features which are the causatives of the observed geomorphological features within and around Lake Ercek. The focal parameters of the 56 aftershocks with magnitude range 3.5 <= Mw <= 5.5 were obtained using the regional broadband seismic data from the network of Kandilli Observatory and Earthquake Research Institute (KOERI) through the Centroid Moment Tensor (CMT) inversion of locally observed broadband-waves. The implications for the lateral variation of the stress field in the lake area were based on information derived from integration of seismicity and stress tensor inversion results. The major outcomes of our analyses were as follows: (a) The maximum principal stress axis is close to horizontal and aligned in NNW-SSE direction, the minimum compressional stress axis is close to vertical and aligned in an ENE-WSW direction, the intermediate stress axis is close to horizontal and is parallel to the strike of the ruptured fault plane, while the stress ratio is (R = 0.6); (b) the lake basin is mostly deformed by dip-slip faults (dips >= 45-60 degrees) in and along its margins where the various folds, basement highs with eroded surfaces and several low-angle normal faults are evident; (c) the majority of the aftershocks have reverse-thrust mechanism with some strike-slip components in the Lake Ercek area (N- and S-margins). Strike-slip and normal faulting events are also evident (E- and W-margins, respectively); (d) the normal and reverse mechanisms with some strike-slip component are mostly confined in the upper crust (0-15 kin), compatible with NS-striking normal and WE-striking reverse faults in the marginal sections of the lake; (e) the WE-striking reverse faulting in the N- and S-margins seems to have the seismic potential to generate destructive earthquakes in the lake area; (f) the tectonic implications of the regional stress field and ongoing deformation processes in the lake are defined and supported by field observations and seismic reflection data. The result of these analyses reveals N -S compression/W-E extension in the lake and also the collisional/compressional regime taking place in a region of Lakes Van and Ercek.