Tectonic stress around the South Caspian basin deduced from earthquake focal mechanisms

Abstract

ABSTRACT In this paper, we study the tectonic stress around the South Caspian Basin (SCB), which includes the Kopeh Dagh, Alborz, Talesh, eastern Greater Caucasus mountain belts, the Apsheron sill, and the Balkhan-Ashkabad fault zone. We apply the stress inversion to focal mechanisms of 410 mainshocks that have occurred over the last 69 years. These mechanisms indicate that the surrounding fault zones of the SCB exhibit diverse types of faulting, ranging from thrust to strike-slip, normal, and their combinations. The results of the stress inversion align with the kinematics of the major fault zones bounding the SCB and emphasize the spatial heterogeneity of the stress field in this region. The region is predominantly under compression, but transpressive and strike-slip regimes are also present. This highlights the role of obliquely oriented basement faults with respect to the maximum horizontal compressive stress (S Hmax ) in accommodating deformation through convergent zones. The orientation of the S Hmax is in the NE to NNE direction in the Kopeh Dagh, Alborz, Talesh, and Ashkabad-Balkhan fault zones, being rotated to NNE-N in the Greater Caucasus. The orientation of the S Hmax relative to the convergence direction of the Arabian and Eurasian plates indicates that the Arabian-Eurasian oblique convergence-derived tectonic stress is the primary contributor to the total stress and deformation in this region.