124Stress field and tectonic regime of Northwestern Iran from inversion of earthquake focal mechanisms

Crustal stress field inferred from focal mechanism solutions is a key parameter for understanding active tectonics and assessing the occurrence of large earthquakes. In this study, we investigate the present-day stress field in northwestern Iran and its tectonic regime. As an active tectonic region, northwestern Iran is situated within the central part of the Arabia-Eurasia collision zone, an area that has experienced destructive historical and instrumental earthquakes. We compiled 216 focal mechanism data collected from the seismological centers and published literature. Also, we determined the focal mechanisms of 48 small earthquakes with magnitudes 3.8 ≤ M w ≤ 4.8. We grouped the 264 focal mechanisms in 11 distinct zones based on their geographical proximity, kinematic homogeneity, tectonic setting, relatively similar earthquake focal mechanisms, and geodetic data. For each zone, focal mechanism solutions were analyzed using the STRESSINVERSE and Win-Tensor packages to establish the present-day stress field and characterize the active tectonics. Both methods show approximately similar results for horizontal stress axes orientations and the tectonic regimes. Stress inversion provides a first-order stress field with an NNW-SSE (N162°) maximum horizontal compressive stress (SH max ) direction corresponding to a general strike-slip tectonic regime related to the Arabia-Eurasia convergence. This stress field is responsible for the accommodation of shortening and strike-slip movement along the NW-trending and NE-trending strike-slip faults. Underthrusting rigid South Caspian Block, block rotation, escape tectonics, and strain partitioning occurred due to second and third-order stress fields. Our result provides more details of the stress field for a better understanding of the mechanisms of seismic activity and crustal deformation in northwestern Iran and the nearby regions.