Abstract

The Zagros Fold and Thrust Belt (ZFTB) in southern Iran is a seismically active tectonic zone, where SW-vergent thrust faults and NW–SE- and NE–SW-oriented strike-slip fault systems accommodate crustal shortening, resulting from the active Arabia–Eurasia collision. The majority of earthquakes in Iran occur within the ZFTB, posing a major hazard for society. The 9 April 2013 Kaki Earthquake (Mw 6.3) in the southern part of the ZFTB took place along a fault that was previously unknown regarding its surface expression, geometry and kinematics. We have used surface–subsurface distributions and focal mechanism solutions of the Kaki Earthquake aftershocks to characterize the fault system responsible for the quake. Our results indicate that it was a NE-vergent thrust fault with a minor dextral component that slipped c. 7–17 km at depth, causing the Kaki Earthquake. There were no surface ruptures, although some surface fissures developed in fluvial terraces during the main shock. We interpret this fault as a blind backthrust, which probably represents a reactivated Mesozoic basement fault emanating from the Zagros detachment surface. An upper shallow décollement zone within the Miocene Gachsaran Salt facilitated its upward propagation on the back-limb of an overturned syncline.