Microearthquake seismicity at the intersection between the Kazerun fault and the Main Recent Fault (Zagros, Iran)

Seismicity and fault plane solutions of earthquakes at the intersection between the Main Recent Fault (a right-lateral strike-slip fault that bounds the Zagros to the NE) and the Kazerun Fault system (another right-lateral zone that crosses the Zagros) show slip to be partitioned into nearly pure strike-slip at shallow depths and nearly pure thrust slip below 12 km. Such slip partitioning is commonly observed where oblique convergence occurs, but in general faults of different styles lie adjacent to one another, not at different depths with one below the other. We provide evidence for this partitioning in a microearthquake study in which we deployed a temporary network of 29 seismographs for 7 weeks. We located no activity north of the Main Zagros Reverse Fault (MZRF), which separates the Zagros fold belt from Central Iran. Most earthquakes occurred between the northern termination of the Kazerun Fault and the MZRF, but not near to known major faults. Activity is limited to the upper crust, between 2 and 16 km. Most of the focal mechanisms show strike-slip faulting, dextral if the NS striking plane is the active plane, but a few for the deepest events show reverse faulting, distributed between the Kazerun Fault and the MZRF, with P-axis trending consistently similar to NS. This partitioning of the deformation with depth suggests that the brittle upper crust deforms by slip on pre-existing faults that strike obliquely but that the lower crust accommodates the shortening by reverse faulting. We infer that the deformation in the upper part of the crust reflects a stiffer medium in which pre-existing faults localize the deformation. The largest event recorded during this experiment, located at the same place as the destructive 1977 Naghan earthquake (Mw similar to 5.9, 348 victims), shows reverse faulting, likely related to the Dopolan High Zagros Fault. The crustal thickness deduced from receiver function analysis does not show a marked difference across the Kazerun fault, which suggests a pure strike-slip motion on the fault.