TRIGGERED SHALLOW FAULT SLIP BY THE 2017 MW 7.3 IRAN-IRAQ EARTHQUAKE

Earthquakes can cause significant hazards particularly along active plate margins. Following a main shock, aftershocks with much smaller magnitudes generally occur within the same fault system. Sometimes an earthquake can also trigger other fault systems at distance to slip seismically or aseismically. This process is commonly explained by a change of dynamic or static Coulomb stress on the receiver faults from the main shock that decreases fault stability and promotes fault slip. In this study, we use interferometric synthetic aperture radar (InSAR) to map shallow fault slip that were likely triggered by the 2017 Mw 7.3 Iran-Iraq earthquake. Through mapping, we found 230 shallow triggered fault slips. There are two dominant fault orientations, the first (135 faults) oriented at 310˚ with a mean length of 4 km. A second orientation (48 faults) was found oriented at roughly 060˚ with a mean length of ~2 km. The expression of this second orientation is located due south from the epicenter ranging from 57.5-95.5 km in distance from the epicenter. Both static and dynamic Coulomb stress changes for this event were calculated using a coseismic slip distribution provided by the USGS NEIC finite fault solutions. The static Coulomb stress change model suggests that the receiver faults in this area were strengthened against slip, which is unlikely the cause of the triggered shallow slip. To estimate the dynamic Coulomb stress change, we first calculate ground motion during the main shock within 150 km radius from the epicenter using the same finite fault model with a 1D layered Earth structure same as in the USGS finite fault solution. We then calculate peak dynamic Coulomb stress using the same receiver fault geometry as for the static stress change case. The results show that in the region south of the earthquake, despite static Coulomb stress change strengthening the receiver faults, dynamic stress change in this area was estimated to be up to 1 MPa due to a southward directivity of the fault slip process during the main shock. The spatial extent and the amount of stress change could be the cause of these shallow triggered slip. This work demonstrates the importance of taking both static and dynamic stress change into account for earthquake events with unilateral slip history can better evaluate triggered slip in neighboring fault systems.