Ramp-flat basement structures of the Zagros Mountains inferred from co-seismic slip and afterslip of the 2017 Mw7.3 Darbandikhan, Iran/Iraq earthquake

Abstract

The November 2017 Mw7.3 Darbandikhan earthquake in the northern Zagros Mountains of Iran/Iraq was the largest instrumentally recorded earthquake to date in this mountain belt. Despite extensive research in this region, the nature of active faults in the Arabian basement beneath the Zagros Mountains is largely unresolved. Additionally, there is active debate regarding the interaction of thin- and thick-skin shortening of the Zagros, and whether these processes are accommodated seismogenically. Here, we use interferometric synthetic aperture radar (InSAR) of co- and post-seismic surface displacements to document the source of the Darbandikhan earthquake and the first 120 days of afterslip following the earthquake. We apply a fault inversion approach that finds permissible geometries, locations, and slip depths as a means to image subsurface structure with remote sensing geodesy. We find that the Darbandikhan earthquake ruptured the shallowly dipping (14°–19°) Mountain Front Fault within the Arabian crystalline basement (∼12–22 km deep). We additionally find that afterslip propagated up-dip of the earthquake onto the nearly horizontal (dip = 1°–5°) basal decollement of the Zagros Mountains, indicating that the earthquake and its initial afterslip activated a ramp-flat structure. We also inverted co-seismic InSAR displacements of the January 11, 2018 Mandali earthquake sequence that occurred ∼80 km south of the Darbandikhan earthquake and find that these moderate magnitude events (Mw5.0–5.5) ruptured the lower Phanerozoic cover of the Zagros. Together, the Darbandikhan and Mandali sequences demonstrate that the Zagros Mountains actively shorten via both thin- and thick-skin styles, and that these modes of shortening are accommodated in part by earthquakes. Moreover, the Darbandikhan earthquake and afterslip highlight the presence of seismically active low-angle thrust faults beneath the Zagros that were not previously recognized, highlighting sources for potential future large earthquakes. We compare these results to the 2015 Gorkha, Nepal earthquake to argue that frictional characteristics of decollement govern spatial relationships between seismic and aseismic slip in ramp-flat geometries.