Probing Fault Frictional Properties During Afterslip Updip and Downdip of the 2017 Mw 7.3 Sarpol‐e Zahab Earthquake With Space Geodesy

Abstract

AbstractWe use interferometric synthetic aperture radar (InSAR) data collected by the Sentinel‐1 mission to study the coseismic and postseismic deformation due to the 2017 Mw 7.3 Sarpol‐e Zahab earthquake that occurred near the Iran‐Iraq border in Northwest Zagros. We find that most of the coseismic moment release is between 15‐ and 21‐km depths, well beneath the boundary between the sedimentary cover and underlying basement. Data from four satellite tracks reveal robust postseismic deformation during ~12 months after the mainshock (from November 2017 to December 2018). Kinematic inversions show that the observed postseismic InSAR line‐of‐sight (LOS) displacements are well explained by oblique (thrust + dextral) afterslip both updip and downdip of the coseismic peak slip area. The dip angle of the shallow afterslip fault plane is found to be significantly smaller than that of the coseismic rupture, corresponding to a shallowly dipping detachment located near the base of the sediments or within the basement, depending on the thickness of the sedimentary cover, which is not well constrained over the epicentral area. Aftershocks during the same time period exhibit a similar temporal evolution as the InSAR time series, with most of aftershocks being located within and around the area of maximum surface deformation. The postseismic deformation data are consistent with stress‐driven afterslip models, assuming that the afterslip evolution is governed by rate‐strengthening friction. The inferred frictional properties updip and downdip of the coseismic rupture are significantly different, which likely reflect differences in fault zone material at different depths along the Zagros.