Transient aseismic slip and crustal shortening following 2017 Iran-Iraq (Sarpol-e Zahab) Mw 7.3 Earthquake Inferred from 3 years of InSAR Observations

The 2017 Mw 7.3 Sarpol-e Zahab earthquake is the largest instrumentally recorded event to have ruptured in the Zagros Fold-thrust belt. In this study, we perform multi-temporal interferometry analysis using Sentinel-1 SAR data to investigate changes in postseismic ground deformation at the Earth’s surface and interpret this change in terms of various models including kinematic afterslip, stress-driven afterslip and viscoelastic response. We show that the kinematic afterslip model can explain the postseismic deformation spatiotemporally, while the stress-driven afterslip model tends to underestimate the earlier deformation in the western part of the postseismic deformation field. The viscoelastic response, however, is negligible with the best-fitting viscosity which is on the order of 10¹⁹ Pa s. By an integrated analysis of geodetic inversion results, geological cross-section data, regional stratigraphic column and local structures, we infer that the spatial heterogeneity of frictional property of fault plane and/or more complex geological structures may explain the underfitting between the earlier postseismic deformation and the corresponding stress-driven afterslip models. Because the coseismic rupture propagated along a basement-involved fault while the postseismic slip was more likely activated the frontal structures and/or shallower detachments in the sedimentary cover, the 2017 Sarpol-e Zahab earthquake may be evidence of a typical event which contributes both of the thick- and thin-skin shortening of Zagros in both seismic and aseismic way.