Depth-Varying Friction on a Ramp-Flat Fault Illuminated by ∼3-Year InSAR Observations Following the 2017 Mw 7.3 Sarpol-e Zahab Earthquake

We use interferometric synthetic aperture radar observations to investigate the fault geometry and afterslip evolution within 3 years after a mainshock. The postseismic observations favor a ramp-flat structure in which the flat angle should be lower than 10 degrees. The postseismic deformation is dominated by afterslip, while the viscoelastic response is negligible. A multisegment, stress-driven afterslip model (hereafter called the SA-2 model) with depth-varying frictional properties better explains the spatiotemporal evolution of the postseismic deformation than a two-segment, stress-driven afterslip model (hereafter called the SA-1 model). Although the SA-2 model does not improve the misfit significantly, this multisegment fault with depth-varying friction is more physically plausible given the depth-varying mechanical stratigraphy in the region. Compared to the kinematic afterslip model, the mechanical afterslip models with friction variation tend to underestimate early postseismic deformation to the west, which may indicate more complex fault friction than we expected. Both the kinematic and stress-driven models can resolve downdip afterslip, although it could be affected by data noise and model resolution. The transition depth of the sedimentary cover basement interface inferred by afterslip models is similar to 12 km in the seismogenic zone, which coincides with the regional stratigraphic profile. Because the coseismic rupture propagated along a basement-involved fault while the postseismic slip may activate the frontal structures and/or shallower detachments in the sedimentary cover, the 2017 Sarpol-e Zahab earthquake may have acted as a typical event that contributed to both thick- and thin-skinned shortening of the Zagros in both seismic and aseismic ways. Plain Language Summary The 2017 Mw 7.3 Sarpol-e Zahab earthquake is the largest instrumentally recorded event to have ruptured in the Zagros fold thrust belt. Although much work has been conducted for a better understanding of the relationship between crustal shortening and seismic and aseismic slip of the earthquakes in the Zagros, active debate remains. Here, we use interferometric synthetic aperture radar observations to study the fault geometry and afterslip evolution within 3 years after the 2017 Mw 7.3 Sarpol-e Zahab earthquake. For postseismic deformation sources, afterslip and viscoelastic relaxation are considered to be possible causes of postseismic deformation. Our results show that the kinematic afterslip model can spatiotemporally explain the postseismic deformation. However, the mechanical afterslip models tend to underestimate the earlier western part of the postseismic deformation, which may indicate a more complex spatial heterogeneity of the frictional property of the fault plane. We find that there is deep afterslip downdip of coseismic slip from both the kinematic and stress-driven afterslip models, although it could be affected by data noise and model resolution. We additionally find that the viscoelastic response is negligible. Postseismic slip on more complex geological structures may also be reactivated and triggered, combined with geodetic inversions, geological cross-section data and local structures in the Zagros.