Ramp-Flat and Splay Faulting Illuminated by Frictional Afterslip Following the 2017 Mw 7.3 Sarpol-e Zahab Earthquake

As the largest instrumentally recorded earthquake in the fold-and-thrust belt of the northwestern Zagros mountain so far, the fault structure of the 2017 Mw 7.3 Sarpol-e Zahab earthquake and its contribution to regional crustal shortening remain controversial. Here, we utilize the integration of Interferometric Synthetic Aperture Radar observations and 2D finite element models incorporating various fault geometries such as planar faults, ramp-flat faults, and the combined models of ramp-flat and splay faults to explore frictional afterslip process due to coseismic stress changes following the mainshock. Our findings suggest that a ramp-flat frictional afterslip model, characterized by the maximum afterslip of ∼1.0 m and frictional variations (Δμ) of ∼0.001 and ∼0.0002 for the up-dip and down-dip portions, respectively, better explains the long-wavelength postseismic deformation than planar fault models. However, an integration model of a ramp-flat and a splay fault further improves the model fit, although the splay fault’s frictional slip is limited to <0.2 m, which is much smaller than that on the ramp-flat part (∼0.9 m). Considering the relocated aftershocks and structural cross-sections, the combined model could be best attributed to fault slip on the blind Mountain Front fault. Our findings thus suggest the complexity of the fault interactions between the basement and sedimentary cover in the Zagros, and that this largest basement-involved event in the region contributes to both thick- and thin-skinned shortening via seismic and aseismic behaviors, respectively.