Coseismic fault-propagation folding during the 2015 Mw 5.7 Dajal earthquake on the Sulaiman Fold and Thrust belt from Sentinel 1A

The continental collision at the western boundary of the Indian continent formed the tectonically complex transpressional zones of the Sulaiman Fold Thrust (SFT) and Kirthar Fold Thrust (KFT) belts. Seismic hazard around the SFT is considered elevated, but shortening across its eastern boundary is poorly understood because of the scarcity of moderate-sized earthquakes in the last decades. Here, we use Sentinel-1A interferometry to analyze the coseismic crustal deformation and source fault geometry associated with the 2015 moment magnitude (Mw) 5.7 Dajal earthquake that occurred on the boundary thrust in the SFT belt. The line-of-sight displacements amount to ~45 and ~50 mm for the ascending and descending interferograms, respectively, due to thrust-dominated slip over a blind fault. The inverted InSAR measurements fit well with USGS and ISC model parameters, except for the depth of the fault that is found to be shallower. A simplified fault model shows a dip of 41°, strike of 194° and rake of 79°, with a large aspect ratio of the seismogenic fault, due to a narrow ruptured fault width compared to the ruptured length. The InSAR inverted fault source parameters are used to determine the seismic moment (Mo = 3.94 × 1017 Nm), with the shear modulus (μ = 32 GPa), corresponding to a moment magnitude Mw = 5.7 that is consistent with seismological estimates. We also perform the sensitivity analysis to understand the geometry of a possible ramp-décollement or décollement-ramp-décollement fault system using finite fault inversion. Finite fault modeling indicates the presence of a shallow décollement branching into a shallow ramp and décollement at approximately 7 km depth. We also incorporate the flexural slip over axial surface during coseismic deformation. The Dajal earthquake seems to have propagated along the base of the ramp and associated axial surface, that stops at the tip of the ramp. A few aftershocks occurred in the surrounding anticline. We suggest that folding of the shallow sediments by flexural slip at the eastern boundary of the SFT belt may have restricted the rupture propagation.