Abstract
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 few decades. Here, we use
Sentinel-1A ascending and descending interferometry to analyze the
coseismic crustal deformation associated with the 2015 moment magnitude
(Mw) 5.7 Dajal earthquake that occurred on the boundary thrust in the
SFT belt. The surface displacement was caused by slip on a blind thrust
and coseismic folding in the hanging wall. We use kinematic inversions
to determine the distribution of slip on the frontal ramp and flexural
slip along active axial surfaces for two end-member models of fault
geometry. We first consider a double fault-bend fold system involving
two sub-horizontal décollements separated by a ramp. Second, we consider
a fault-propagation fold system where the frontal ramp terminates below
a thick sediment layer. The fault-bend fold model includes slip on the
décollement-ramp-décollement and flexural slip on two active axial
surfaces initiated at the fault bends. For the fault-propagation fold,
the model includes slip on the décollement-ramp system and flexural slip
on the lower axial surface. In a preliminary step, the geometry of the
ramp is optimized using a Monte Carlo method using a single asperity
model. The geometry of the décollement and active axial surfaces is
inferred based on balanced cross-sections, whereby the fold axes bisect
the sediment layers across a fault bend. In both end-member models, a
shallow décollement branches into a shallow ramp at approximately 7 km
depth. However, the undeformed sediment of the overlying floodplain
indicates that a ramp is the recent geomorphic feature. We conclude that
the Dajal earthquake propagated along the base of the ramp, representing
coseismic ramp failure over fault-propagation folding.