STRAIN PATTERNS IN THE MAKRAN ACCRETIONARY PRISM, PAKISTAN FROM SURFACE FAULTING

Strain Patterns in the Makran Accretionary Prism, Pakistan from Surface Faulting

The September 24, 2013 M_w7.7 Baluchistan earthquake in the Makran accretionary prism of southern Pakistan ruptured ~200 km of the Hoshab reverse fault. This earthquake was notable because slip was primarily of a strike-slip sense (6:1 or greater strike-slip to dip-slip ratio) despite occurring on a reverse fault and at a high angle to the plate convergence direction. Consequently, this earthquake left a dip-slip deficit of several meters. Internal deformation of the Hoshab fault hanging wall is one potential means to accommodate this slip deficit. Here, a broad network of conjugate NW and NE striking strike slip faults offset topography and are apparent in satellite optical imagery. To address this possibility, we undertake a regional strain analysis using Kostrov-style fault plane summations to characterize both the magnitude and orientations of principal strains reflected in the Hoshab fault hanging wall. We first map all strike-slip faults in the hanging wall of the Hoshab fault, their fault lengths, and peak surface offsets using high resolution (0.3 – 0.5m) panchromatic optical imagery (2059 total faults). Next we calculate infinitesimal strain tensors (ε_ij) after making several assumptions: that the faults accommodate pure strike-slip motion and that they dip vertically. The strain tensors are derived over a grid system, and we adjust the gridding size to account for potential biases introduced by this approach. The maximum strain orientation in the southern region of the Hoshab fault hanging wall is predominantly sub-perpendicular to the plate convergence direction. The NE portion of the Hoshab fault hanging wall shows a maximum strain orientation ~35° different than in the Southern end of the Hoshab fault hanging wall. These strain orientations suggest post-deformation rotation of the hanging wall. We believe the distinct change in maximum strain orientations following the Hoshab fault up to the northeast is a result of the dextral continental transform margin between the Eurasian and Indian plates.