QUATERNARY GEOMORPHIC HISTORY OF THE HOSHAB FAULT, SOUTHWESTERN PAKISTAN

The 2013 M7.7 Balochistan earthquake in southern Pakistan ruptured ~200 km of the north-dipping Hoshab reverse fault with primarily left-lateral strike slip displacements (~6:1 strike-slip to dip-slip ratio). Despite nearly pure strike-slip motion in 2013, the area surrounding the fault shows signs of past reverse slip ruptures. Two recent hypotheses attempt to reconcile this observation: that this event was consistent with past fault movements in the late Quaternary, or that the Hoshab fault exhibits bimodal slip—the ability of a single fault to slip in multiple directions at different times. To better characterize the slip behavior across multiple earthquake cycles, we map Quaternary alluvial surfaces adjacent to the 2013 rupture from high-resolution (0.3-0.5 m spatial resolution) WorldView imagery. We use morphological characterization and surface appearance as mapping criteria. The color of alluvial surfaces, including fans, pediments, and terraces, reflects the weathering and development of desert pavements while the texture expresses the relief on the surface, indicative of the degree of dissection following uplift or base level fall. From these characteristics, we develop a relative stratigraphy near the 2013 fault trace. Preliminary results suggest that, over the history of fan/pediment evolution on the foot and hanging walls, fault displacements are predominantly reverse sense in the southwest and strike-slip sense in the northeast – consistent with slip senses expected from regional plate motions. The oldest surfaces in the northeast reveal predominantly strike-slip motion where stream incision generally outpaces near-fault footwall offsets. The oldest surfaces in the southwest are generally less incised and locally buried by younger fans near the fault, suggesting relative subsidence of the footwall. Other southwestern bajadas show incision of older surfaces and strike-slip offset of stream courses. We use offset surfaces and the relative stratigraphy to evaluate the change in offset over time and space. Surfaces cut by strike-slip or reverse offset indicate the relative age and type of motion; however, reverse sense offsets change the evolution of surface topography, making across-fault age correlation difficult.