SURFACE SLIP AND OFF-FAULT DEFORMATION PATTERNS IN THE 2013 MW 7.7 BALOCHISTAN, PAKISTAN EARTHQUAKE

Comparison of fault offsets measured by visual analysis of WorldView high-resolution satellite imagery with deformation maps produced by COSI-Corr analysis of Landsat-8 and SPOT5 imagery reveals significant complexity and distributed deformation along the 2013 M_w 7.7 Balochistan, Pakistan earthquake. We record an average slip of 3.7 m, with local maximum offsets up to 11.4 m and as low as 0 m along the main trace of the fault. Comparison of our geologic measurements, which represent the component of localized slip along the main strand of the fault, to the total displacement across the entire zone of surface deformation obtained from COSI-Corr sub-pixel image correlation reveals ~50 % off-fault deformation. Whereas previous studies have shown that the structural maturity of the fault exerts a primary control on the total percentage of off-fault surface deformation, large along-strike variations in the percentage of strain localization observed in the 2013 rupture imply the influence of important secondary controls. One possible such secondary control is the type of near-surface material through which the rupture propagated. We therefore compare the percentage off-fault deformation to the type of material (bedrock, old alluvium, young alluvium) at the surface and the distance of the fault to the nearest bedrock outcrop (as a proxy for sediment thickness) at many locations along the fault. We find that the age and inferred relative thickness of different sediment deposits can significantly affect local percentages of off-fault deformation, with more off-fault deformation in younger and/or thicker sediments. Accounting for and predicting such off-fault deformation patterns has important implications for the proper interpretation of geologic slip rates as well as the development of next-generation seismic hazard assessment analyses and microzonation protocols for the built environment.