Surface slip and off‐fault deformation patterns in the 2013 M_W 7.7 <scp>B</scp>alochistan, <scp>P</scp>akistan earthquake: Implications for controls on the distribution of near‐surface coseismic slip

Abstract

Abstract Comparison of 398 fault offsets measured by visual analysis of WorldView high‐resolution satellite imagery with deformation maps produced by COSI‐Corr subpixel image correlation of Landsat‐8 and SPOT5 imagery reveals significant complexity and distributed deformation along the 2013 M w 7.7 Balochistan, Pakistan earthquake. Average slip along the main trace of the fault was 4.2 m, with local maximum offsets up to 11.4 m. Comparison of slip measured from offset geomorphic features, which record localized slip along the main strand of the fault, to the total displacement across the entire width of the surface deformation zone from COSI‐Corr reveals ∼45% off‐fault deformation. While 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 such possible secondary control is the type of near‐surface material through which the rupture propagated. We therefore compared the percentage off‐fault deformation to the type of material (bedrock, old alluvium, and young alluvium) at the surface and the distance of the fault to the nearest bedrock outcrop (a proxy for sediment thickness along this hybrid strike slip/reverse slip fault). We find significantly more off‐fault deformation in younger and/or thicker sediments. Accounting for and predicting such off‐fault deformation patterns has important implications for the interpretation of geologic slip rates, especially for their use in probabilistic seismic hazard assessments, the behavior of near‐surface materials during coseismic deformation, and the future development of microzonation protocols for the built environment.