Optical Offsets 7To determine the co-seismic horizontal displacement field due to the Gorkha earthquake, we use optical 8 image correlation to measure the displacement of pixels between pre-and post-earthquake satellite im-9 ages. We are able to resolve sub-pixel displacements of less than 1/15th of the Landsat8 pixel resolu-10 tion (i.e. < 1 m) using the COSI-Corr software package images, which helps to increase the signal-to-noise ratio, (4) the deformation field is resolved perpendicular to 16 the look angle (i.e. the horizontal plane for nadir images), thereby providing measurements complementary 17to InSAR (which is sensitive to vertical displacements), (5) the nadir look angle is insensitive to topographic 18 residuals produced during orthorectification of the satellite images (such residuals are produced when a lower 19 resolution digital elevation model, DEM, is used during the orthorectification process), and (5) Landsat8 20 images are freely available from the USGS as an orthorectified product -see 5 for additional details. 21Landsat8 images are typically acquired at 10am each morning. Consequently, the illumination charac-22 teristics (i.e. shadows) vary in every image acquired throughout the year according to the position of the 23 sun. Because shadows produce sharp edges in satellite images, they strongly influence the correlation. There-24 fore, to reduce the effect of differing shadows biasing the displacement field, we correlate Landsat8 images 25 acquired at a similar time of year, thereby yielding images with similar illumination characteristics (i.e. sun 26 azimuth and elevation). In addition to having similar illumination characteristics, we also require images with 27 minimal cloud cover. From the Landsat8 archive, we found two suitable images from the (pre-earthquake) 2813th May 2014 (sun azimuth: 109• , sun elevation: 68

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Coseismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near-field coseismic surface deformation or lack preevent data. Consequently, the characteristics of off-fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near-field, coseismic deformation patterns at high resolution using the COSI-Corr program by correlating pairs of aerial photographs taken before and after the 1992 M w 7.3 Landers earthquake. COSI-Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off-fault deformation. We show that 46% of the total surface displacement occurred as off-fault deformation, over a mean deformation width of 154 m. The magnitude and width of off-fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near-surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self-affine fractal with dimension of 1.56.

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.