Co-seismic Surface Dilatation and Shear Strain Fields Derived from SPOT-5 Displacements

Repeated earthquake cycles produce topography, fault damage zones, and other geologic structures along faults. These geomorphic and structural features indicate the presence of co-seismic permanent (inelastic) surface deformation, yet a long-standing question in earthquake research is how much of the co-seismic deformation field is elastic versus inelastic. These questions arise in part because it is unclear what measurable co-seismic characteristics, such as off-fault or distributed surface deformation and cracking, represent true unrecoverable deformation. One emerging descriptor of permanent co-seismic deformation is surface strain magnitudes inferred from imaging geodesy observations. In this study, we present the surface strain field of the 2013 Mw7.7 Baluchistan strike-slip earthquake in southern Pakistan. We invert co-seismic displacement fields generated from pixel-tracking of SPOT-5 and WorldView optical imagery for co-seismic surface horizontal strain tensors. We observe that co-seismic strain field is dominated by negative dilatation strains, indicating that the co-seismic fault zone contracted during the earthquake. We show that co-seismic inelastic failure exhibits a relatively consistent width along the rupture that is localized to a zone 100-200 m wide on the hanging wall side. The width of co-seismic permanent deformation does not correlate with variations in off-fault deformation or surface geology. Based on comparisons to other recent earthquakes, we posit that the permanent surface strains reflect inelastic deformation of the faults inner damage zone, and that the width of this zone reflects fault maturity.