Permanent Co-seismic Deformation of the 2013 Mw7.7 Baluchistan, Pakistan Earthquake From High-resolution Surface Strain Tensors

Earthquakes produce topography, fault damage zones, co-seismic surface cracks and other geologic structures along faults. These structures 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. One emerging descriptor of permanent co-seismic deformation is off-fault deformation, which is characterized by off-fault offsets, surface cracking, or other observations of distributed surface deformation. It remains unclear though if off-fault deformation results from permanent yielding, if it is indeed elastic deformation related to shallow variations in fault geometry and slip magnitude, or if it is induced by dynamic shaking at the surface. An alternative way to quantify the magnitude and spatial extent of co-seismic permanent surface deformation is to derive co-seismic surface strain fields. In this study, we use high spatial resolution (~10m) displacement fields generated from sub-pixel cross-correlation of WorldView optical imagery to derive co-seismic surface 2D strain fields for the 2013 M _w 7.7 Baluchistan strike-slip earthquake in southern Pakistan. We undertake two meshing schemes on the displacement fields: Delaunay triangles and gridded rectangles, each of which applies a mesh-by-mesh weighted least square inversion for strain components and yields different spatial resolutions. We apply a combination of Gaussian and local mean filters to the derived co-seismic strain fields to mitigate the noise introduced in the inversion process. Our preliminary results show a narrow zone (50-100 m) of inelastic deformation where strain magnitudes exceed 0.5%. This inelastic deformation zone systematically spans a narrow er region than the off-fault deformation zone of this earthquake. These results suggest that there is not a clear connection between the derived permanent deformation zones and the surface deformation features that define the co-seismic off-fault deformation zone of this earthquake . The apparent mismatch implies that other co-seismic processes, such as ground shaking or deep fault slip, may have contributed to the formation of off-fault deformation features .