Off-Fault Deformation in Regions of Complex Fault Geometries: The 2013, Mw7.7, Baluchistan Rupture (Pakistan)

Observations of recent earthquake surface ruptures show that ground deformations include a localized component occurring on faults, and an off-fault component affecting the surrounding medium. This second component is also referred to as off-fault deformation (OFD). The localized component generally occurs on complex networks of faults that connect at depth onto a unique fault plane, whereas OFD consists of distributed fracturing and diffuse deformation of the bulk volume, and occurs over scales of hundreds of meters to kilometers around the faults. High-resolution optical image correlation presents a unique potential to characterize the complexity of the surface displacements, including on-fault displacements and OFDs. In this study, we used sub-pixel correlation of 0.5-m resolution optical images to measure the surface displacement field with a M(w)7.7 Baluchistan, Pakistan, rupture. Our results document significant variability in the fault displacements, associated with large proportions of OFD in regions of fault geometrical complexity. Conversely, in regions where the fault geometry is simple, surface deformation is entirely accommodated by the primary faults with 0% OFD. When combining the localized deformation on faults with the OFD, we show that the total surface displacement budget is constant along the strike of the rupture, despite strong variations observed in the rupture geometry. Based on this analysis, we propose an idealized scenario of earthquake surface deformation as a function of the rupture geometrical variations.

Automated summary

Observations of recent earthquake surface ruptures reveal that ground deformations consist of localized fault displacements and off-fault deformations affecting the surrounding medium. High-resolution optical image correlation is used in this study to measure surface displacement fields. The results show that in regions with complex fault geometries, off-fault deformations play a significant role, while in regions with simple fault geometries, surface deformation is entirely accommodated by fault displacements. The total surface displacement remains constant along the strike of the rupture, despite variations in rupture geometry.