Viscoelastic relaxation of the Makran accretionary prism following the 2013 Baluchistan, Pakistan earthquake

Geodetic observations of post-seismic transients following earthquakes commonly help to make inferences about the rheology of the lower crust and mantle, frictional properties of faults, and the kinematics of deformation across the earthquake cycle. Here, we use interferometric synthetic aperture radar (InSAR) time series observations of post-seismic deformation following the 2013 M_w 7.7 Baluchistan, Pakistan earthquake to investigate the rheology of the Makran accretionary prism. Using observations that began 15 months after the earthquake and a suite of potential fault geometries, we show that the ongoing deformation transient in the vicinity of the earthquake cannot be explained by afterslip alone. Instead, the broad spatial and temporal characteristics of the transient can be explained by viscoelastic relaxation of a shallow (>6 km) weak zone bounded by the Baluchistan earthquake and the underthrusting Arabian oceanic plate. Our first order results show that the viscoelastic lower accretionary prism has a thickness of 8-12 km and exhibits power-law (n=3.5) behavior with viscosities on the order of 10¹⁷-10¹⁸ Pa s. These estimated viscosities may be higher if afterslip occurred during the initial period of post-seismic deformation that is not constrained by our observations. The power-law rheology suggests that creep processes common at lower crustal and mantle temperatures (diffusion and dislocation creep) may be active at comparatively lower temperatures in the Makran. We argue that dislocation creep within wet quartz-rich siliciclastics is a potential source of low temperature plasticity within the prism, and that the weak nature of the Makran accretionary prism is driven by high pore fluid pressures introduced by underplated sediments and/or hydrocarbon development. Last, we show that a mechanically weak, non-elastic accretionary prism may lead to overestimates of plate coupling in purely elastic backslip models.