Absence of shallow slip deficit during the Balochistan earthquake (2013, Mw 7.7, Pakistan): insights from SAR and optical-based coseismic slip model

The 2013 Mw 7.7 Balochistan earthquake occurred in the complex tectonic setting of the triple junction between Arabia, Eurasia and India tectonic plates. It ruptured a 200-km-long curved section of the Hoshab fault. Coseismic motion was dominated by left-lateral strike slip together with some reverse motion.

By combining optical satellite data from Landsat 8 and SPOT 5 and radar satellite data from TerraSAR X and RADARSAT 2, we derive the full 3D coseismic displacement field and estimate the slip distribution at the surface. Following a statistical analysis of the coseismic surface trace, the fault is discretized into 15 segments. Then, to determine the dip and down-dip width for these 15 segments we perform a non-linear elastic inversion of the geodetic dataset following Okada's formulation. Using output of this first model, we then prescribe the fault geometry and linearly invert for slip at depth with enhanced discretization.

Results show a dip varying from 54 to 89° to the NW. Steeper dips are inferred at each termination, whereas the fault flattens in its middle part. Strike slip component peaks at 12.7 m near the epicenter, decays slowly southward, before terminating abruptly, forming a continuous slip patch confined between the surface and 8 km depth. The dip slip component is distributed on several patches that seem to get more shallow near the southern termination where the amplitude peaks at 5.7 m.

Our joint model uses near-fault and far-field data and shows no shallow slip deficit. On the contrary, we infer for nearly all fault segments an increase of slip toward the surface. In line with recent studies (Vallage et al. 2015, Xu et al. 2016, Marchandon et al. 2018), we argue that no shallow slip deficit occurred during this earthquake.