The 2013 Mw 7.7 Balochistan earthquake: Seismic potential of an accretionary wedge

Romain Jolivet, Zacharie Duputel, Bryan Riel, Mark Simons, Luis Rivera, Sarah E. Minson, Haijiang Zhang, M. Aivazis, Francois Ayoub, Sebastien Leprince, Sergey S. Samsonov, M. Motagh, & Eric J. Fielding

Published 2014, SCEC Contribution #1899

Great earthquakes rarely occur within active accretionary prisms, despite the intense long-term deformation associated with the formation of these geologic structures. This paucity of earthquakes is often attributed to partitioning of deformation across multiple structures as well as aseismic deformation within and at the base of the prism (Davis et al., 1983). We use tele-seismic data and satellite optical and radar imaging of the September 24th 2013
Mw 7.7 earthquake that occurred on the southeastern edge of the Makran plate boundary zone to infer the distribution of fault slip of study this unexpected earthquake. We first compute a multiple point source solution from the W-phase waveforms to estimate fault geometry and rupture duration and timing. We then derive the distribution of subsurface fault slip from geodetic coseismic offsets. We sample for the slip posterior probability density function using a fully Bayesian approach, including a full description of the data covariance and accounting for errors in the elastic structure of the crust. The rupture nucleated on a sub-vertical segment, branching out of the Chaman Fault system, and grew into a major earthquake along a 50° north-dipping thrust fault with significant along-strike curvature. Fault slip propagated at an average speed of 3.0 km/s for about 180 km and is concentrated in the top 10 km with no displacement on the underlying décollement. This earthquake does not exhibit significant slip deficit near the surface, nor is there significant segmentation of the rupture. We propose that complex interaction between the subduction accommodating the Arabia-Eurasia convergence to the south and the Ornach Nal Fault plate boundary between India and Eurasia resulted in the significant strain gradient observed prior to this earthquake. Convergence in this region is accommodated both along the subduction megathrust and as internal deformation of the accretionary wedge.

Citation
Jolivet, R., Duputel, Z., Riel, B., Simons, M., Rivera, L., Minson, S. E., Zhang, H., Aivazis, M., Ayoub, F., Leprince, S., Samsonov, S. S., Motagh, M., & Fielding, E. J. (2014). The 2013 Mw 7.7 Balochistan earthquake: Seismic potential of an accretionary wedge. Bulletin of the Seismological Society of America, 104, 1020-1030. doi: 10.1785/0120130313.