Velocity Structure of the Northeastern End of the Bayan Har Block, China, and the Seismogenic Environment of the Jiuzhaigou and Songpan-Pingwu Earthquakes: Inferences from Double-Difference Tomography

ABSTRACT The 2017 Mw 6.5 Jiuzhaigou mainshock hit the northeastern end of the Bayan Har block, which has experienced many historical earthquakes, including the 1976 M 7.2 Songpan-Pingwu earthquake swarm. We used the double-difference tomography method to perform a joint inversion of the seismic source and P-wave velocity (VP) structure of the Jiuzhaigou-Songpan-Pingwu region. The results show significant lateral heterogeneity in the VP in the mid-upper crust. The velocity structure in the shallow crust correlates well with the surface geology. The Jiuzhaigou mainshock and Songpan-Pingwu earthquake swarm both occurred at the boundary between high- and low-VP anomalies. The Songpan-Pingwu earthquake swarm may be related to the eastward flow of low-viscosity material in the mid-lower crust of the Tibetan plateau. Low-viscosity material intrudes into the bedrock when it encounters the rigid Motianling massif, resulting in surface uplift and thrust earthquakes. By contrast, the Jiuzhaigou earthquake is associated with strain energy accumulating at the boundary between high- and low-VP anomalies related to the different movement rates of the low-VP material in the mid-lower crust and the high-VP body in the mid-upper crust. In this case, the high-VP body ruptures with a strike-slip sense to the southeast.