P-wave anisotropic tomography in Southeast Tibet: New insight into the lower crustal flow and seismotectonics

We determined the first 3-D P-wave anisotropy tomography beneath Southeast Tibet and adjacent regions using 63,773 P-wave arrivals from 2866 local earthquakes and 55,457 arrivals from 2802 teleseismic events. A remarkable low-velocity layer with a thickness of about 20 km is revealed in the lower crust, which may reflect a mechanically weak zone capable of flow on a geological timescale. Our seismic anisotropy results suggest that the flow direction changes when it encounters the mechanically strong Sichuan basin. Most of the large earthquakes including the 2008 Wenchuan earthquake (M 8.0) and the 2013 Lushan earthquake (M 7.0) occurred at the margin of the ductile flow in the lower crust, suggesting that the seismogenesis is controlled by the deep dynamic processes. In the upper mantle, the subducting Indian plate is imaged clearly as a high-velocity zone which has reached near the Jinsha River suture. In addition, our results show significant variations of seismic anisotropy with depth, implying that the upper crust and the lithospheric mantle deform separately beneath most parts of the study region.