Anisotropic tomography and seismotectonics of the longmenshan fault zone in east tibet

Summary We invert 122 147 P, S, and PmP phase arrival-times from 1549 local earthquakes for both isotropic and azimuthally anisotropic lithospheric P-wave velocity structure beneath the region of the Longmenshan Fault zone, China. The use of PmP data significantly improves the spatial resolution of the middle-lower crust tomography. Our results show that widespread low-Vp anomalies exist in the middle and lower crust of the Songpan-Ganzi block and the Chuandian block, which contribute most crustal anisotropy. Moderate and strong earthquakes mainly occurred in the high-Vp and low-Vp transition zone, and obvious low-Vp anomalies appear below the seismogenic zone, indicating that the occurrence of earthquakes is affected by crustal fluids. The upper-crust anisotropy is mainly controlled by the stress field and local faults. The fast Vp directions (FVDs) on the Longmenshan fault zone are NE-SW in the lower crust and uppermost mantle, suggesting that the material flow is blocked by the Sichuan basin, so the flow moves in the NE-SW direction. The FVDs in the Longmenshan fault zone are different from SKS splitting measurements, suggesting that the crust and lithospheric mantle are decoupled there. Our anisotropy results also suggest that the thickening deformation of the upper crust and the middle-lower crustal flow jointly control the uplift and deformation of the Longmenshan mountain.