S wave tomography of the crust and uppermost mantle in China

We use 280,000 local and regional arrival times from 13,105 local earthquakes recorded by 220 seismic stations to determine a three-dimensional S wave velocity structure of the crust and uppermost mantle under China and surrounding regions. We use a traveltime tomography code that accommodates a heterogeneous crust and varying Moho depth. The grid for tomographic inversion is 1 degrees x 1 degrees in the horizontal direction and 10 km in depth. Our results show that large velocity variations of more than 6% exist in the crust and upper mantle in the China region. The velocity image of the upper crust correlates with surface geological features. Crustal heterogeneity is clearly observed, and velocity changes are visible across some of the large fault zones. In Tibet, low velocities exist in the thick crust, and high Sn velocities exist in the southern and eastern parts. Low-velocity zones beneath volcanic sites and the rifts are clearly observed in our tomographic results. Under the Tengchong, Hainan, and Changbai volcanic areas, strong low velocity zones are visible down to the upper mantle, indicating the existence of magma chambers beneath the volcanoes.