Upper mantle beneath Southeast Asia from <i>S</i> velocity tomography

We present a three‐dimensional, S velocity model of the SE Asian‐western Pacific upper mantle with 400‐km lateral resolution. Using the novel Automated Multimode Inversion technique, we processed 4038 vertical‐component seismograms and extracted 22,708 linear equations with uncorrelated uncertainties that constrain upper mantle structure. We used time‐frequency windows to select signal with negligible proportion of scattered energy. The windows included the fundamental Rayleigh mode and S and multiple S waves. The observed range of S velocity variations is the widest (17–18%) in the upper 150 km of the mantle. High‐velocity continental roots can reach beyond the present extent of the overlying Archean‐Proterozoic crust by 500 km. Beneath some Precambrian units the roots are absent, which can be attributed to deformation and gradual destruction of the ancient lithosphere. At 120–150 km, S velocity beneath some cratons reaches 4.8 km/s; this can be accounted for by thermal and compositional effects. Beneath the Hainan Island area a low‐velocity anomaly is observed from near the surface to the bottom of our model; the hot spot‐type volcanism here may be caused by the deep‐mantle Hainan plume. A low‐velocity mantle domain underlies the south central Sea of Japan, surrounded on the surface by intraplate volcanoes. A deep‐seismicity gap is present near 40°N in the Pacific slab subducting below and may result from a plume‐slab interaction. A high‐velocity anomaly is present in the transition zone beneath the northern boundary of the Yangtze Craton. We propose that the anomaly corresponds to subducted continental lithosphere, stagnant atop the 660‐km discontinuity.