The S-velocity structure of the East Asian mantle from inversion of shear and surface waveforms

The structure of the mantle beneath East Asia down to 800 km depth is investigated using full waveforms of seismic shear and surface waves. Epicentral distances are limited to less than 40degrees. In contrast with previous waveform inversions, we avoid ray-theoretical or path-integral approaches. Instead, we use (1) exact 3-D waveform sensitivity kernels that correctly reflect off-path sensitivity and the existence of Fresnel zones. We apply (2) an accurate 3-D forward modelling technique based on a coupled-mode, multiple-forward-scattering approach, allowing us (3) to iterate the inversion procedure through several 3-D models and (4) to evaluate the true misfit between the data and the synthetics for the 3-D model. Average lateral resolution of the model in regions with good path coverage is 400 km throughout the upper mantle. In the depth range from 100 to 250 km the lateral resolution even approaches 200 km. Since wave front healing is taken into account, the amplitudes of velocity perturbations are larger than in other tomographic models. Moreover, the waveform sensitivity kernels provide an intrinsic physical smoothing of the model, stabilizing the inversion. Finally, owing to the use of exact sensitivities, better resolution can be achieved with a given set of seismograms. Major features of the model are high-velocity subducting slabs along the West Pacific subduction zones stagnating at the 660 km discontinuity, a strong low-velocity zone between 80 and 250 km depth in the West Pacific backarc regions, a plume-like low-velocity feature beneath the southern tip of the Baikal rift zone extending into the transition zone, a low-velocity region under the Tien Shan with connection into the transition zone and thick crust under Tibet reaching its maximum thickness of approximately 80 km close to the 35th parallel. The lithospheric mantle underneath southern Tibet is very fast indicating underthrusting of the Indian lithosphere. In contrast, the upper mantle beneath northern Tibet exhibits average-to-slow values from the Moho down to the 660 km discontinuity. Very high S velocities are again observed beneath the Tarim Basin. The high-velocity mantle lithospheres under southern Tibet and the Tarim join beneath the Karakorum range at the western tip of the Tibet Plateau.