Radial anisotropy of the upper mantle beneath the northeast Asia from Bayesian inversions of ambient noise data

The eastern margin of the Eurasian continental plate has suffered multiple tectonic events including interactions with the subducting Pacific and Philippine Sea plates. Therefore, heterogeneous and distributed extensional basins and intraplate volcanoes are not easily explained with a simple mechanism. Radial anisotropy (i.e., velocity differences between horizontally (VSH) and vertically (VSV) polarized shear waves), provides reliable information about tectonic processes in the lithospheric and asthenospheric upper mantle, and the associated variation of the lithosphere-asthenosphere boundary (LAB). We conducted joint inversion of Love and Rayleigh wave dispersion data using hierachical and transdimensional Bayesian inversion techniques. Using continuous seismic records of 237 broad-band seismic stations, more than 55,000 group and phase velocities of Love wave fundamental mode are extracted for periods of 5-60 s. Rayleigh wave dispersion data obtained from a previous study (Kim et al., 2016; for periods of 8-70 s). In addition we combined longer period Love and Rayleigh wave dispersion data from the global dispersion model (Ekstrom, 2011; for periods of 70-200 s). Therefore, the constructed 3-D radial anisotropy model provides details about the crustal and upper mantle anisotropic structures to the depth of 200 km. The most prominent feature of the model is the variation in the thickness and depth of a layer with strong positive radial anisotropy (VSH > VSV), which indicates horizontal upper mantle flows at the top of asthenosphere. The layer exists at depths between 70 and 150 km beneath continental regions compared to the thinner and shallower (30 60 km) structure beneath regions subjected to extension during the Cenozoic. These upper mantle variations represent the undulation of the LAB attributed to complex geodynamic processes in interactions with the preexisting continental lithosphere.