Rayleigh Wave Group Velocity Distributions for East Asia from Ambient Seismic Noise Tomography

We have collected continuous vertical-component broadband data from 1109 seismic stations in regional networks across China, Korea, and Japan for the year 2011 to perform the largest surface wave tomography study in the region. Using this data set, we have measured over half a million Rayleigh wave group velocity dispersion curves from 1-year stacks of station-pair ambient seismic noise cross-correlations. Quality control is performed by measuring the coherency of the positive and negative lag time sides of the cross-correlations. If the coherency is below an empirically determined threshold, the dispersion curve is measured on the side of the highest SNR. Otherwise, the positive and negative sides of the cross-correlation are averaged before dispersion curve measurement. Group velocity measurements for which the SNR was less than 10 are discarded. The Rayleigh wave group velocity dispersion curves are regionalized on a tessellated spherical shell grid in the period range 10 to 50 s to produce maps of Rayleigh wave group velocity distributions. Preliminary maps at 10 seconds period match well with geologic features at the surface. In particular, we observe low group velocities in the Songliao, Bohai Bay, Sichuan, Ordos, Tarim, and Junggar Basins in China, and the Ulleung and Yamato Basins in the East Sea (Sea of Japan). Higher group velocities are observed in regions with less sediment cover. At periods around 30 s, we observe group velocity decreases going from east to west in China, representing an overall trend of crustal thickening due to the collision between the Indian and Eurasian plates. The Ordos and Sichuan blocks show higher group velocities relative to the eastern margin of the Tibetan Plateau, possibly reflecting low temperatures in these cratons.