Lateral variations of the upper mantle structure in Eurasia from group velocities of surface waves

Abstract

Group velocities of both Rayleigh and Love waves in the 20–70 s period range along paths across Asia and the Eastern Europe are used to obtain group velocity maps of the region. For this purpose, we developed a method for tomographic inversion on spherical surface, which is practically free of a priori assumptions. Checker-board test shows that velocity anomalies of 10°×10° size can be resolved. The ‘local’ dispersion curves are inverted to velocity-depth curves up to 160 km depth along three profiles: (1) Tibet, the South China Platform and the southern part of the Japan Sea; (2) the West Siberian Platform, the Tienshan and the Pamir; (3) the East European Platform and the Black Sea Basin. Lateral S-wave velocity variations are presented at the depths of 60, 90 and 120 km. A low velocity zone in asthenosphere under Japan Sea is extended under the South China and the North China–Korean Platforms. The upper mantle structure of the Russian (East European) and the West Siberian Platforms is different: a low-velocity zone at 120–140 km depths is observed under the West Siberian Platform, whereas the velocity under the Russian Platform increases monotonously with depth. A low velocity layer appears under the Black Sea indicating an oceanic structure of the Basin. Low surface wave velocities in Tibet are caused not only by a thick crust, but also by low S-wave velocities in the upper mantle. The Precambrian West Siberian, North Indian and North China–Korean Platforms are characterized by high velocities at the depth of 120 km unlike the younger tectonic units.