Rayleigh-wave Phase-velocity Maps beneath Eastern China

Eastern China is a geologically complex region with strong lateral changes in Moho depth. It is also a tectonically active region with active faults and protocratonic units. We investigated the variations of isotropic and anisotropic Rayleigh-wave phase velocity beneath eastern China using broadband records at 38 stations with roughly even distribution from the China National Seismic Network. Rayleigh-wave dispersion curves are manually measured by the two-station technique for a total of 741 inter-station paths from the vertical-component waveforms. We complemented this dataset with 599 automated inter-station measurements. When selecting the data, we imposed an upper bound of 10° for the angle between the great circle connecting a pair of stations and the great circle connecting the stations and the event. The inter-station distances are in the range 250-2500 km, enabling phase-velocity measurements over a broad period range, 8-200 s. We extracted 59306 records from 438 events with epicentral distances between 10° and 170°. These dispersion curves are then inverted using the LSQR algorithm for the high-resolution isotropic and azimuthally anisotropic phase-velocity maps at selected periods between 16 and 200 s. The isotropic as well as anisotropic models of Rayleigh-wave phase velocities we obtain are consistent with the tectonic features observed in this region. Furthermore, the anisotropic anomalies we observe are compatible with previous SKS splitting measurements. Interestingly, we observe different azimuthal anisotropy patterns in several distinct period ranges, suggesting both lateral and depth variations of azimuthal anisotropy in this region. At crustal depths, the isotropic structure exhibits a clear contrast between the Yangtze Craton in the southeast, which appears faster than regional average by up to 5%, and the northwest region, which is slower than average by about 3-4%. The Jiangnan Belt separates regions with different velocity expressions, indicating a possible suture between the Yangtze Craton and Cathaysia Unit. Geodynamical interpretations for azimuthal anisotropy patterns are proposed in specific regions, in particular an abrupt change in Moho depth in the eastern border of the Tibetan Plateau, and a lithospheric thinning beneath the Cathaysia Unit. The anisotropic structures are in agreement with absolute plate motion and the overall upper-mantle flow. However, more local variations are related to the tectonics of the region, such as the border between the Ordos and Sichuan Basins.