关键词: 背景噪声, 相速度, 频散曲线, S波速度, 壳-幔速度结构

Abstract: We use continuous vertical component broadband seismic data between January 1, 2010 and December 31, 2012 from the regional networks of 609 stations to extract the empirical Green's function between each station pair by waveform cross-correlation and stack calculation. The phase velocity dispersion curves of Rayleigh surface wave were extracted by Frequency Time Analysis method, and the three-dimensional S-wave velocity structure of South China block and its adjacent area was obtained by using nonlinear Bayesian Monte Carlo inversion method. The results show that the shallow S wave velocity distribution maps generally demonstrate good correlations with surface geological and tectonic features，and they also clearly revealed the lateral velocity variation in the crust．The results show that the S wave in the shallow basin and graben area area shows low velocity anomaly due to the influence of sedimentary layers, and there are high velocity anomaly in the middle and lower crust of Jianghan and Sichuan Basin, which indicates that the middle and lower crust of the basin are cold and hard. Due to the upper mantle arching phenomenon, the S wave velocity of the upper mantle in the Sichuan Basin is relatively high, and the S wave velocity of the central region of the Sichuan Basin is higher than that of the marginal region. Both of the Yangtze block and the Cathaysian block belong to the South China block, but their upper mantle S wave velocity structure of are quite different, which indicates that the evolution process of the two blocks is different. The S wave velocity of the Yangtze block is relatively high, which indicates that the internal structure of the block is relatively stable and the S wave of the Cathaysian block shows obvious low velocity anomaly, which indicates that the magmatic activity was more intense during the evolution of the block.The crust-mantle S-wave velocities located west of the southwest boundary of the South China block show low velocity anomalies, which may indicate the existence of asthenosphere in the middle and lower crust of the eastern margin of the Qinghai-Tibet Plateau. The S wave velocity structure of the eastern and western parts of the Qinling-Dabie orogenic belt is quite different, and the distribution features are high in the east and low in the west with the crust thickness transition zone as the boundary. The crust-mantle velocity structure of the Ordos block is relatively high, which indicates that the internal structure of the Ordos block is relatively stable. However, the S-wave low velocity anomaly in the upper mantle at the southwest corner of the Ordos Basin may indicate that the heat flow of the upper mantle of the North China Craton has begun to "invade" the Ordos lithosphere.

Key words: Ambient noise, Phase velocity, Dispersion curve, S wave velocity, Crustal and uppermost mantle structure