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:

The South China block, located in the east of the Eurasian plate, mainly consists of the Yangtze block and the Cathaysia block. The South China block is bounded by the eastern margin of the Qinghai-Tibet Plateau in the west, the Qinling-Dabie orogenic belt in the north, and its eastern boundary extends from the southeast coast to the north, through the Taiwan Strait, and then along the Ryukyu Island arc to the west direction. The neotectonic movement of the South China block is intense. It is not only the continental margin with the most active crustal growth and continental accretion, but also the tectonic belt with the most intense core-mantle mass transfer and the coupling zone of the inner layers of the Earth. Therefore, the crust-mantle velocity structure of the South China block and its formation and evolution have always been a hot topic in earth science research.

In this paper, we collected continuous vertical component broadband seismic data between January 1, 2010 and December 31, 2012 from the regional networks of 609 stations and used ambient noise tomography method to inverse the three-dimensional S-wave velocity structure of South China block and its adjacent area. Firstly, the seismograms are cut into daily segments and decimated at a sampling rate of 1Hz. After the removal of the mean, trend, and instrument response, a 3~150s band-pass filter is applied. In order to reduce the effect of earthquakes and instrumental irregularities on cross-correlations, we normalized the seismograms with a time-frequency normalization method. Then, we computed daily cross-correlations for each station pairs and stacked all of them by using normalized linear stacking method to obtain cross-correlation functions. Next, the phase velocity dispersion curves of Rayleigh surface wave were extracted by frequency-time analysis method. Finally, the three-dimensional S-wave velocity structure of the study area was obtained by using nonlinear Bayesian Monte Carlo inversion method.

The results show that the S-wave velocity distribution has a good correlation with surface geological and tectonic features, and could clearly reveal the lateral velocity variation in the crustal. The shallow S-wave velocity in basin and graben area presents low velocity anomaly due to the influence of sedimentary layer. The high velocity anomaly exists in the middle and lower crust of Jianghan Basin and Sichuan Basin, indicating that the middle and lower crust of these basins are cold and hard. Due to the phenomenon of arching existing in the upper mantle of Sichuan Basin, the S-wave velocity of the crust and mantle is relatively high in the upper mantle, meanwhile, the S-wave velocity in the center of the basin is higher than that in the edge. Although both the Yangtze block and Cathaysia block are located in the South China block, their upper mantle S-wave velocity structures are quite different due to their different evolutionary processes. The high S-wave velocity of the Yangtze block indicates the internal structure of the block is relatively stable, while the low S-wave velocity of the Cathaysia block indicates the strong magmatic activity during its evolution. The crust-mantle S-wave velocities in the 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 structures of the eastern and western parts of the Qinling-Dabie orogenic belt are quite different, and the crustal thickness transition zone is the boundary of the S-wave velocity structure, which is high in the east and low in the west. The crust-mantle S-wave velocity of Ordos block is relatively high, indicating that the inner structure of 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, crust and upper mantle velocity structure