S-wave Velocity Structure Of The South China From Ambient Noise Tomography

The subduction of the Indian Ocean, Philippine sea and western Pacific plate beneath the Eurasian plate makes the geological tectonic of south of China(99°E-122°E,18°N-33°N) complicated. The construction of a crust-mantle velocity model of south of China has important significance to understand the tectonic evolution characteristics of the study area. Whereas, due to the great inhomogeneity in seismicity, the traditional method based on earthquake tomography is difficult to obtain the shallow crustal structure of the entire study area. In this article, ambient noise tomography which has developed in recent years has been used to invert the high-resolution S-wave velocity structure of South China.We collect vertical component continuous waveform data from January to December 2013 recorded by 244 broadband seismic stations of national networks and regional seismic networks of 14 provinces in south China such as Anhui, Chongqing,Fujian, Guangdong, Guangxi, Guizhou, Hubei, Hainan, Hunan, Jiangxi, Sichuan,Xizang, Yunnan and Zhejiang. Firstly, group velocity dispersion curves of Rayleigh wave with different periods are obtained by single data preprocessing,cross-correlation and stack, group velocity dispersion measurement and quality evaluation and so on. Secondly, the Rayleigh wave’s group velocity distribution of8-35 s are inverted by Torantola nonlinear inversion method. Finally, the S-wave velocity structures of the study area are obtained with the depth range of 0-60 km from pure path dispersion inversion.We obtain the following results and understanding:(1) in the shallow crust, due to the effect of the surface topography, Sichuan Basin and Jianghan Basin are characterized by low velocity anomaly, and South China Fold Belt characterized by high velocity anomaly. With the increase of depth, the low velocity anomaly caused by sediments within basin area gradually reduce.(2) the crustal thickness becomes nonuniformly thinner and thinner from west to east. However, that of eastern Yangtze Craton and South China Fold Belt almost unchanged.(3) There is obvious low velocity anomaly in the shallow crust of north of Sichuan Basin, which reflects that there are thicker sediments than other areas of Sichuan Basin(up to about 10 km). In the center of Sichuan Basin, there are high velocity anomaly in the depth range of 10-15 km, which may indicate the base’s uplift in the center of basin.(4) In the depthrange from surface to middle-low crust, Chuan-dian Block is generally characterized by low velocity anomaly, which may be associated with the high heat flow values in the crust of Chuan-dian Block. Whereas, obvious low velocity anomaly appears in the border areas between south of Chuan-dian Block and west of Yangtze Craton, which may be associated with the eastward crustal flow of Tibetan Plateau.