Surface Wave Velocity And Azimuthal Anisotropy In South China

The data used in this study are continuous seismic waveforms recorded at broadband seismic stations include the 387 stations of China National and Regional Seismic Network in 2009 and 2010.Cross-correlations are computed in daily segment to retrieve the reliably empirical Rayleigh wave Green's function.Rayleigh wave phase velocity dispersion curves are measured using a phase-matched filter and frequency time analysis technique.The Eikonal tomography is used to obtain the azimuthal anisotropy of the different period(8-35s)in South China and seismic ambient noise tomography is used to image the different period phase velocity maps(8-35s)of South China.The results of phase velocity distribution indicated that the phase velocity of the whole lithosphere in South China is relative uniformity,which also reflect South China as a whole that is characterized of the relative stability.Our results agree with that the South China since late Mesozoic has not happened intensely tectonic activity.Although velocity structure of the whole lithosphere in South China is relatively uniform,distinct velocity differences appear in the west of Yangtze Block,Sichuan Basin,the east of Yangtze Block and Cathaysia Block,namely,the 8-10 s period phase velocity of Cathaysia Block is higher than the west of Yangtze Block,Sichuan-Yunnan regions and Sichuan Basin,The Sichuan Basin is the lowest velocity because of the relatively thick sedimentary layer.The velocity of the Yangtze Block,the Cathaysia Block and the Sichuan Basin that is higher than Sichuan-Yunnan regions are basically the same in the 35s period,which is consistent with the crustal thickness in South China that is obviously lower than Sichuan-Yunnan regions.The results of azimuthal anisotropy distribution show that the fast wave polarization in the middle and upper crust of the Yangtze Block(excluding Sichuan Basin)from west to east trends NW,EW and NE,which is basically consistent with the collision of the Cathaysian Block along NW towards the Yangtze Block during Neoproterozoic era,the collision forming the EW fold orogenic belt of the Yangtze Block and the North China Block along Qinling Mountains-Dabie orogenic belt during early Mesozoic era and the subduction of paleo Pacific plate along NE to South China Block in low angle way during late Mesozoic era.In the lower crust or upper mantle(30s period),the fast wave direction in South China mainly trends NW,with a small amount of NW and NE;There is no obvious difference in fast wave direction between the Yangtze Block and Cathaysia Block and its anisotropic amplitude is basically the same,which mainly reflect the collision of the Cathaysia Block along NW towards the Yangtze Block during Neoproterozoic era and reflect two tectonic action of South China that result in deformation of the Yangtze Block and Cathaysia Block in the middle and upper crust during the late Mesozoic and early Mesozoic era,However,the lower crust or upper mantle(30s period)in the whole South China is less affected by two tectonic action in this time.The fast wave direction in Sichuan-Yunnan regions from north to south shows a change of NW,NS and NW,which reflects the process of the Qinghai-Tibet Block materials flowing east,and Sichuan-Yunnan regions is strongly squeezed by this process,the preferred orientation of the fast wave polarization in Sichuan-Yunnan regions is approximately the same as GPS horizontal field in the area,which indicates that the direction of plate movement in the region is approximately the same or parallel.The anisotropic characteristics of different periods in Sichuan Basin are totally different from those in the Sichuan-Yunnan regions,the Yangtze Block and the Cathaysia Block,It can't be explain four changes of geological period experienced by South China and it can't also explain mechanism that Qinghai-Tibet Block squeeze Sichuan-Yunnan regions or South China.