Inversion Of Rayleigh Wave Velocity And Anisotropy In Eastern China From Ambient Noise

The tectonic structure of eastern China is strongly affected by the collision of the Eurasian plate with the Indian plate and the Pacific plate.The area mainly includes the Northeast Block,North China Craton and South China Block which consists of Yangtze Craton and Cathaysia Block.Previous studies on anisotropy showed that the Yangtze Craton,the North China Craton,and the eastern Tibetan Plateau collided with each other.However,due to the few stations used in the previous study and the limitation of the method,the result resolution was low and the deformation of the interior and boundary of the craton was not detailed enough,so the further research is needed.Using the ambient noise data recorded by dense broadband stations in the study area,we obtaine the velocity and azimuthal anisotropy of Rayleigh wave in eastern China.This study is based on the continuous ambient noise data of 899 stations in the eastern part of China Seismological Network Center from September 2009 to August2011.The data processing process can be summarized as the following four steps:(1)single data preprocessing;(2)noise cross-correlation;(3)phase weighted stacking;(4)dispersion curve measurement.Finally,the ambient noise azimuth anisotropy inversion technology is used to obtain the velocity and azimuthal anisotropy of Rayleigh wave of 5 ? 125 s in eastern China,with analyzing the residuals of the calculation results,and selecting the appropriate spatial correlation length and error parameters.For short periods such as 10 s,30 s,and 40 s,the fast propagation directions near the Tan-Lu fault is mainly parallel to the fault zone.The fast propagation directions in the Ordos Basin,the western Qinling-Dabie Orogenic Belt and the northern Sichuan Basin with the change of a low velocity anomaly to the high are mainly N-S,which may be related to the collision and extrusion tectonic activities.With the high velocity anomaly,the anisotropy directions in the eastern and central parts of the Yangtze Craton-Huaxia block are mainly NE-SW and E-W,respectively,which are consistentwith the trend of the suture zone.The anisotropy in the west of the suture zone is more complicated.At long periods such as 60 s,100 s,and 120 s,there are two main fast propagation directions in the eastern Tibetan Plateau with a high velocity anomaly,NEE-SWW and NWW-SEE,which may explain the direction of material escaping in the eastern Tibetan Plateau and was affected by the Sichuan Basin during the escaping.The anisotropy in the southern Ordos Basin is also in the NWW-SEE direction,which may indicate that the southern part of the basin was destroyed under the tectonic background of the Qinghai-Tibet Plateau extrusion and the plateau material eastward flow.The anisotropy directions of the Cathaysia block and the Yangtze craton with different velocity anomaly are EW and NE-SW,respectively,with a magnitude of2-2.5%,which may indicate that there are more active asthenospheric material activitys in the eastern area and it is the asthenospheric hyperthermal gradient place.The overlying mantle lithosphere undergoes strong structural deformation when heated.The magnitude of anisotropy in southeastern North China is about 2%.There are two main directions,NW-SE and NEE-SWW,and with a high velocity anomaly,perhaps,because of the eastward flow of the mantle flow caused by the subduction of the Pacific plate.At long period,the magnitude of anisotropy in central and eastern North China with a low velocity anomaly weakened by about 1.5-2%,mainly in the direction of NNW-SSE,possibly due to the lithospheric thinning of the central and eastern North China Craton.