Study On Crust-mantle Azimuthal Anisotropy In Sichuan-yunnan Region Based On Background Noise And Teleseisms

Since the Cenozoic,India-Eurasian plate continuous collision and extrusion caused the Tibet plateau uplift deformation,with the plate collision action and its gravity,the plateau materials exruded to the southeastern.The Sichuan-Yunnan region is located in the southeast margin of the Tibetan Plateau,where neotectonic deformation is intense and the strong earthquake activity frequency is high.The Sichuan-Yunnan region is an important place for us to study the south-east expansion of the Tibetan Plateau.It is of great significance to study the structure and dynamics of the crust and upper mantle in Sichuan-Yunnan region to understand the tectonic evolution of the Tibet Plateau and the causes of continental interior earthquakes.The project China Array I developed 190 broadband seismic stations in SichuanYunnan region from 2011 to 2013,with an average station spacing of 35 km.In this paper,we obtain the high resolution Rayleigh surface wave apparent phase velocity and azimuthal anisotropy from 6 S to 60 S with the background noise imaging and surface wave imaging based on eikonal equation.Furthermore,We applied the Helmholtz imaging to correct the visual phase velocity including the finite frequency effect and obtain the structural phase velocity distribution from 6 S-60 S.Combining the results of previous research studies in this region,we analyzed the velocity structure and causes.Following are the results:The short period(6 S-12 S)Rayleigh surface wave fast direction is almost NS direction.The fast direction of the upper and middle crust is consistent with the strikes of the faults.There is a diffrence in the the Red River fault,where the fast direction is NS direction that is not consistent with strikes of the faults of NW direction.The fast direction of the upper and middle crust of the Yunnan–Myanmar–Thailand block in the study area shows that it rotates clockwise around the Himalayan tectonic junction.There is a low velocity anomaly with strong anisotropy in the middle and lower crust of Sichuan Yunnan block,which supports the middle and lower crust flow model.The direction of the fast wave near the Red River fault zone begins to be parallel to the strike of the fault zone in the middle and lower crust.The anisotropy direction of the crust and mantle in the south of the study area is inconsistent,so it is considered that the crust and mantle in this area are decoupled.The anisotropy direction of the asthenosphere in the southern part of Sichuan Yunnan region is near EW,which may be caused by the mantle flow caused by the westward retreat of the Indian plate subducted along the Myanmar arc.