Ambient Noise Tomography In Anninghe-zemuhe Fault Zone And Its Surrounding Areas

The Anninghe-Zemuhe fault zone is located at the junction of the Chuandian block,the Bayanhar block and the Southern China block,and is an important eastern boundary of the Chuandian block.Due to the collision between the India plate and the Eurasian plate,the Tibet Plateau continued to rise and the Chuandian block moved south-south-eastwards under the uplift background,resulting in complex structural deformation and strong seismic activity in the Anninghe-Zemuhe fault.Many destructive earthquakes have occurred in the fault zone,and it is one of the key seismic monitoring areas in China.Therefore,studying the velocity structure of the fault zone is of great significance for understanding the geological structure background,seismogenic process and potential seismic risk in the area.The Rayleigh wave group and phase velocity maps of 4-20 s in this area are obtained with ambient noise tomography method,using2-3 years' vertical component ambient noise data recorded by 187 broadband seismic stations around the Anninghe-Zemuhe fault zone,and its distribution characteristics are discussed,and its tectonic significance is explained.Compared with previous studies,the lateral resolution has been improved greatly in this paper.The resolution is about 20 km in the central Anninghe-Zemuhe fault zone,and 20-40 km in other regions.The imaging results show that the velocity structure of the middle-upper crust in the Anninghe-Zemuhe fault area has obvious lateral inhomogeneity.The velocity structure is basically consistent with the regional geological structure,and the velocity maps differ little at different periods.The Yanyuan basin,the Xichang basin and the southwest margin of the Sichuan basin are characterized by low velocity anomalies,which reflects that these basins have thicker sedimentary layers.The velocity of the Xichang basin is lower than that of the Yanyuan basin,indicating that the basement of the Xichang basin is deeper than the Yanyuan basin,which is corresponded to the shallow geological structure.The apparent high-speed anomalies around the Jiulong region are inferred to be related to the exposed Indosinian granite bodies and metamorphic core complexes.The high-speed anomalies near the Yanbian-Dechang-Qiaojia region on the southern part of the study area may be related to the magmatic activity of the Emeishan basalt.In the southern segment of Anninghe fault and the northern segment of the Zemuhe fault zone,there are obvious velocity contrasts between the two sides of the fault,most of the historical strong earthquakes inthe area occurred on the belt,and this velocity contrast between the two sides of the other fault zones are not obvious.The middle and upper crust in the vicinity of the Gongga Mountain is characterized by obvious low velocity anomalies,and a small high-speed abnormal body is on the southeast side.The low velocity anomalies in Gongga mountain area may be related to the abundant geothermal resources in the region and the younger late Cenozoic granites.During the south-east movement of material in the Tibetan Plateau,the obstruction of the high speed bodies in the east and the southwest sides of the Gongga Mountain,and the changes in the strike of the Xianshuihe fault and Anninghe fault,forming a compression bending section in this area,making the south-east horizontal movement of the Chuandian block transfer to compress effect perpendicular to the fracture and vertical deformation,resulting in rapid vertical uplift of the Gongga Mountain.