3D P-wave Velocity Structure Of Crust In The Eryuan Area

Western Yunnan is the area with the most frequent tectonic activity in our country,with frequent earthquakes,staggered distribution of fault zones,diverse topography,and complex geological tectonic activities.Many scholars have carried out a lot of deep exploration research work in the western Yunnan area,using different geophysical methods.And they have made many research results and revealed the basic characteristics of the crustal structure in the area.Before and after the Eryuan Earthquake,geophysicists conducted a lot of research on the velocity structure of the crust and upper mantle in this area and its immediate areas,and obtained many meaningful research results.However,due to the limitation of data distribution,the existing research results have not given a small-scale fine velocity structure to the medium environment of the source area of the Eryuan earthquake.This poses difficulties for understanding the deep media environment of the Eryuan earthquake and the deep-seated formation mechanism of the rupture process.In our study,we use microseismic data and double-difference tomography to determine the small-scale three-dimensional P-wave velocity structure in Eryuan area,which is important for understanding the deep structure and genetic mechanism under Eryuan area.In addition,our research focuses on the relationship between the seismic distribution after relocation and the three-dimensional P-wave velocity anomaly.The double difference seismic tomography method is applied to 11968 P wave arrival times and 73987 high quality relative P arrival times of 1987 events of Eryuan seismic sequence to determine the detailed crustal 3D P wave velocity structure and the hypocenter parameters in the Eryuan seismic area.According to the distribution of the source location after relocation,the earthquake is mainly distributed along the fault or occurs in the transition zone of high and low velocity anomalies.The results of seismic tomography are obviously related to the surface geological structure.The P-wave velocity anomaly near the surface is closely related to the geological structure.The shallow low-speed area corresponds to the depression area,and the high-speed area corresponds to the high mountain or uplift area.The faults in the study area control the distribution of high-and low-speed anomalies in the upper crust.The basin is surrounded by faults and presents low-speed anomalies in the Jianchuan,Heqing,Eryuan,Midu,and Xiangyun basins.We speculate that the low-velocity anomalies may be affected by shallow sediments.At 4km,the low-velocity area shifted to the northeast direction,and the basin showed a tendency to northeast in the deep part.At the depths of 8km and 12 km,along the parallel Tongdian-Weishan fault direction,there are beaded strips,and the velocity structure shows the distribution characteristics of high and low velocities.As the depth increases to a depth of 24 km,the velocity structure has changed significantly compared with the previous layer,and the overall study area presents a high-speed anomaly.The low-velocity body Lv1 exists in the middle and lower crust beneath the Tongdian-Weishan fault to the Lancangjiang fault,and the low-velocity body Lv1 beneath the Tongdian-Weishan fault becomes shallower in the northwest of the fault.Our research suggests that the low-velocity body of the middle and lower crust may be the result of stress concentration under the fault after being blocked by the rigid Yangtze plate due to the pushing from the huge pressure of the Qinghai-Tibet Plateau.