Three-dimensional fine structure imaging and tectonic characteristics of the crust in western Yunnan

This paper used the seismic data from the fixed stations of the China Earthquake Networks Center, the temporary stations of the Institute of Geophysics, CEA and the Institute of Geology and Geophysics, CAS in western Yunnan and its surrounding areas to invert the threedimensional fine structure models of VP, Vs and Vp/Vs at depths of less than 50 km in western Yunnan by the consistency constrained double-difference tomography method. The results show that earthquakes are mainly concentrated in the weak area of the high-speed body or the transition zone of the high-low-velocity body is biased to the high-speed body, with good correspondence between the spatial distribution and the velocity structure. The distribution of low-velocity anomalous bodies is very complicated and restricted by fault structure, since the maximum control depth even reaches the top of the upper mantle. The crustal structure model shows that the depth of the Moho surface in western Yunnan varies little in the east-west direction, which it changes from deep to shallow in north-south direction with huge fluctuations. The deepest part is greater than 55 km, and the shallowest part is about 29 km. There is an obvious transition zone at the crust-mantle boundary, ranging from 1. 5 to 1. 9 in the inner crust and 1. 9 to 2. 1 at the top of the upper mantle, which is consistent with the variation trend of the Moho surface. Under the Tengchong volcanic area, there are high-velocity bodies represented by cooling and consolidated magmatic intrusions or unvolatilized high -density residues in the early volcanic channels, unconsolidated magma capsules or low speed abnormal bodies of molten materials, and heat flow channels extending to the top of the upper mantle. There are low-velocity anomalous bands in the North-South direction in the Dali area, which may be the crustal material migrating southward in the lower Tibetan Plateau. Under the action of regional tectonic stress, frequent seismic activity occurs in the transition zone of high and low velocity anomalies. Although the four strong earthquakes of magnitude 7. 0 or above in Longling, Lancang and Gengma are far away, they have the same seismogenic structure and environment. This model can provide detailed crustmantle structural data support for analyzing the deep tectonic background of other key areas in western Yunnan.