3-D crustal P-wave velocity structure in western Yunnan area and its tectonic implications

Using the absolute relocation method, we relocate 1593 earthquakes from the seismic waveform data recorded by the dense temporary seismic array from August 2011 to August 2012, which is deployed in the southern segment of the north-south seismic belt. Then seismic traveltime tomography method is applied to obtain the 3-D crustal P-wave velocity structure in the western Yunnan area. The inversion results indicate that high-velocity anomaly extends to the middle crust from the surface in the Panzhihua area, with the high velocity on its west merging into a large-scale high-velocity block. We speculate that the high-velocity block plays a certain impediment to southward escaping of the Tibetan plateau materials, which caused the rapid uplift of the northern sub-block of the Sichuan-Yunnan active block. In the south of the Jinshajiang-Red River Fault, low-velocity anomaly exsists in the lower crust of the Tengchong block and Baoshan block. Since it is located in the subduction boundary between the Indian plate and Eurasian plate, we consider that the low-velocity anomaly may be caused by the high temperature derived from the upper mantle due to the Indian plate's eastward subduction. Jinshajiang-Red River Fault is the southern boundary of the Sichuan-Yunnan block. The high-velocity crustal structure and the weak seismic activities in the middle segment of the Red River Fault(Midu to Red River areas)imply that this region is locked currently. However, as the boundary between the blocks, it should be one of the key monitoring areas in the future.