TOMOGRAPHIC INVESTIGATION OF THE UPPER CRUSTAL STRUCTURE AND SEISMOTECTONIC ENVIRONMENTS IN YUNNAN PROVINCE
Bai Zhiming Wang Chunyong (Institute of Geophysics, China Seismological Bureau, Beijing 100081 , China)
Investigation has been made for the upper crust structure and seismotectonic environments in Yunnan Province by using the plentiful DSS data of the four profiles. The derived velocity model has a good relationship with the exposed basins, uplifts and faults, and the low velocities anomaly corresponding to the volcanoes also has been revealed. There exits a prominent lateral inhomogeneity within the upper crust of Yunnan region. The depth of crystalline basement generally ranges from 0 km to 5 km, and the bedrock are exposed on the ground directly in some places, nevertheless the thickness of sedimentary cover also can reach to 8 km or even 12 km at some large depressions. Although the Changning-Shuangjiang fault is a boundary between two first class tectonic units, its incision depth within the crust however maybe shallow. On the other hand, known as the plates' seam, the Honghe fault has a distinct evidence of extending into the mid-lower crust. The widely spread activity of the volcanoes in the geological era has a closed relationship with the earthquake's occurrence up-to-date. Despite of the stop of the volcanoes in some places on the ground, the material in the mid-lower crust is still active, and there still exists strong upward stress. For the ceasing of the volcanoes on the surface, most parts of the power from the lower crust and upper mantle cannot be released, therefore it accumulates at some beneficial tectonic locations. Moreover, saturation of the water from the basin and the action of other fluids, and the affection of outer stress maybe another direct reason account for the strong earthquakes' occurrence in Yunnan region.