Structure of Crust and Upper Mantle in Tianshui-Wudu Strong Earthquake Region of North-South Tectonic Belt

The structure of crust and upper mantle in Tianshui-Wudu strong earthquake region is studied by using DSS data from two cross high resolution refraction /wide angle reflection profiles and corresponding fan profiles.The 2-D result shows that the crust can be vertically divided into two layers:upper crust and lower crust.There exists a low velocity layer in the upper crust with velocity difference of 0.3-0.5 km/s.The depth of Moho is about 46-48 km.There is a strong lateral variation in the lower crust along the NE Tianshui-Wudu profile.And along the NW Chengxian-Wushan profile,there are some evidences showing that the Moho and C discontinuities are changed by substances from the deep layer.The 3-D result shows that there is a fault with near NS strike at the depth of 7-11 km near 105°E.The velocities at the two sides of this fault are quite different:low velocity at the west side and high velocity at the east side.This NS fault is very close to the deep fault of 2-D profile.All the large earthquakes occurred near 105°E and formed a NS belt.