The Research Of Three-dimensional Velocity Structure In Cangzhou And Yushu Area

Since tomography was introduced to study the Earth’s internal structure, seismic tomography has been developed rapidly, and now has become one of the most effective methods of researching the deep Earth’s deep structure. This article introduces the theory and methods of local and teleseismic tomography techniques in detail, and discusses four important steps in the tomographic imaging processes. We study the underground velocity structure in two regions:Cangzhou District, Hebei province and Yushu, Qinghai Province with using Professor Zhao Dapeng’s tomography metnhod (TOMOG3D). The main results of this study are as follows:(1) We used2308P wave arrival times from271teleseismic events recorded by the20portable seismic stations during December2006to July2010in Cangzhou region, Hebei Province. By using seismic tomography method, we got the3-D crustal and upper mantle P wave velocity structure in Cangzhou and its adjacent area (38.0°N—39.0°N,116.5°E—117.5°E). Teleseismic tomography results reveal that the significant lateral variations for the velocity distributions in crust on both sides of Cangdong fault zone. The shallow velocity distribution is consistent with the distribution of the surface geological structure. Cangxian uplift in the northwest of Cangdong fault zone is with the higher velocity values in the crust, indicating that it has the uplift basement. Huanghua depression in the southeast of the fault zone is with lower velocity, indicating the substrate buried deeper. The combination of these results and previous gravity, magnetic and artificial seismic exploration results show that the Cangdong fault zone is in obvious changes in the geological structure and geophysical properties, which could be displayed in whole crust.(2) The Ms7.1Yushu earthquake on April14,2010occurred in the Ganzi—Yushu fault, which is thought to be caused by the sudden release of stress, accumulated by the southeastward differential movement between the Bayan Har block and the Qiangtang block. After the earthquake,22portable seismic stations were deployed near the seismogenic region by Institute of Geophysics, Chinese Earthquake Administration. The total of1345local events and803teleseismic events were analyzed from May9to October30,2010.7687Pg arrivals and9494 teleseismic P arrivals were collected from the dataset. The joint tomographic imaging of local and teleseismic arrivals was applied to obtain the three-dimensional velocity structure in the Yushu and its adjacent area (31°N—35°N,95°E—98°E). The tomographic result indicates that there are significant velocity variations across the Ganzi—Yushu fault, especially within the depth of60km. The Qiangtang block to the south of the fault is characterized by low velocity, in contrast to the high velocity in the Bayan Har block to the north. The significantly lower velocity in shallow of Qiangtang block, may reflect that the Qiangtang block sediment thickness is much larger than the Bayan Har block. The Qiangtang block low velocity area in crust may be caused by fluid water and partial melting materials. Partial melting due to friction of subduction and collision between Indian plate and Eurasian plate. Combination of local and teleseismic tomography result in this study and previous gravity, magnetic, electrical detection results show that whether in the plane or profile, Yushu fault zone is in the obvious change in the nature of geological formations and deep geophysical surroundings. The distinctive velocity contrast of Bayan Har block in northeast and Qiangtang block in southwese may indicate that the subduction of the Indian plate to the Eurasian plate is possibly blocked by the relatively rigid Bayan Har block, induces the plateau’s deep material flow southeastward, weakens the northward squeezing effect of the Qiangtang block, causes the instability of its boundaries (Yushu fault).