THE 3D VELOCITY STRUCTURE INVERSION IN BEIJING-TIANJIN-TANGSHAN AREA BY USING LOCAL EARTHQUAKE DATA

The better resolution in horizontal and vertical compenents for the crustal velocity structure can be obtained by using local earthquake data. For events occurred in Beijing, Tianjin and Tangshan area, the earthquake location and velocity structure inversion were calculated iteratedly. 147 earthquakes occurred in this area were relocated with ACH algorithm by using the data from Beijing Telemetered Seismic Network. Using SIRT algorithm, the crustal 3D velocity structure was inversed from the P and Pn arrival times. The SIRT algorithm is characteristic of clear physical meaning and fast calculation. The inversion with a large amount of data can be done in PC micro-computer. The results of digital modeling show that the SIRT algorithm is desirable.In order to inverse the crustal structure of Beijing-Tianjin-Tangshan area, we divided the crust into three layers, and divided each layer into 13×13 homogeneous blocks. The velocity values of the blocks were inversed by 3310 P and Pn arrival times of 147 earthquakes. The inrersion results indicate that: (1) the velocities vary slightly in the upper crust; (2) for middle layer of the crust, the velocities are lowre in the plain and higher in the mountain area, and (3) there is a low velocity zone between Beijing and Tangshan in the lower crust. The low velocity zone in the lower crust is coincident with the position of upper mantle low velocity zone obtained by teleseismic data. It means that there have been material intrusion from deeper mantle, which affects the seismicity in North China.