THREE-DIMENSIONAL P VELOCITY STRUCTURE OF THE CRUST AND UPPER MANTLE UNDER BEIJING REGION
Jin An-shu (Institute of Geophysics, StateSeismological Bureau}Liu Fu-tian Sun Yong-zhi(Institute of Geophysics,Aeademia Sinica)
By use of the teleseismic P arrival times at 15 stations of Beijing network for 120 events distributing over various azimuths, we studied the three-dimensional P velocity structure under Beijing region. In calculating the theoretic traver-time we adopted the soure parameters given in BISC, and used the J-B model as the stan-dard model of earth. On inversion, we a dopted singular value decomposition as a generalized inversion package, which can be used for solving very large over-deter-mined system of equations Gm = t without resort to normal equations GTGm = GTt. The results is that within the crust and upper mantle under Beijing region there are clear lateral differences. Thers the results obtained by use of data 1972-1975, it can be seen that there are three different zones of P-velocity. In the south-east of Beijing region, P velocity is lower than that of the normal model by 10-14% within the crust, and by 8-9% within the upper mantle. The north-west of Beijing region is a higher-velocity zone, within which the average P-velocity is faster than that of the normal model by about 9%. It disappears after entering into the upper mantle. The central part of this region is a normal zone. On the surface, the distri-bution of these P velocity variations corresponds approximately with the distribution of the overburden. But. in the deeper region, the distribution of velocity variation agrees with the distribution of seismicity. It is interesting to note that the hypo-centers of several major earthquakes in this region, e.g. the Sanhe-Pinggu earthquake (1679, M = 8), the Shacheng earthquake (1730, M=63/4 and the Tangshan earth-quake (1976, M = 7.8) are all located very close to this boundary of these P-velocity variation zones.