北京地区地壳和上地幔的三维P波速度结构

用分布在北京地区各方位上,震中距在20°-100°范围内的120次远震在本区15个地震台上的P波到时,研究北京地区地壳和上地幔的三维速度结构。 在正演时,远震的震源参数（λ、φ、h、o）采用BISC的数据,以J-B模型作为标准地球模型。 在反演中,采用奇异值分解的方法直接解大型超定方程组Gm=t,而不借助于正规方程组G^TGm=G^Tt。 结果表明:北京地区地壳和上地幔的P波速度存在明显的横向差异。就1972年-1975年的资料所得的结果来看,存在东南的相对低速区（地壳中P波速度低10-14％,在上地幔中低8-9％）,西北部的相对高速区（在地壳中P波速度高9％左右,进入上地幔后差异逐渐消失）,中部是正常区。 在地表,速度差异带的分布与覆盖层的分布大体一致。而在深部,这种差异带的分布与地震活动性分布相当符合。而且,本区的几次强震:三河-平谷地震（1679年,M=8）、沙城地震（1730年,M=6 3/4）和唐山地震（1976年,M=7.8）的震源正好在这些速度差异带的边界附近。

THREE-DIMENSIONAL P VELOCITY STRUCTURE OF THE CRUST AND UPPER MANTLE UNDER BEIJING REGION

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 G^TGm = G^Tt. 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.