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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。
Using P wave arrival at 15 seismic stations in this area with 120 teleseismic epicenters located in the range of 20 ° -100 ° epoch in all directions of Beijing area, the 3D velocity structure of crust and upper mantle in Beijing area was studied. In forward modeling, the teleseismic source parameters (λ, φ, h, o) use BISC data and the J-B model as the standard earth model. In the inversion, we use the method of singular value decomposition to directly solve the large overdetermined system Gm = t without resorting to the normal system G ~ TGm = G ~ Tt. The results show that the P wave velocity in the crust and the upper mantle in Beijing has obvious lateral differences. According to the data from 1972 to 1975, there are relatively low velocity regions in the southeast (10-14% of the P wave velocity in the crust and 8-9% in the upper mantle), relatively high velocity regions in the northwest In the crust, the P wave velocity is about 9% higher and the difference gradually disappears after entering the upper mantle.) The middle part is normal. On the surface, the distribution of velocity difference bands is generally consistent with the distribution of overburden. In the deep, the distribution of this difference zone is quite consistent with the distribution of seismicity. Moreover, several strong earthquakes in this area coincided well with the hypocenter of the Sanhe-Pinggu earthquake (1679, M = 8), Shacheng earthquake (1730, M = 6 3/4) and Tangshan earthquake (1976, M = 7.8) Near the boundaries of these speed difference bands.