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通过区域地震和远震记录的走时残差,测定华北盆地中部和周围山区的三维弹性波速度结构。我们计算了1975年唐山地震的200个余震的P波和S波的走时残差,以及从1976年至1983年间北京台网记录的194个远震的P波走时残差。仅基于余震走时残差,我们完成了地壳和上地幔结构一维和三维的块状反演迭代。我们还基于远震走时残差以及把余震和远震走时残差相结合,完成了对地壳和上地幔的块状反演迭代。我们测定出华北盆地和周围山区的地壳平均厚度接近35km,然而华北盆地沿海岸部分的地壳厚度大约为32～33km。地壳上部大约20km的速度结构与地质露头完全相关,并且相似于表面地形的短波长(几十公里)特点。低地震速度的岩石在盆地之下而高地震速度的岩石存在于山区之下。地幔顶部,也许下地壳在内,好象是上地壳和地幔之间速度结构的过渡带。关于速度结构的相对高低趋势,地幔与地壳几乎完全相反,华北盆地下伏的大部分地幔物质具有比平均地震速度要高的速度,而西部山区之下物质的速度都低于平均速度。地幔速度结构与华北盆地的重力和地形的长波(成十到成百公里)特征显然相应。盆地之下的高速地幔物质或许说明了地幔过程与华北盆地无直接关系。我们推断,短波与长波速度特点之间的过渡带表明了岩石层与软流层无相互影响的范围。尤其是华北地块的岩石层相对软流层正在向东运动。
Based on the travel time residuals of regional earthquakes and teleseismic records, the 3D elastic wave velocity structures in the central and surrounding areas of the North China Basin were determined. We calculated the traveltime residuals of P-wave and S-wave of the 200 aftershocks of the 1975 Tangshan earthquake and the P-wave traveltime residuals of 194 teleseismic records recorded by Beijing Network in the period from 1976 to 1983. Based solely on the residual travel time of the aftershocks, we completed one-dimensional and three-dimensional massive inversion iterations of the crust and upper mantle structures. We have also completed an iterative block-inversion of the crust and upper mantle based on the residual travel from the teleseismic strike and the combination of aftershocks and remnant travels from telekinesis. We determined that the average crustal thickness in the North China Basin and the surrounding mountainous areas is close to 35 km, whereas the crustal thickness along the coast of North China Basin is about 32-33 km. The velocity structure of about 20 km above the crust is completely related to geological outcrops and similar to the short wavelength (tens of kilometers) of surface topography. Rocks of low seismic velocity are under the basin and rocks of high seismic velocities exist below the mountainous area. The top of the mantle, perhaps beneath the crust, appears to be the transitional zone of velocity structure between the crust and the mantle. With regard to the relative trend of velocity structure, the mantle and the crust are almost exactly the opposite. Most of the mantle material underlying the Huabei Basin has a higher velocity than the average seismic velocity, while the material velocities below the western mountain are lower than the average velocity. The velocity structure of the mantle clearly corresponds to the longwave (tens to hundreds of kilometers) features of the gravity and topography of the North China Basin. The high-velocity mantle material beneath the basin may indicate that the mantle process is not directly related to the North China Basin. We conclude that the transition between shortwave and longwave velocities indicates the extent to which the lithosphere does not interact with the asthenosphere. In particular, the relatively stratum of the rock layer in the North China block is moving eastward.