Abstract: Seismic tomographic data show that crustal low-velocity layers and mantle uplift do exist in a vast area along Beijing-Tianjin-Tangshan-Zhangjiakou, where a lot of large earthquakes had occurred. However, such low-velocity layer and mantle uplift can not be found in Ordos region, where no strong earthquake had occurred. Geological study shows that the compositions of the crust are similar in these two areas. The main difference of the two areas is that rifting had occurred in Beijing-Tianjin-Tangshan-Zhangjiakou area at Cenozoic. We suggest, therefore, that the crustal low velocity layers in North China were controlled by the Cenozoic rifting and mantle uplifting, which caused the rise of temperature and hence giving rise to plastic deformation of lower part of the middle crust and lower crust. The lattice preferred orientation of minerals formed by plastic deformation can cause wave anisotropy, while subgrain substructure produced by dislocation creep may reduce the elastic wave velocity of rocks. The low velocity layers in lower part of the middle crust and lower crust, therefore, should be the result of plastic deformation of rocks. The low velocity layers in the upper part of the middle crust, however, might represent the low angle detachment or ductile shear zones formed during rifting, while low velocity might be resulted from anisotropy of rocks and possibly the effect of fluid. The crustal structure of Ordos region is similar to "ice-water" structure, and that of Beijing-Tianjin-Tangshan-Zhangjiakou area is like "sandwich". A large number of studies show that the crustal weak layers (lower velocity and plastic deformation layers) enhance the decoupling of stronger layers, and this may play an important role in the generation of large earthquake.

Key words: Low velocity layers, Plastic deformation, Strong earthquake generation, North China

摘要： 京津唐张地区普遍存在壳内低速层,鄂尔多斯块体内部没有发现低速层,壳内低速层的这种分布受新生代裂陷伸展的控制.华北地区中地壳下部和下地壳低速层是岩石塑性流变的结果,中地壳上部低速层是地壳裂陷伸展时形成的水平拆离带和韧性剪切带,岩石各向异性和流体作用可能是引起低速的原因.壳内软弱层(低速和塑性流变层)增强了块体层间的解耦作用,对地震孕育起着重要作用.

关键词: 低速层, 塑性流变, 强震孕育, 华北地区