关键词: 低速层, 塑性流变, 强震孕育, 华北地区

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