A MECHANICAL MODEL OF THE FORMATION MECHANISM OF THE SHANXl GRABEN ZONE AND THE CHARACTERIS-TICS OF SHANXl EARTHQUAKE ZONE

The Shanxi Graben zone in North China is a small continental rift valley. Its principal characteristics are the same as that of the other continental rife valleys of the world. Based on the observed facts that the formation and development of the rift valleys and grabens are the results of the propagation of the fracture system existing in the crust, it is possible to study the mechanism of their mechanical formation by using the method of fracture mechanics. In this paper, a three-dimensional mechanical model is presented in order to study the formation and development processes of the Shanxi graben and explain the zigzag pattern of the Shanxi graben system. The angles between the segments of shear and tension are calculated from this model and their magnitude depends on such factors as the direction of the regional stress field, the aspect ratio between the depth and length of the graben fracture zone, the mechanical properties of the materials etc. The general direction of the tension segments always directs to that of the maximum principal compression stress of this region, so that the direction of the maximum principal compression stress of the region can be given from the general direction of the tension segments and vice versa. The characteristics of earthquake distribution in the Shanxi graben zone are also analysed by means of the seismicity data.Besides the Shanxi graben system, the Rhine graben, the Rio Grande rift valley and the Baikal rift valley are also analysed by using the same method. The results show that there also exist obvious segments of shear and tension in grabens and rift valleys.