关键词: 几何障碍, 断层几何结构, 剑川-洱源伸展带, 地震的发生和迁移

Abstract: The neotectonics and seismicity in Jianchuan-Eryuuan area, northwestern Yunnan exhibit a model of the geometric barriers of the faults relative to the earthquake rupturings.Field investigations have revealed that the modern Jianchan-Eryuan extensional zone is made up of two longitudinal zigzag rupture belts which experienced two important stages of development in the past: pulling-apart within the overlapping area of NE-trending strike-slip faults in early Quaternary, and zigzag extension in late QuaternaryEarthquake data from 1982 to 1986 show that five earthquake events occurred along the extensional belts corresponding with the trend of the zigzag ruptures. First event happened in July, 1982 at the northern end of the zone, and then the following seismic swarms occurred one by one towards the south, each of which was located at the geometric barrier of the faults constructed by the intersections of faults with different behaviors.In order to describe the mechanism of the seismicity like this, we adopt the concept of barrier proposed by Aki (1979), King and Yielding (1984), and set a numerical model to calculate the change of stresses along the zigzag rupture. The results reveal that the. maximum shear stress and the displacement of the faults usually focus on the geometric barriers, which were formed by the intersections of faults with different behaviors or trends and broken most easilly. When the stress on a barrier is released, a mu-tuation of stress at next barrier must be induced and so it will be broken following the first in a short time which will cause the third barrier fractured after that, and so on. Thus, the earthquake events appear along the zone one by one untill the induced strain energy can not produce a new rupture.By comparision, the seismicity of extensional zones in other places of China is similar to that of Jianchuan-Eryuan zone. We can conclude that the migration and magnitude of earthquakes in modern structures are not random but depend on the geometry of the structures.

Key words: Geometric barrier, Fault geometry, Jianchuan-Eryuan extensional zone, Occurrence and migration of earthquakes