Formed in Mesozoic-Cenozoic time, the Kuche depression on the south side of Tianshan and the Urumqi depression on the north side of Tianshan demonstrate striking similarities in geometry, kinematics and dynamics. However, the seismic activities, especially middle and large earthquakes show evident differences between the two depressions. This paper analyses the primary factors that cause the differences. They include: 1. the difference in deformation extent between the two depressions; 2. the difference in fine structure of gophysics between the two depressions; 3. the difference in the environment of geodynamics between the two depressions.

The Xianshuihe fault zone is a well known active fault located at the north eastern boundary of the Tibetan plate. The micro texture and microstructures of fault gouge, the eroded micro texture and microstructures on the surface of quartz fragments from the fault zone are measured by Scanning Electron Microscope. The results of statistics and analysis show that the fault zone has experienced the intense oriented stress field and displays the stick slip movement mode. Its activity was still continuing from late Pleistocene to Holocene. Both the feature of the grain size distribution from fault gouge and the analysis of the fractal dimension have the obvious segmentation along the fault zone. This is consistent with the result from geodetic surveying along the fault zone in recent years.