A PRELIMINARY DISCUSSION ON THE RELATIONSHIP OF DEEP ENVIRONMENT TO SEISMICITY IN NORTH-SOURH SEISMIC BELT (ITS NORTHERN AND MIDDLE SECTIONS), CHINA
Mao Tongen and Yao Jialiu(National Bureau of Seismology)
The North-Soath Seismic Belt is one of the most seismically active belts in China. It stretches from the Yinchuan depression in the northern end through the Liupangshan Mts., Qinling Mt., Wenxian, Maowen and along the Hengduan Mts., and far beyond the frontier.In this paper a preliminary discussion is made on the structural features of the crust and upper mantle under the northern (Holangshan-Liupangshan seismic zone) and middle sections (Tianshui-Wudu, Wenxian-North-West-Sichuan seismic zones) of the North-South seismic belt through the geophysical data.The analysis and study of the materials and data available make enable us to recognize the main features of deep seismic environment in northern and middle sections of this seismic belt.1. Top surface of the upper mantle under this seismic belt appears to be in zonal uplift. The axial orientation of the zonal uplift of upper mantle is in NNE direction in the Yinchuan graben area and in NW direction in Xiji, Haiyuan and Guyuan areas, and in sublatitudinal direction in Tianshui-Wudu area.2. The strike of faults along which strong earthquakes occurred and direction of migration of epicenters are coincident with the orientation of isobaths of the top surface of the asthenosphere.3. Strong earthquakes usually took place in the conjunction portion of deep-seated structures with shallow-seated structures at depth in the earth, such as in Tianshui-Wudu area.4. The occurrence of strong earthquakes is related to the local uplifts and depression of the high conductivity layers in the crust and upper mantle.A tentative analysis is made on the relation of the deep-seated environment to preparation of earthquakes. At the end of this paper a discussion is made on the significance of the deep seismic environment in research on medium and long-term earthquake prediction and causes of earthquakes.