关键词: 垂直形变速率梯度, 梯度信息, 地震活动

Abstract: In this paper we illustrate the gradient of vertical deformation rates, seismotectonics and seismicity in Chinas continent, as well as their relationship. Basing on the map of vertical deformation rate of Chinas continent (1951～1990), we have derived the gradients of vertical deformation rates on a GIS platform. We find that the variation of the gradients reflects basically the magnitudes of vertical crustal shear strain and the corresponding crustal shear stress. In the eastern part of Chinas continent, the gradient of vertical deformation rates of North-China Block is obviously higher than those of Northeast China and South China Blocks. In the western part, the gradients are higher in Qinghai-Tibet and Tianshan Blocks, which are the regions of strong current tectonic activities. The NE-trending Chayu-Tonghua high gradient zone obliquely passes through Chinas continent, and the Yinchuan-Wuli high gradient zone on its west side is the region of intense current vertical differential movement. The current strong earthquakes occur mainly along the anomalous zones with high gradient and their surrounding areas. In the period from 1966 to 1976, most of the strong earthquakes occurred along the NE-trending Chayu-Tonghua high gradient zone, such as the Haicheng M_S 7.3, Tangshan M_S 7.8, and Xingtai M_S 7.2 earthquakes in the northeast section of the zone, and the Songpan M_S 7.2, Pingwu M_S 7.2, Luhuo M_S 7.9, and Longling M_S 7.3 earthquakes in the southwest section of the zone. The gradient of vertical deformation rates is a mirror of the vertical crustal shear strain. High gradient zone of vertical deformation rates is therefore the zone of high vertical shear strain, forming the active zone of strong earthquakes. In the western part of China, however, the distribution of strong earthquakes is not accordant completely with the high gradient zone. This is partly because that the leveling data for the western part of China is relatively incomplete, which may cause the lost of information about the vertical deformation rates. In addition, strong earthquake may also be developed in the region where horizontal deformation predominates over vertical deformation. In the evaluation of active tectonics, therefore, an overall consideration should be given to both vertical and horizontal crustal deformations. In a word, the analysis of the gradient of vertical crustal deformation rates has great application potential for the assessment of seismic risk areas.

Key words: Vertical deformation rate, Gradient information, Seismicity