Formation Of Main Regions Of Strong Earthquakes: Mechanism And Numerical Simulation

Studies (Ma Zongjin et al, 1986; Zhang Zhaocheng et al, 1994; Hong Hanjin et al, 1994, 1997; Huang Shengmu et al, 1997) on strong earthquake distribution in China and its vicinity have led to a recognition that the activities of continental strong earthquakes in China and its surrounding areas may concentrate in a certain active region during a certain time period. Hong Han-jing (1994, 1997) suggested that the seismic activities in China and its surrounding areas in the last century can be grouped into several micro-dynamic periods, while in an individual period the activity of strong earthquakes may be fixed in a main active region. Moreover, in different periods the patterns of strong earthquake distributions may vary significantly in space and the main active region of strong earthquakes may migrate for a long distance.The main regions of strong earthquakes reflect the quick change of the stress field in the Chinese continent. To explain this change some questions must be answered, they are: what is the driving force? How the stress immigrates and accumulates in the crust? And what are the main factors to affect the change of the stress field?Dynamic analysis demonstrates that the dynamic activities of the three surrounding plates, i.e. the Indian plate, the Pacific plate and the Philippine plate, affect the stress field in the China continent. However the cracked upper crust could not transfer the stress in long distance, and it could not help to explain the migration of the main regions of strong earthquakes. Therefore this work takes into account the rheological stratification of the whole lithosphere, and modifies the boundary condition of the two-layers visco-elastic lithosphere model of Kusznir & Bott's (1977) to a constant velocity boundary condition, and gives the analytical resolution. When the two layers are assumed to be welded, the greater the differences of the viscosities and the ratio of the thickness between two layers are, the faster the stress concentrates in the layer with greater viscosity. The result shows that under the same boundary condition, if the accumulation of stress for one-layer homogenous model needs 950 years, then for a two-layer visco-elastic model (with the ratio of the two layers of 1/100 and the viscosities of 1018 Pa-s and 1022Pa-s, respectively), it needs only 38 years. The above results prove that the stress could be transferred from plate boundaries to the inner part of the continent and concentrates in the crust caused by creep in underlying ductile layers of the lithosphere.In order to understand the regional media of the China's continent, the crustal heat production is inferred from seismic velocity (vp) by using the Cermak's method(1982, 1989, 1995). The data needed are collected from several published geoscience transects, deep seismic sounding profiles and the three compiles of published heat flow data. The physical parameters for the active blocks in China, such as the viscosity, temperature and Young's module for individual crustal layers are calculated, and the corresponding isoline maps of these parameters are compiled.The strain energy background map released from strong earthquakes before several periods is made using time, location and degree of the earthquakes as the function, to emphase the effect that earthquakes exerts on the properties of the crust.Based on the above analyses, a 3D non-uniform visco-elastic finite element model is set up to simulate the variations of the stress filed in the continent. In this model, diverse properties of media are given to the crust of the lithosphere according to the media calculations of the China's continent. According to the earthquake activities on plate boundaries, the fluctuation of movement at plate boundaries is analyzed, and then the boundary conditions of the model are adjusted correspondingly according to the result. And the crust media are adjusted slightly to accord to the strain energy background released from the strong earthquakes before a certain time, which represents the effect that form...