关键词: 应力传播, 黏弹性, 大陆内部, 常速度, 时间尺度

Abstract: According to the researches of strong earthquake distribution in China's continent and its vicinity, many researchers proposed that the strong earthquake activities in China and its neighboring areas may concentrate mainly in a certain seismic region during a certain time period. Hong Han-jing(1997)suggested that the seismic activities in China and its surrounding areas in the past century can be divided into several micro dynamic periods, and in different periods there should be a relatively stable region of seismicity. Moreover, in different periods the features of strong earthquake distributions are different in space and the main seismic regions may migrate for a long distance. In the research of this large-scale seimogenic model, the transfer of stress in various layers of the lithosphere has attracted more and more attention of most researchers. The dynamic analy~ses demonstrate that it is difficult to explain the long distance migration of the main seismic regions in the micro dynamic periods by considering only the brittle layer of the crust. Therefore, the layered rheological structures of the whole lithosphere and the process of stress accumulation must be taken into consideration. The layered structure of the lithosphere has been given in many previous studies, and the rupture in brittle layer of the crust was proved to initiate at deeper layer of greater strength and propagate to the shallow layer of lower strength. Kusznir & Bott(1977)have introduced a simplified model of two layers visco elastic lithosphere, which comprises an upper layer with higher viscosity coefficient and a lower layer with lower coefficient. As a boundary condition, they applied uniform horizontal boundary stress at one side of the model and maintain the other side to be fixed. The simulation proved that after the application of stress at one side of the model the stress relaxes in lower part of the model and concentrates in the upper part as a function of loading time. However, there are various kinds of boundary conditions in the nature, and the movement of Plate boundaries is a dynamic process. Sometime plates move at a certain rate(such as the squeezing rate of the Indian Plate), but do not maintain a constant force on the adjacent plate. In this case, the stress on the boundaries of the plate is not a constant stress, but within a certain time scale and precision scale the velocity can be considered as a constant velocity. We modify, therefore, the boundary condition of the lithospheric model of Kusznir & Bott to constant velocity boundary condition, and give the analytical resolution. It is proved that if the thickness of high viscosity coefficient layer, the Young's modulus and viscosity coefficients of each layer are given, then the rate of stress accumulation in the high viscosity coefficient layer increase with increasing thickness of the high viscosity coefficient layer. According to the result, under the same boundary condition, if the accumulation of stress for one layer homogenous model needs 950 years, then for two layers visco elastic model(with the ratio of the two layers of 1/100 and the viscosities of 10¹⁸ Pa·s and 10²² Pa·s, respectively), it needs only 38 years. Obviously, the accumulation time of stress is significantly shortened. The above discussions prove that the low viscosity coefficient layer plays an important role in the transfer and concentration of stress in different media. This result may provide a new insight into the understanding of the short term geological problems. In a word, by analyzing the transfer condition of stress in lithosphere and the dynamic boundary conditions surrounding the continent, we suggest that under the effect of dynamic boundary conditions, the accumulation time of stress in lithosphere can be shortened significantly.

Key words: stress transfer, viscoelastic, continent, constant velocity, timescale