环境刚度效应——影响震源应力降的重要因素

固体围压岩石三轴实验和理论分析表明,震源环境刚度(包括轴压系统和围压系统刚度)是影响应力降的重要因素。在三轴实验条件下,轴向应力降和围压系统刚度呈双曲线关系,前者随后者的增大而减小。实验应力降显著偏大的基本原因之一在于环境刚度效应。地壳内震源应力降△τ=2λ(S_s—S_k)+(μ_s—μ_kσ_(n.0)],λ为震源的力学环境因数,主要取决于环境刚度和应力场方向。如果滑移过程中正压力大小和主应力方向无明显变化,则λ大致为0—0.9

ENVIRONMENTAL STIFFNESS EFFECT——AN IMPORTANT FACTOR INFLUENCING STRESS DROP IN HYPOCENTER

The results of the triaxial compression tests with solid confining pressure media and the theoretical analyses show that the "environmental stiffness", including the stiffness of both axial and confining pressure systems, is an important factor which influences the stress drop. Under the condition of triaxial compression tests the axial stress drop is a hyperbolical function of the stiffness of confining pressure system, and the former decreases with increasing the later. Thus, the problem, that the experimental stress drop is much greater than that estimated from an earthquake, may be explained.The expression of shear stress drop of hypocenter in the crust iswhere λ is the environmental factor which depends mainly upon the environmental stiffn- ess and the orientation of stress field. The values of the environmental factor may be 0-0.9 for the conditions without obvious change in the magnitude of normal pressure and the orientation of principal stresses.The environmental stiffness not only influences the magnitude of stress drop, but also determines whether the sudden stress drop will occur or not. Fully taking account of the effect of environmental stiffness and other factors such as temperature, confining pressure, pore pressure, orientation of stress field, fault materials, etc., it will finally be possible to recognize the total law of stress drop in hypocenter.It is also indicated in this paper that the direct shear tests and triaxial compression tests with gas or liquid confining media can only model the pressure condition of hypocenter, but cannot reflect completely the "environmental stiffness" which can be controlled to a large extent with solid confining media as presented.