Abstract: There are two modes of sudden instabilities in rocks,i.e.low-and high-pressure sudden instabilities.The substantial difference in mechanism between them is the "nonuni-form-" or "uniform-resistance" during sliding,and the asperity of sliding surface is one of the important factors which results in the non-uniform resistance and influences the mode of instability.The corresponding criteria for different modes of instabilities have been established in this paper to reflect the effects of environmental stiffness,contact stiffness and nonuniformity of sliding resistance.The strong shallow earthquake is principally a process of high-pressure sudden instability.It is emphasized that apart from the quasi-static strength condition the environmental stiffness,the uniform-resistance and other conditions are necessary for the occurrence of strong earthquake.