Abstract: In this paper, the earthquake process is taken as an unstable phenomenon of a mechanical system which consists of fault zone and elastic surrounding subjected to external disturbance. Based on the mechanics of continuum, a general mathematical description of a work-softening constitution law for the fault zone medium and an instability model of earthquakes are given. Vsing an instability model in an uniform field, we have discussed the concept of elastic rebound, derived a formula of earthquake efficiency, demonstrated the generality of the instability criterion in energy form and indicated the importance of acoustic emission investigation in a stiff testing machine. Finally, a general F. B. M. program for the study of earthquake instability in a generally non-uniform field is presented and an analysis of the 1976. Tangshan earthquake is used as an example.