This paper investigates models for the inception of earth faulting based on the deformaion of a rock mass containing an embedded weakened zone. Constitutive laws appropriate to dilatant, frictional, inelastic behavior are used to characterize the weakened zone material. Two distinct types of instability, corresponding to possible models of seismic mechanisms, are identified. These are 'localization' instabilities, at which no further quasi-static deformation is possible and inertial effects dominate. Conditions derived for the onset of these instabilities demonstrate that the amount of postpeak deformation in the weakened zone prior to instability is stronglyl dependent on the deviatoric state of stress induced with the weakened zone and on the de tailed nature of the inhomogeneities. In particular, instability is predicted much nearer to peak load for very narrow weakened zones and for states of deviatoric pure shear than for states of axisymmetric cpmpression. Hence, the premonitory events predicted by 'dry crack' precursor models, which associate crack closure with the postpeak regime, would be dramatically different for these two cases. More generally, systematic differences may be observed between strike slip and thrust type faults. A discussion of the qualitative effects of coupled streee-pore fluid diffusion on instability suggests a new interpretation of the dilatancy-diffusion model and indicates that premonitory events predicted by this model may also depend on the amount of postpeak deformation prior to instability.