Earthquake precursory effects due to pore fluid stabilization of a weakening fault zone

We report the analysis of two mechanisms by which pore fluids could partially stabilize the earthquake rupture process in natural rock masses. These mechanisms are based on dilatancy strengthening and on the increase of elastic stiffness for undrained as opposed to drained conditions. Both are studied in relation to an inclusion model in which a zone of strain weakening material, possibly representing a highly stressed seismic gap zone, is embedded in nominally elastic surroundings subjected to steadily increasing tectonic stress. Owing to the coupling between deformation and pore fluid diffusion, the inclusion does not exhibit an abrupt rupture instability; rather, a period of self‐driven precursory creep occurs which ultimately accelerates to dynamic instability. The precursory time scale is reported for a wide range of constitutive parameters, including fluid diffusivity, ratio of undrained to drained stiffness, and factors expressive of strain softening and dilatancy. Our conclusions are that the precursory times for a spherical inclusion of 1‐km radius are of the order of 15–240 days for a range of constitutive parameters that we suggest are representative. The predicted times are shorter by a factor of approximately 10 for a flattened ellipsoidal inclusion that we analyze with an 18 : 1 aspect ratio. It is suggested that perhaps only toward the latter part of the precursory period are the effects of accelerating inclusion strain detectable in terms of surface deformation or alteration of transport or seismic properties.