Theory of Precursory Processes in the Inception of Earthquake Rupture

The paper reviews models for inelastic deformation processes preceding earthquake rupture in brittle, natural rock masses. The discussion is organized around three types of failure models: (i) localization bifurcations, whereby a uniform or nearly uniform pattern of deformation gives way to highly localized deformations within a fault zone; (ii) non-homogeneous deformations due to an inclusion-like zone of different mechanical properties than its surroundings, possibly representing a seismic gap, which is driven ultimately to a "runaway" dynamic instability; and (iii) a slipping region on an existing fault, which spreads on the fault plane under initially quasi-static conditions until limiting conditions for rapid shear-crack propagation are met. It is emphasized that failure conditions are sensitive to strain-softening behavior of nominally coherent rock or fault gouge, and that a general precursor is provided by the acceleration of local, by comparison to remote tectonic, deformations as instability conditions are approached. More striking precursors may arise from the presence of an infiltrating pore fluid. This may cause time-dependent surface-chemical effects, such as reductions of resistance to microcrack growth and post-fault strength recovery in compacted gouge, and also mechanical effects due to dilatant strengthening and time-dependent stiffness of elastic response at conditions intermediate between the drained and undrained limits. Some calculations of stabilization against abrupt earthquake rupture by these mechanical effects are given for the above models.