Numerical simulation of localisation phenomena in a fault zone

A lattice solid model was developed to study the physics of rocks and the nonlinear dynamics of earthquakes and is applied here to the study of fault zone evolution. The numerical experiments involve shearing a transform fault model initialised with a weak and heterogeneous fault zone. During the experiments, a fault gouge layer forms with features that are similar to those observed in recent laboratory experiments involving simulated fault gouge (BEELER ct al., 1996). This includes formation of Reidel (R-1) shears and localisation of shear into bands. During the numerical experiments, as in the laboratory experiments, decreases of the gouge layer strength correlate with decreases in gouge layer thickness. After a large displacement, a re-organisation of the model fault gouge is observed with slip becoming highly localised in a very narrow basal shear zone. This zone is such that it enhances the rolling-type micro-physical mechanism that was responsible for the low heat and fault strength observed in previous numerical experiments (MORA and PLACE, 1998, 1999) and proposed as an explanation of the heat flow paradox (HFP). The long time required for the self-organisation process is a possible reason why the weak gouge layers predicted by the numerical experiments, and which could explain the HFP, have not yet been observed in the laboratory. The energy balance of a typical rupture event is studied. The seismic efficiency of ruptures of the gouge layer is found to be low (approximately 4%), substantially lower than previous estimates and compatible with typical field-based estimates.