A three-dimensional fluid-controlled earthquake model: Behavior and implications

We describe the behavior of a three-dimensional, fluid-controlled fault mod el that couples the dominant mechanical effects of fluid within a cellular fault zone with shear stress accumulation from constant plate motion applie d at the downward continuation of the fault. Improvements from a previous m odel include long-term plate motion loading and porosity creation through d ilatant slip, which allow the model to evolve to its steady state dynamic e quilibrium. The examined results include slip and slip-deficit accumulation , pore pressure buildup and release, stress states, the emergence of seismi c scaling relationships, and frequency-size statistics of model earthquakes . We find that asperities develop naturally within the model, reflecting th e disorganization of the evolving stress state in Mohr space. The dynamical interaction of shear stress and effective normal stress perturbs the initi al uniform stress state to a complex state that produces transient asperity development along the fault. These "Mohr-space" asperities spontaneously e volve, disintegrate, reemerge, and migrate along the fault plane. The gener al model behavior is independent of the state of the fluid pressure. In fou r examined cases, which span the range of possible fault zone overpressures , the equilibrium condition is that which occupies all of the available Moh r space. Maximum slip deficits along the fault depend on the degree of faul t weakness, ranging from about 3 m for a weak fault to over 30 m for a stro ng fault after 4000 years of model evolution. For events that breach the su rface the seismic moment scales with the cube of the source dimension M-o s imilar to L-3, which reflects the slipped area times the depth extent of th e rupture. This scaling crosses isolines of stress drop. For confined event s, M-o similar to L-2 along isolines of stress drop, but no general scaling emerges. Clusters emerge between stress drop versus seismic moment and str ess drop versus source dimension, with large events converging to average s tress drops of about 8 MPa for a weak fault and about 20 MPa for a strong f ault.