Modeling seismicity clustering and fault weakness due to high pore pressures

Abstract

An earthquake model that couples shear stress and zones of high pore pressure produces many features of observed seismicity patterns and geodetic observations. The model shows earthquake clustering, repeated events, large spatial and temporal variations in stress drop, complex slip distributions, and a power law distribution of sizefrequency. This dynamical model tracks the evolution of the stress state in a 2D fault plane of discrete rectangular dislocations subjected to heterogeneous increases in pore pressure and embedded in a 3D elastic solid. The source of increasing pore pressure can include a time-dependent porosity reduction mechanism or fluid sources from dehydration reactions. Increased fluid pressure within the fault plane reduces the effective confining stress until cells slip by simple frictional sliding. Changes in the fault plane shear stresses associated with a slipped cell are calculated using the solution for a rectangular dislocation in an elastic solid, and pore pressures are redistributed using a cellular automaton model of migrating fluid pressures. The frictional resistance of the system is shown to evolve non-linearly from a uniform shear stress on a strong fault, to a strongly heterogeneous shear stress distribution on a weak fault. Features of the non-linear temporal response of the system frictional resistance are discussed in terms of calculated seismicity patterns and increased seismicity clustering.