Hydrodynamic response to strike- and dip-slip faulting in a half-space

Field observations have shown strong coupling between earthquake-induced stress-strain fields and subsurface hydrodynamics, reflected by water level change in wells and stream flow fluctuations. Various models have been used in an attempt to interpret the coseismic fluctuations in groundwater level, predict water table rise in the event of an earthquake, and explain stream flow variations. However; a general model integrating earthquake-induced stress-strain fields, coseismic pore pressure generation, and postseismic pore pressure diffusion is still lacking. This paper presents such a general framework with which one can approach the general problem of postseismic pore pressure diffusion in three dimensions. We first use an earthquake strain model to generate the stress-strain field. We then discuss the linkage coupling stress and strain with pore pressure and present an analytic solution of time-dependent pore pressure diffusion. Finally, we use two examples, a strike-slip and a dip-slip fault, to demonstrate the application of the analytical model and the effects of earthquakes on fluid now. The application to the two fault systems shows that the diffusion time is shorter than conventional estimates, which are based on a diffusivity and a length scale. We find that the diffusion time is predominately a function of the diffusivity of the system, while the length scale influences the magnitude of the initial pore pressure. A diffusion time based on the diffusivity and a length may be misleading because significant localized flow occurs in complex three-dimensional systems. Furthermore, the induced patterns of a pore pressure change resemble the strain field when shear stress effects are neglected but are significantly modified when shear stresses are included in the coupling relation. The theoretical basis of this work is developed assuming a single episode dislocation. However, the methodology and the results can be readily applied to studying pore pressure conditions after multifaulting events by simple superposition.