Pearce, F.D. & Kattenhorn, S.A (2001)
Earthquake-Driven Fluid Redistribution Around Mechanically Interacting, Segmented Normal Faults: Predictions of 3-D Numerical Models
Observed hydrologic responses to major normal fault earthquakes demonstrate that such seismic events significantly perturb the regional groundwater flow field. We attempt to characterize this process in three dimensions using numerical models of the hydrologic response to slip along segmented normal fault systems. We propose that sudden perturbations in mean normal stresses adjacent to mechanically interacting fault segments induce pressure gradients that provide the primary driving force for fluid migration. Such fluids migrate from regions of increased mean normal stress (i.e., more compressive) to regions of reduced mean normal stress. Accordingly, we predict the three-dimensional fluid flow direction to be perpendicular to mean normal stress contours in the direction of mean normal stress reduction.
We used a three-dimensional boundary element numerical model based upon the principles of linear elastic fracture mechanics to calculate perturbed mean normal stress distributions for both individual and segmented semi-elliptical normal faults embedded in an elastic half-space having material properties and boundary conditions appropriate for extending crustal rocks. Fault surfaces were assumed to represent low-permeability zones that inhibit fluid movement across the fault plane. For an isolated normal fault, the model predicts regions of increased mean normal stress near the Earth's surface in the footwall and near the bottom of the fault in the hanging wall. Reductions in mean normal stress occur near the fault tips at the surface and near the bottom of the fault in the footwall. The three-dimensional distribution of the perturbed mean normal stresses is such that near surface fluids in the footwall will migrate downward towards the lower tip line. Conversely, fluids in the hanging wall will flow from the deepest portions of the fault up towards the surface and outward towards the lateral tips.
Segmented normal fault model results resembled those of isolated faults in the regions away from the relay zone separating the fault segments, however, stresses close to the relay zone are influenced by both overlap ratio (ratio of overlap distance to fault length) and perpendicular spacing. Underlapping geometries tend to focus fluids towards the relay zone whereas overlapping geometries expel fluids into the hanging wall region adjacent to the relay zone. As perpendicular spacing decreases in overlapping faults, there is an increasing tendency for fluids to be forced through the relay zone from the footwall to the hanging wall of the fault system, converging towards the fault tip on the hanging wall side of the relay zone.
We compared the results of our analysis to the documented hydrologic response to the 1983 Borah Peak earthquake (M7.3) in south-central Idaho. Following this earthquake, spring flow increased almost exclusively in the hanging wall of the Lost River fault system and near the distal tips of the surface rupture. Using geodetic, geophysical, and field observations to constrain model input parameters, our predicted earthquake hydrologic response and fluid sink locations compared favorably to documented sites of increased fluid flow. We conclude that the perturbation of the mean stress field during slip along normal faults strongly controls post-seismic fluid flow. In the case of the Borah Peak earthquake, redistribution of fluids may have played a significant role in the distribution of aftershocks as certain regions experienced increases in fluid pressure and corresponding decreases in effective stress along potential failure planes.