Coupled deformation-diffusion effects on water-level changes due to propagating creep events

Abstract

Water-well-level fluctuations associated with episodic creep are studied using a coupled deformation-diffusion solution for the pore pressure produced by a plane-strain shear dislocation moving steadily at a speedV in a linear elastic, saturated porous medium. For largeVr/2c, wherer is distance from the dislocation andc is diffusivity, the solution approaches the form of the uncoupled elastic solution used by Wesson (1981) to analyze water-level changes due to creep events. The differences between the two solutions are significant within 10 diffusion lengths (20c/V) from the fault plane. More specifically, the pore pressure predicted by the coupled solution reverses sign behind the dislocation and is much smaller in magnitude than that predicted by the uncoupled solution. For an undrained Poisson ratio of 0.3, Skempton's coefficient of 0.8 and a shear modulus of 30 GPa, the coupled solution predicts a peak pore-pressure change of 13.7 kPa (137 mbar) per millimeter of slip forV=1 km/day andc=1.0 m2/sec. The spectrum of the coupled solution is limited to a band of frequencies, centered at a value proportional toV and approximately inversely proportional to the distance from the observation point to the fault plant. Thus, close to the fault plane the frequency band occupied by the coupled solution may lie above the range at which water wells can respond. The coupled solution is used in interpreting the same creep-associated water-level change observed by Johnson (1973) and modeled by Wesson (1981) using the uncoupled solution. Although there are uncertainties in properties of the rock material and in the speed of the creep event, the coupled solution predicts a water-level change comparable in magnitude to the observed change.