3D Finite Element Modeling of Slip on a Well-Oriented, Critically Stressed Fault due to Pore Pressure Increase

English Abstract:

Simulation of activation of slip on a pre-existing fault due to a perturbation in pore pressure is important for characterizing and understanding potential induced-seismicity hazards. We present a 3D model using a widely used finite-element package, ABAQUS, to simulate activation of slip on a pre-existing fault due to an imposed perturbation in pore pressure on a subregion of the fault. The model inputs are 3 initial principal background stresses, elastic properties of the medium, coefficient of friction and geometry of the fault. Initial stresses are assigned according to Mohr Coulomb theory based on the assumption that the crust is in a critically stressed state. Our proposed method is shown to be stable, consistent with displacement calculations using Okada’s analytical formula, and predicts rupture characteristics and stress changes that are generally compatible with observed values. Our results indicate that the fault rupture area is generally larger than the initially perturbed area; moreover, the rupture area, and thus the earthquake magnitude, increases with the size of the perturbed area, irrespective of the overall fault dimensions. For one realization, our model predicts that a 2km by 2km area perturbed by a relatively small pore pressure increase results in a magnitude 4.4 earthquake with 1 MPa (10 bar) of shear stress drop. For a particular fault geometry, stress state and perturbation parameters, we interpret our calculations to constitute an upper limit for earthquake magnitude, as the equivalent stress release could be achieved with a sequence of smaller events.