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Off-Fault Plasticity and Earthquake Rupture Dynamics, 2. Effects of Fluid Saturation

Robert C. Viesca, Elizabeth L. Templeton, & James R. Rice

Published September 16, 2008, SCEC Contribution #1196

We present an analysis of inelastic off-fault response in fluid-saturated material during earthquake shear rupture. The analysis is conducted for 2D plane strain deformation using an explicit dynamic finite-element formulation. Along the fault, linear slip-weakening behavior is specified, and off the fault the material is described using an elastic-plastic description of the Drucker-Prager form, which characterizes the brittle behavior of rocks under compressive stress when the primary mode of inelastic deformation is frictional sliding of fissure surfaces, microcracking and granular flow. In Part 1, materials bordering the fault were considered dry or at least effects of pore pressure changes were neglected. In Part 2, we more fully address the effects of fluid saturation. <br/><br/>During the rapid stressing by a propagating rupture, the associated undrained response of the surrounding fluid-saturated material may be either strengthened or weakened against inelastic deformation. We consider poroelastic-plastic materials with and without plastic dilation. During non-dilatant undrained response near a propagating rupture, large increases in pore pressure on the compressional side of the fault decrease the effective normal stress and weaken the material, and decreases in pore pressure on the extensional side strengthen the material. Positive plastic dilatancy reduces pore pressure, universally strengthening the material. Dilatantly strengthened undrained deformation has a diffusive instability on a long enough time scale when the underlying drained deformation is unstable. Neglecting this instability on the short time scale of plastic straining, we show that undrained deformation is notably more resistant to shear localization than predicted by neglect of pore pressure changes.

Key Words
pore pressure, poroelastic-plastic, shear crack, earthquakes, dilatancy, strike-slip faults

Viesca, R. C., Templeton, E. L., & Rice, J. R. (2008). Off-Fault Plasticity and Earthquake Rupture Dynamics, 2. Effects of Fluid Saturation. Journal of Geophysical Research: Solid Earth, 113, B09307. doi: 10.1029/2007JB005530.