Poroelastic Bimaterial Effects in Rupture Dynamics
(research report from SCEC 2006 annual meeting)
Objective: Investigate rupture along interface separating materials with different elastic and poroelastic properties. Non-uniform slip alters effective normal stress; sign of change reverses for opposite propagation direction. Mismatch need only be present over hydraulic diffusion length (several mm). Elastic bimaterial effect and poroelastic response can either oppose or enhance each other.
Poroelastic Response: Slip compresses material on one side of the fault and extends material on the other. When the strained material is poroelastic, pore pressure within this material either increases or decreases. Fluid diffuses across the fault to ensure continuity of pore pressure and fluid flux. Any asymmetry between the two sides (either in the magnitude of induced pore pressure change, as occurs when the elastic properties are mismatched, or in hydraulic diffusivities) results in alteration of pore pressure on the fault itself (Rudnicki and Rice, J. Geophys. Res., 2006). Assuming that faults obey the effective stress law &tau=f(&sigma-p), changes in p affect the shear strength &tau of the fault. The effect is similar to that from a mismatch in elastic properties, which alters normal stress &sigma. The two effects can either enhance or oppose each other.
Project Summary:

1. Stability analysis of steady sliding at constant coefficient of friction (not shown) Steady sliding at constant f is unstable for most seismologically relevant situations (any f, any poroelastic mismatch, and wave-speed mismatch less than about 30%).

2. Spontaneous rupture dynamics (shown in Figs. 1 and 2) The poroelastic response is incorporated into both our spectral boundary integral code, which has been generalized to the elastic bimaterial formulation, and into our staggered-grid finite difference code. The coefficient of friction obeys a (regularized) linear slip-weakening law in studies done thus far.

Fig. 1: Example from 2D spontaneous rupture calculation. This simple case of rupture between identical elastic solids, but with a contrast in poroelastic properties in damage fringes along the fault, illustrates the poroelastic response in isolation from changes in &sigma. Rupture, nucleated from an overstressed asperity in the center, becomes asymmetric since pore pressure increases to the left and decreases to the right.

Fig. 2: Plot of shear stress vs. slip (main figure) and pore pressure vs. time (inset) at two points, one in each direction. The asymmetry is clearly evident in that the strength drop is larger in the direction of increased p (and smaller in the direction of decreased p).

Last Modified: August 31, 2006