HR: 0800h
AN: S51B-02    [Abstracts]
TI: Effects of Pore Fluids on Off-Fault Plasticity During Earthquake Rupture Propagation
AU: * Templeton, E L
EM: templet@fas.harvard.edu
AF: Harvard School of Engineering and Applied Sciences, 29 Oxford St., Cambridge, MA 02138, United States
AU: Viesca, R C
EM: viesca@fas.harvard.edu
AF: Harvard School of Engineering and Applied Sciences, 29 Oxford St., Cambridge, MA 02138, United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Harvard School of Engineering and Applied Sciences, 29 Oxford St., Cambridge, MA 02138, United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Harvard University Department of Earth and Planetary Sciences, 29 Oxford St, Cambridge, MA 02138, United States
AB: We examine factors that determine the extent and distribution of off-fault Coulomb plasticity during earthquake rupture propagations in regions where pore fluids are present at full saturation. The dynamic finite element method, in the form of ABAQUS Explicit, is used with linear slip-weakening behavior along the fault. Material surrounding the fault is described by Drucker-Prager poroelastic-plastic properties which describe the brittle behavior of rocks under compressive stress when the primary mode of inlelastic deformation is frictional sliding on fissure surfaces and microcracking. The microcracking and local uplifts at sliding asperities that occur during brittle deformation are features that lead to microscopic dilatancy. We assume that when the Drucker-Prager yield criteiron is violated, the plastic deformation either has no hardening or positive hardening and allow the possibility of dilatant plastic strains. In plane strain, the Drucker-Prager model is coincident with the Mohr-Coulomb model when the out of plane normal stress is equal to the average of the two in-plane normal stresses, but it is not precisely coincident otherwise. We incorporate pore-fluid effects into the model assuming locally drained conditions on the fault and undrained conditions off the fault. The undrained pore pressure change is calculated from the poroelastic-plastic constitutive description in terms of a Skempton coefficient, B, times the earthquake-induced change in the mean compressive normal stress, with the additional effects of dilational plastic straining included too. The few- second time scale of significant stress pulsing near the rupture front implies that poroelastic fluid diffusion effects will be active over scales of only a few mm to a few cm, much shorter than the expected multi-meter scale lengths of the slip-weakening zone (whose size sets the scale of the region of large stressing rapid rupture), allowing the approximation of undrained conditions off the fault. When the material on each side of the fault has identical poroelastic properties and permeability, the assumption of no change in pore pressure on the fault can be made, although such conditions might not generally be met [Rudnicki and Rice, JGR, 2006]. The dilatant and pressure sensitive nature of brittle rock deformation results in elastic-plastic constitutive relations in which strain localization can occur in static situations, as shown by Rudnicki and Rice [JMPS, 1975]. Our results show localizations for cases where the critical hardening required, as predicted by Rudnicki and Rice, is greater than zero. These localizations can be supressed by prescribing hardenning above the critical value in the elastic-plastic material description. We extend the studies of Templeton and Rice, [Eos. Trans. AGU, 2006] and Viesca et al. [Eos. Trans. AGU, 2006] which investigate how off-fault elastic-plastic and poroelastic-plastic response is controlled by the initial stress state, in the form of Ψ, the angle that the most compressive stress makes with the fault, and the seismic S ratio, to include the effects of dilatant plastic straining, and we investigate a range of Skempton coefficients. We show the effects of dilation and B for high and low initial angles of most compressive stress, Ψ on the extent of plastic straining and the residual stress state in the region where plastic deformation has occured. In the plastic zone, the residual fault-parallel stress is altered from its initial value close to the fault, and this change in stress can influence future rupture events.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics (1242)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Seismology [S]
MN: 2007 Joint Assembly