HR: 09:00h
AN: H21L-05    [Abstracts]
TI: Incorporating Undrained Pore Fuid Pressurization Into Analyses of Off-Fault Plasticity During Dynamic Rupture
AU: * Viesca, R C
EM: viesca@esag.harvard.edu
AF: Sch. Engin. Appl. Sci., Harvard Univ., Cambridge, MA 02138, United States
AU: Templeton, E L
EM: templet@fas.harvard.edu
AF: Sch. Engin. Appl. Sci., Harvard Univ., Cambridge, MA 02138, United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Dept. Earth Planet. Sci. and Sch. Engin. Appl. Sci., Harvard Univ., Cambridge, MA 02138, United States
AB: When considering dynamic fault rupture in fluid-saturated elastic-plastic materials, it is sensible to assume locally undrained behavior everywhere except in small diffusive boundary layers along the rupture surface. To evaluate undrained pore pressure changes, we consider here not just the linear poroelastic effect expressed in terms of the Skempton coefficient B, like in our previous work [Viesca et al., AGU Fall 2006], but also include plastic dilatancy, which, when positive, induces a fluid suction. We work in the context of Mohr-Coulomb-like plasticity, but with a Drucker-Prager type model. Plastic parts of strain increments are controlled by the Terzaghi effective stress, elastic parts by the Biot stress combination. Following earlier work of Rudnicki, the incremental elastic-plastic constitutive relation for undrained deformation has precisely the same form as for drained deformation, so long as we change the drained constitutive parameters into new undrained ones under transformation rules that we present. Spontaneous slip-weakening fault rupture is analyzed using the dynamic finite element procedures with ABAQUS Explicit, and undrained elastic-plastic properties. Results are shown for plastic zones and effects on rupture propagation, and how they are influenced by such parameters as B and ratio β of dilatant to shear plastic strains, for a range of principal orientations and magnitudes (relative to yield) of the pre-stress state. The undrained approximation must fail in diffusive boundary layers along the slip surface [Rudnicki and Rice, JGR 2006; Dunham and Rice, AGU Fall 2006] because the predicted pore pressures will be discontinuous at the fault. We show how to extend the Rudnicki and Rice calculation of the actual pore pressure on the fault in terms of the undrained predictions to the two sides. However, because of difficulties thus far in representing this within the ABAQUS program, all results obtained as of the time of writing neglect effects of such pore pressure changes on the fault slip-weakening strength during rupture.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 1822 Geomechanics
DE: 8004 Dynamics and mechanics of faulting (8118)
SC: Hydrology [H]
MN: 2007 Fall Meeting