HR: 09:10h
AN: T11E-05    [Abstracts]
TI: Pore Pressure Evolution in Shallow Subduction Earthquake Sequences and Effects on Aseismic Slip Transients -- Numerical Modeling With Rate and State Friction
AU: * Liu, Y
EM: liu@esag.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard Univeristy, 327 Pierce Hall, 29 Oxford Street, Cambridge, MA 02138 United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard Univeristy, 327 Pierce Hall, 29 Oxford Street, Cambridge, MA 02138 United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Division of Engineering and Applied Sciences, Harvard University, 224 Pierce Hall, 29 Oxford Street, Cambridge, MA 02138 United States
AB: In 3D modeling of long tectonic loading and earthquake sequences on a shallow subduction fault [Liu and Rice, 2005], with depth-variable rate and state friction properties, we found that aseismic transient slip episodes emerge spontaneously with only a simplified representation of effects of metamorphic fluid release. That involved assumption of a constant in time but uniformly low effective normal stress in the downdip region. As suggested by observations in several major subduction zones [Obara, 2002; Rogers and Dragert, 2003; Kodaira et al, 2004], the presence of fluids, possibly released from dehydration reactions beneath the seismogenic zone, and their pressurization within the fault zone may play an important role in causing aseismic transients and associated non-volcanic tremors. To investigate the effects of fluids in the subduction zone, particularly on the generation of aseismic transients and their various features, we develop a more complete physical description of the pore pressure evolution (specifically, pore pressure increase due to supply from dehydration reactions and shear heating, decrease due to transport and dilatancy during slip), and incorporate that into the rate and state based 3D modeling. We first incorporated two important factors, dilatancy and shear heating, following Segall and Rice [1995, 2004] and Taylor [1998]. In the 2D simulations (slip varies with depth only), a dilatancy-stabilizing effect is seen which slows down the seismic rupture front and can prevent rapid slip from extending all the way to the trench, similarly to Taylor [1998]. Shear heating increases the pore pressure, and results in faster coseismic rupture propagation and larger final slips. In the 3D simulations, dilatancy also stabilizes the along-strike rupture propagation of both seismic and aseismic slips. That is, aseismic slip transients migrate along the strike faster with a shorter Tp (the characteristic time for pore pressure in the fault core to re-equilibrate with that of its surroundings). This is consistent with our previous simulations, which show that the aseismic transients migrate along the strike at a higher speed under a lower, constant in time, effective normal stress. As a combination of the two factors, we show the pore pressure evolution with drops (due to dilatancy during slip) and then rises (due to shear heating) on the fault over multiple time scales. We next plan to formulate, and merge with the slip-rupture analysis, fuller fluid release models based on phase equilibria and models of transport in which the average fault-parallel permeability is a decreasing function of the effective normal stress. The thrust fault zone, at seismogenic depths and slightly downdip, is represented in a conceptually similar manner to the well-studied major continental faults, assuming the fault core materials have a lower permeability than the neighboring damaged zone. Heat diffusion in the fault core and damaged zone will also be considered in the modeling. The simulation results may help to improve our understanding of the processes of the aseismic transients observed within a transform plate boundary along the SAF near Cholame, California [Nadeau and Dolenc, 2005].
DE: 3225 Numerical approximations and analysis (4260)
DE: 5114 Permeability and porosity
DE: 8045 Role of fluids
DE: 8163 Rheology and friction of fault zones (8034)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005