HR: 15:25h
AN: T13F-08    [Abstracts]
TI: Dilatancy Stabilization of Frictional Sliding as a Mechanism for Slow Slip Events
AU: * Segall, P
EM: segall@stanford.edu
AF: Geophysics, Stanford University, Stanford, CA 94305, United States
AU: Rubin, A
EM: arubin@princeton.edu
AF: Geosciences, Princeton University, Princeton, NJ 08544, United States
AB: Slow slip events and associated non-volcanic tremor have been discovered in a number of tectonic settings, yet the processes giving rise to these phenomena are as yet not understood. Transient slip in subduction zones appears to occur between the locked megathrust and the steadily creeping fault below, suggesting that slow slip occurs in regions near frictionally neutral stability. However, the transiently slipping zone must be large enough to allow non-steady slip but not so large that the rupture becomes dynamic. The size range for which transient, quasi-static slip occurs is small, particularly for the slip-law form of rate-state friction. We suggest that rate-state friction nucleates slip under drained conditions but that as slip accelerates deformation becomes effectively undrained, and dilatancy induced pore-pressure reductions quench the instability. We study this process assuming 2D elasticity, rate-state friction and the Segall-Rice [1995, JGR] constitutive law for dilatancy. Pore-pressure is treated mainly with simplified membrane diffusion: dp/dt = (p^∞ - p)/t_f + (1/β) d φ/dt, where p and p^∞ are fault and remote pore-pressure, t_f a characteristic diffusion time, β pore and fluid compressibility and φ fault zone porosity. For a step change in slip speed, v, the peak dilatant suction scales with (ε/β) \log (v θ/d_c) g(v t_f/d_c), where ε is the dilatancy parameter, θ the state in front of the rupture, and g(v t_f/d_c) a function of the ratio of diffusion time to that for state evolution. Using drained results of Rubin -Ampuero [2005, JGR] we find that the ratio of dilatant strengthening to frictional weakening scales with E \equiv f₀ ε/ β b (σ - p^∞), where f₀ is nominal friction and b the rate-state parameter. Indeed, numerical simulations with E ~ 1 exhibit slip that accelerates to limiting speeds well below inertial, followed by stable propagation. Simulations with E < ~ 0.1 accelerate to radiation damping limits. This suggests that stable slip is favored by low effective stress, consistent with some seismic observations. Transient sip nucleates at the down-dip end of the velocity weakening region, below which creep is imposed at v^∞, and propagates updip toward the locked region. We observe stable transient slip for an effectively unbounded range of W/h^* >~ 2, where W is the width of the velocity weakening region and h^* is the drained critical nucleation dimension. With increasing W/h^* the behavior transitions from periodic, to complex via period doubling. In the complex domain multiple slip events are spawned at the down-dip end, some overtaking early formed events. Increasing v^∞ t_f/d_c inhibits drainage, thereby increasing the time between events while decreasing their amplitude. We have also developed finite difference methods that allow for one-dimensional fluid diffusion normal to the fault. In this case we have obtained analytical solutions only by excluding elastic fluid storage in the shearing zone. The peak suction scales similarly to the membrane diffusion case, although the decay in pore-pressure change is far slower, consistent with numerical simulations.
DE: 7209 Earthquake dynamics (1242)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting