HR: 09:30h
AN: S11E-07 [Abstracts]
TI: Self-Healing Slip Pulse Driven by Shear Heating of a Fluid-Saturated Fault Zone
AU: * Garagash, D I
EM: garagash@dal.ca
AF: Dalhousie University
Dept of Civil and Resource Eng, Sexton Campus
1360 Barrington Street, D215, Halifax, NS B3J 1Z1, Canada
AB:
Shear heating in a fluid-saturated fault zone results in an increase of pore pressure p and corresponding
reduction of frictional resistance τ=f(σ-p) when dilatancy and transport of pore fluid and heat away from
the shearing fault core are limited. Influence of these effects on propagation of a self-healing slip pulse are
examined in two limit cases when (I) frictional heat and pressurized pore fluid are trapped inside the shearing
fault core of finite thickness h (small compared to the length of the slipping patch ℓ); and (II) all frictional
heat is dissipated from the shearing fault core to the surroundings, rendering irrelevant the thickness of the fault
core, h=0.
Solutions for a steady propagation of a self-healing slip pulse driven by uniform remote stress τ∞ are
considered in the framework of antiplane elastodynamics; friction coefficient f and normal stress σ are
uniform and their alterations with the slip are neglected; shear deformation and corresponding temperature and
pore pressure alterations are uniform across the fault core thickness.
In the case of adiabatic, undrained slip (case I) the slip pulse solution corresponds to the gradual frictional stress
drop from the maximum value τo=f(σ-po) at the leading rupture edge to a value below the remote
stress τ∞ at the trailing edge. Behind the trailing edge, consistent with the no-slip requirement, the
recovery of shear stress takes place slower than that of the frictional resistance due to the pore pressure diffusion
in the trail of the pulse. We find that distribution of the shear stress and slip with the distance from the leading
edge normalized by their respective characteristic values τo, δc=(ρ c/fΛ)h, and
ℓc=(μ/τo)δc, depends on a single parameter - stress ratio τ ∞/τo. (Above, ρ c
and Λ=dp/dT are effective heat capacity and undrained thermal pressurization factor of the fault gouge,
respectively, and μ is elastic shear modulus of the surrounding rock). Furthermore, the normalized length of
the slipping patch is a function of the pulse velocity fraction β<1 of the shear wave speed and of the stress
ratio: ℓ/ℓc=\sqrt{1-β2}B(τ∞/τo), where preliminary numerical results for B suggest that it
is a monotone function of its argument with B(1)=2.32 and B(0)=+∞. We observe, that (i) generally, the
length of the pulse is decreasing function of the propagation velocity, vanishing when the latter tends to the shear
wave speed limit; (ii) at a fixed value of pulse velocity β, pulse length ℓ, slip rate, and accumulated slip
increase with decreasing remote stress (consistent with increasing value of the stress drop over the duration of
the pulse), diverging at zero stress. The latter zero stress limit considered at a fixed length of the pulse implies
that the propagation velocity approaches the shear wave speed. Furthermore, with decreasing stress ratio, the
shape of the slip-rate distribution evolves from the symmetric to highly antisymmetric with the peak approaching
the leading edge.
In the case of slip on a plane, h=0, (case II), when diffusion of temperature and pore pressure from the slip
plane to the surroundings is dominant, we prove that no slip pulse solutions are possible in the considered
idealized model. The latter suggests the thickness of the fault core may play a crucial role in the development of
self-healing modes of slip.
DE: 3200 MATHEMATICAL GEOPHYSICS (0500, 4400, 7833)
DE: 7209 Earthquake dynamics (1242)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Seismology [S]
MN: 2007 Fall Meeting