Publication Date:
2006
Description:
Earthquake nucleation requires reduction of frictional strength t = µ (s -
p) with slip or slip rate, where µ, s n , and p are the friction coefficient, normal
stress, and fluid pressure, respectively. For rate state µ at fixed (s - p),
instabilities can occur when d µ ss /dv 〈 0, where µ ss is the steady state
friction and v is slip rate. Shear heating increases p and, if dilatancy and pore
pressure diffusion are limited, will cause t to decrease. We examine how frictional
weakening, shear heating, and dilatancy determine stability in simplified fault models.
Mature faults have a thin (〈1 mm) shear zone on which slip is concentrated,
embedded within a ~0.1 m wide fault core with permeability of order 10-21 to 10-19 m2,
surrounded by rock of variable but higher permeability. Faults with dµ ss /dv 〉
0 are linearly stable at all wavelengths to adiabatic perturbations when v is near a
plate rate if the wall rock permeability exceeds a critical value that is orders of
magnitude less than inferred. Thus shear heating alone cannot then nucleate unstable
slip; frictional weakening is required. However, shear heating can produce inertial
instability on velocity strengthening faults following strong stress perturbations. On
faults with dµ ss /dv 〈 0, shear heating increases pore pressure faster than is
dissipated by Darcy flow at slip speeds of order 1 mm s-1. For faults bounding
half-spaces with uniform thermal and hydraulic properties, µ dot above p exceeds inline
equation (s - p) during nucleation for slip speeds in excess of 10-2 to 101 mm s-1,
depending on parameters chosen. Thus thermal effects are likely to dominate late in the
nucleation process, well before seismic waves are radiated, as well as during fast
seismic slip. By the time shear heating effects dominate, inertial slip is imminent
(~10-1 s), so that time-to-failure calculations based on rate state friction are not
biased by thermal pressurization.
Keywords:
Rock mechanics
;
Geothermics
;
Friction
;
Source
;
Fracture
;
7209
;
Seismology:
;
Earthquake
;
dynamics
;
8118
;
Tectonophysics:
;
Dynamics
;
and
;
mechanics
;
of
;
faulting
;
5114
;
Physical
;
Properties
;
of
;
Rocks:
;
Permeability
;
and
;
porosity
;
3215
;
Mathematical
;
Geophysics:
;
Instability
;
analysis
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