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Group 2 – FARM | Poster Abstracts
first developed for the friction of ceramics, and has already be reported by e.g. Senda et al., [1999].
This apparatus can slide a simulated fault at 1-500 mm/s for an annular sliding surface with 25 mm
and 15 mm outer and inner diameters. There is an induction coil around the sample assembly
which heats sample holders on which about 5 mm thick rock samples are fixed. A thermocouple is
attached to one of the sample holders about 7 mm from the sliding surface, and the measured
temperature can be controlled up to 1000 degreeC within 1 degreeC in accuracy.
Our preliminary result with the same gabbro as in Tsutsumi and Shimamoto [1997] indicates that
at 0.5 MPa normal stress and 20 mm/s slip rate, the friction coefficient is from 0.7 to 0.8 at room
temperature, decreases with increasing temperature down to 0.55-0.6 at 800 degreeC, and increases
to around 0.8 at 900 degreeC. The range of the friction coefficient agrees with Tsutsumi and
Shimamoto [1997] although the experimental condition is different; they changed the slip rate
without controlling the temperature accurately, and we fix the slip rate and control the
temperature. Our results illuminates the importance of the temperature during seismically rapid
fault sliding.
2-083
ON THE HIGH VELOCITY WEAKENING OF FAULT GOUGES Brown KM, and Fialko Y
We present new experimental data and theory that describe the thermal weakening of fine-grained
gouges during earthquake slip. We postulate that the particles in fine-grained gouges thermally
soften due to an intrinsic decrease in the elastic shear modulus in response to rapid heating of the
gouge layer described by a modified Watchman’s equation. In our initial thermally based model,
after slip has initiated and attained a critical velocity the velocity dependence of the effective
coefficient of friction results from the temperature dependence of the theoretical yield strength of
the contact asperities, rather than sudden loss of the asperity strength at some critical temperature.
Eventual contact melting can occur depending on the effective normal stress and displacement.
Our preliminary results indicate that there is a systematic evolution of the friction coefficient from
~0.6 to as low as 0.2 as velocities increase from 0.1 m/s to 2.5 M/s. The inferred power-law
exponent of the velocity dependence is ~ -0.4 depending on the normal stress, considerably smaller
than the exponent of -1 predicted by the flash weakening hypothesis (Rice, 2006). Our model
successfully explains a significant portion of the observed velocity-weakening relationship in terms
of the temperature dependence of the shear modulus (and, thus, contact shear strength). The model
accounts for the fact that the evolution of contact strength during slip depends on increases in both
the average shear zone temperature and transient contact temperatures. Inspection of the
experimentally produced gouge using SEM images indicates that grain sizes are likely to be power
law distributed the majority less than 1-5 µm in diameter. Thermal weakening is less robust than
predicted from the flash weakening because (1) the observed gouge are be too small to allow
adiabatic heating during transient contact, and (2) the asperity strength, and thus the efficiency of
frictional heating, decrease with increasing temperature. We also note some early evolutionary
weakening also occurs at rates that are too low for significant thermally activated weakening
processes probably due to fabric and other mechanical effects.
2-084
EFFECT OF PRESTRESS AND NUCLEATION PROCEDURE ON RUPTURE MODES
IN LABORATORY EARTHQUAKES Lu X, Rosakis AJ, and Lapusta N
We present experimental observations of pulse-like and crack-like rupture modes, and a systematic
variation between them, on Homalite interfaces prestressed both in compression and in shear,
similarly to faults in the Earth’s crust. A number of explanations for the existence of slip pulses
have been proposed, including velocity-weakening friction, bimaterial effect, and local
2008 SCEC Annual Meeting | 185