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Poster Abstracts | Group 2 – FARM
Model calculations with isothermal pore-pressure diffusion in a homogeneous medium are
explored in the limit of vanishingly thin shearing zone. We have developed finite difference codes
to couple pore-pressure diffusion with elasticity and friction. In the homogeneous diffusion case,
dilatancy does not separate from the transport properties and there is no threshold for fast slip, as
exists in the membrane diffusion case. Slow slip is favored by low effective stress and hydraulic
diffusivity, as expected.
Dimensional analysis of the thermal-dilatancy-friction problem in the limit of vanishing shear zone
thickness shows that the ratio of dilatancy to shear heating efficiency scales with the dilatancy
parameter, and inversely with nominal friction, effective stress squared, and pore-compressibility.
This supports the idea that slow slip is favored by low effective stress. Increasing hydraulic
diffusivity decreases the dilatancy efficiency, but also diminishes the coupling of temperature
changes to pore-pressure.
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LABORATORY EXPERIMENTS TO UNDERSTAND DYNAMIC SLIP WEAKENING IN
ROCKS AND ANALOG MATERIALS AT CO-SEISMIC SLIP RATES Yuan F, and
Prakash V
In the present study plate-impact pressure-shear friction and the modified torsional Kolsky bar
friction experiments are employed to investigate the frictional slip resistance in fine-grained
Arkansas Novaculite rock, quartz and soda lime glass, at relevant normal stresses and co-seismic
slip rates. The motivation of these experimental studies is to gain a better understanding of
dynamic fault weakening due to flash heating of asperity contacts, so as to further delineate the
conditions for which this mechanism is expected to control fault strength. The results of the plate
impact experiments on soda-lime glass indicate that a wide range of frictional slip conditions exist
at the frictional interface. These range from initially no-slip, followed by slip-weakening, slipstrengthening,
and eventually seizure, all during a single slip event. The initial slip-weakening is
most likely due to thermally-induced flash-heating and incipient melting at asperity junctions, and
requires only a fraction of a mm of slip to be effective; the slip strengthening is understood to be
aided by the coalescence and solidification of local softened/melt patches, which leads to
continuous healing and eventual seizure of the slip interface. The maximum bulk temperature rise
is attained at the frictional interface, and occurs during the slip strengthening phase (prior to the
seizure of the interface). It is to be noted that during the slip-weakening phase, even though the
bulk temperature rise is small, the flash temperatures at the asperity contacts are expected to
approach near-melt temperatures of soda-lime glass. As slip precedes these soft near-melt asperity
junctions continuously increase in size by local plastic flow leading to an increase in effective area
of contact, leading to healing and eventual seizure of the slip interface. Seizure of the interface is
also aided by the increase in shear-strength of the flattened asperity junctions as they are rapidly
quenched by the surrounding lower temperature material. Re-initiation of slip occurs with the
drop in normal stress, leading to considerable frictional heat generation. It is interesting to note that
the “healed-state” is much stronger than the initial state, with the coefficient of friction being in
excess of 0.8 after the re-initiation of slip. The Kolsky bar experiments show similar slip
characteristics as those observed in the case of the plate impact friction experiments. The coefficient
of kinetic friction during the early part of slip shows slip weakening and then decreases from an
average value of 0.31 to 0.15; following this initial slip weakening the friction coefficient increases
to 0.26 with an increasing slip distance -- indicating slip strengthening.
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