APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012;
Boston, Massachusetts
Session H26: Focus Session: Friction, Fracture and Deformation Across Length Scales - Friction Across Length Scales
8:00 AM–11:00 AM,
Tuesday, February 28, 2012
Room: 257B
Sponsoring
Units:
DCOMP GSNP
Chair: Izabela Szlufarska, University of Wisconsin
Abstract ID: BAPS.2012.MAR.H26.1
Abstract: H26.00001 : Rock Friction from the Nanoscale to the San Andreas Fault
8:00 AM–8:36 AM
Preview Abstract
Abstract
Author:
David L. Goldsby
(Brown University)
Nucleation of earthquakes (EQs) and the resistance of faults to shearing
during EQs are determined by nano-to-micro- scale frictional processes that
occur on tectonic-scale faults. A first-order observation from rock-friction
studies is that of ageing, i.e., the linear increase in friction with the
log of the time of stationary contact, manifest as a positive or negative
dependence of friction on sliding rate. A necessary condition for EQ
nucleation is a negative rate dependence of friction. In spite of the
success of friction `laws' which encapsulate the rate and time dependences
of friction in fitting experimental data and reproducing natural phenomena
in EQ models, these laws lack a physical basis. Atomic force microscope
(AFM) experiments on silica-silica contacts explore the physics of ageing,
more specifically increases in adhesion of nanometers-sized contacts with
time (Li et al., \textit{Nature}, 2011). The experiments reveal prominent ageing which
increases with humidity, as in rock friction tests, without increases in
contact area due to creep (the canonical explanation for ageing in
rock-friction tests). Ageing in the AFM tests is in fact much larger than in
rock-friction tests, a discrepancy explained with a simple multi-asperity
contact model.
At EQ slip rates ($\ge $1 m/s) a variety of dynamic fault-weakening
mechanisms may decrease the shear resistance of faults, which would have
important consequences for the magnitudes of EQ stress drops, strong ground
motions and accelerations, for the EQ energy budget, and for the state of
stress on faults. Experiments on rocks found in the Earth's crust for slip
rates up to $\sim $0.4 m/s over $\sim $40 mm of slip, reveal a dramatic
1/$V$ decrease in frictional strength above a characteristic weakening velocity
$V_{w}$ of $\sim $0.1 m/s (Goldsby and Tullis, \textit{Science}, 2011). Friction is also
revealed to be a nearly pure function of slip rate, i.e., it adjusts to the
ambient slip rate over only microns of slip. The observations are explained
by `flash heating', whereby microscopic asperity contacts become intensely
frictionally heated and weakened above $V_{w}$. Dramatically lower friction
due to flash heating may explain why heat flow along active faults like the
San Andreas Fault is much lower than expected. Strong velocity-weakening
friction and the rapid strength recovery with decreasing slip rate from
flash heating may explain why EQ ruptures propagate as slip pulses rather
than as cracks.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2012.MAR.H26.1