HR: 0800h
AN: T11A-0352    [Abstracts]
TI: Flash Heating and Weakening of Crustal Rocks During Coseismic Fault Slip
AU: * Goldsby, D L
EM: David_Goldsby@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912, United States
AU: Tullis, T E
EM: Terry_Tullis@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912, United States
AB: During fault slip, rocks become heated by the dissipation of friction at transient, microscopic, highly stressed asperity contacts. For sufficiently high slip rates, this 'flash' heating may yield high temperatures and even melting of contacts, causing severe degradation of contact shear stress and dramatic reductions in fault strength. We have reported the results of high-speed friction experiments on a variety of crustal rocks, including quartz rocks, feldspar rocks, granite, gabbro, and calcite marble, for slip rates up to 0.36 m/s and slips of <0.045 m, conditions conducive to extreme heating at asperity contacts but insignificant heating of the entire fault surface. Above ~0.1 m/s, such tests reveal an inverse relationship between friction and slip velocity, and slip weakening distances are on the order of the presumed contact size, in agreement with theory, for all rocks tested except calcite marble. Fits of constitutive relations for flash heating (Rice, 1999, 2006; Beeler et al., 2007) to experimental data yield extrapolated values of the friction coefficient of ~0.2 or less at a seismic slip rate of 1 m/s. Here we report results of high-speed friction experiments on quartz rocks that have larger initial surface roughness, resulting from grinding with #24 grit alumina powder, than smoother quartz rocks tested previously, roughened with #60 and #120 grit alumina. Despite the expectation that flash heating of the rougher samples should be greater - due to larger asperity contacts - experiments on the rougher samples reveal no weakening at slip rates up to 0.36 m/s. The stark contrast in behavior of rough and smooth samples may result from increased interlocking of asperities for the rough samples, resulting in a thicker generated gouge zone. If the gouge layer undergoes distributed shearing, then the contact-scale slip velocity is less than the far-field velocity by a factor N, the number of gouge particles across the gouge layer; equivalently, the weakening velocity for flash weakening is increased by a factor N. Estimation of an apparent weakening velocity for distributed shear of modest gouge thicknesses (<1 mm) using lab-like contact dimensions suggests values of the weakening velocity greater than seismic slip rates, i.e., >1 m/s. Our results emphasize the critical roles of contact size and the degree of slip localization in determining whether dynamic fault weakening due to flash heating occurs during earthquake slip.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics (1242)
DE: 8010 Fractures and faults
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2007 Fall Meeting