HR: 09:00h
AN: S51B-05    [PDF]
TI: Flash Melting of Crustal Rocks at Almost Seismic Slip Rates
AU: Tullis, T E
EM: Terry_Tullis@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AU: * Goldsby, D L
EM: David_Goldsby@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AB: Frictional heat is generated at microscopic contacts between rough surfaces in sliding contact. At high slip velocities over small displacements, as occurs during small earthquakes and at the onset of slip during larger earthquakes, heat generated at highly stressed, microscopic asperities on the fault surface can induce flash melting of the asperities. With continued slip, heat generated at contacts can raise the average fault surface temperature sufficiently to melt the entire fault surface. Both mechanisms may lead to a decrease in shear resistance during earthquakes, and thus have important implications for changes in dynamic stress drop and apparent stress with earthquake size. To investigate flash melting phenomena in rocks, friction experiments were conducted on monominerallic quartz and feldspar rocks, and granite, at near-seismic slip velocities (up to 360 mm/s) but short displacements ($<$ 4.5 cm), i.e., at conditions conducive to flash, but not bulk, melting. Tests were conducted in rotary shear at ambient pressure and temperature at a normal stress of 5 MPa. Each experiment was begun by sliding slowly at velocity {\it V}=10 $\mu$m/s for 2 to 3 mm of slip, then at a velocity of up to 360 mm/s for $\sim$4 cm of slip. At 10 $\mu$m/s, the friction coefficient attains values of 0.6 to 0.8 for all three rocks. At higher velocities, the friction coefficient for quartz and feldspar rocks is purely velocity dependent and falls off as 1/{\it V} above a characteristic weakening velocity {\it V$_{w}$}. The lowest values of the friction coefficient, at 360 mm/s, were 0.25 and 0.45 for quartz and feldspar rocks, respectively. For quartz rocks, the onset of weakening occurs at {\it V$_{w}$}=105 mm/s and for feldspar rocks at 267 mm/s. In contrast, experiments on granite show no significant decrease in friction even at slip speeds of 360 mm/s. Results for quartz and feldspar rocks were compared with predictions of a theoretical model for flash heating/melting ({\it Rice}, 1999). In the model, the weakening velocity {\it V$_{w}$}=({\it $\pi$$\alpha$}/{\it D})[{\it $\rho$c}({\it T$_{w}$-T$_{f}$})/{\it $\tau$$_{c}$}]$^{2}$, where {\it $\alpha$} is thermal diffusivity, {\it D} contact size, {\it $\rho$c} specific heat, {\it T$_{w}$} a weakening temperature above which the contact shear strength {\it $\tau$$_{c}$} is negligible, and {\it T$_{f}$} the average temperature of the sliding surface. Taking {\it $\alpha$}=1.85 (mm)$^{2}$/s, {\it $\rho$c}=2.9 MJ/m$^{3}$ K, {\it D}=25 $\mu$m, {\it T$_{w}$-T$_{f}$}=1700 and 1100 K (melting temperatures minus room temperature for quartz and albite, respectively), and {\it $\tau$$_{c}$}=7 and 3 GPa for quartz and albite, respectively, yields values of {\it V$_{w}$} for quartz and albite of 118 mm/s and 270 mm/s, respectively, in agreement with experimental values. Thus, the onset of weakening in our experiments appears to be consistent with flash melting above {\it V$_{w}$}. The absence of weakening in granite may indicate that flash temperatures are limited by lower contact stresses due to the presence of micas.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5104 Fracture and flow
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2003 Fall Meeting