HR: 08:25h
AN: S41B-02 INVITED     [PDF]
TI: Friction Falls To Zero At Seismic Slip Rates
AU: * Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Universita' di Padova, Via Giotto 1, Padova, 35137 Italy
AU: Goldsby, D L
EM: David_Goldsby@brown.edu
AF: Brown University, 324 Brook Street, Providence, RI 02912-1846 United States
AU: Tullis, T E
EM: Terry_Tullis@brown.edu
AF: Brown University, 324 Brook Street, Providence, RI 02912-1846 United States
AB: Determination of the resistance to slip on faults in the Earth's crust during earthquakes is an important unsolved problem in earthquake mechanics. Knowledge of coseismic slip resistance is critical for understanding the magnitude of shear-stress reduction and hence the near-fault acceleration that can occur during earthquakes, which affects the amount of damage that earthquakes are capable of causing. To investigate the frictional properties of rocks during coseismic slip, we conducted a series of high-speed frictional sliding experiments in a 1-atm rotary shear apparatus at a normal stress of 5 MPa on quartz rocks. Samples were slid at slip velocities {\it v} from 0.001 mm/s to 100 mm/s, for displacements characteristic of large earthquakes (up to 4.5 m). For $0.001 0.75$, as has been observed previously in low speed sliding speed experiments. For $3< v<100$ mm/s, an extraordinary progressive decrease in frictional resistance of quartz rocks occurs with increasing slip speed (d $\mu_{ss}$/ d log$_{10}v = -0.2$), with friction extrapolating to zero at seismic slip rates (actually at $v = 1.2$ m/s). Measurements of the temperature close to the slip surface, estimates of the average sliding surface temperatures by numerical simulations, and estimates of flash temperatures at the asperity contacts all indicate temperatures too low to induce melting, even locally. Instead dramatic weakening appears to be due to the formation of a thin layer of silica gel on the fault surface produced by ultracomminution of quartz in the presence of water (i.e. from air humidity and fluid inclusions). The observation that many seismic faults are mineralised with quartz even when the country rock is not quartz-bearing suggests that quartz behaviour could be important for earthquakes in many fault zones and that, during large earthquakes, the Earth's crust might be weaker than is commonly assumed.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 8045 Role of fluids
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting