HR: 09:30h
AN: T21D-06    [Abstracts]
TI: The Role of Silica Content in Dynamic Fault Weakening Due to Gel Lubrication
AU: Roig Silva, C
EM: isogyre@hotmail.com
AF: Department of Geology, University of Puerto Rico-Mayag\"{u}ez, Box 9017, Mayag\"{u}ez, PR 00681 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
AU: Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Dipartmento di Geologia, Paleontologia e Geofisica, Via Giotto 1, Padova, 35137 Italy
AU: Tullis, T E
EM: Terry_Tullis@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook Street, Providence, RI 02912 United States
AB: Little is known about the frictional behavior of rocks at sliding speeds ($\sim$1 m/s) and slip distances (1-10 m) characteristic of earthquakes, despite the importance of knowing the magnitude of the shear stress during seismic slip for understanding dynamic stress drops and strong ground motions. Recent experiments have demonstrated that the friction coefficient $\mu$ for monominerallic quartz rocks decreases to values as low as 0.1 at slip rates up to 0.1 m/s over meters of slip ({\it Goldsby and Tullis}, 2002; {\it DiToro et al}., 2004). Amazingly, the trend of $\mu$ versus log velocity for quartz rocks extrapolates to a value of zero at 1 m/s ({\it DiToro et al}., 2004). The mechanism deemed responsible for the low friction of quartz rocks is the formation of hydrated silica ('silica gel') on the sliding surface, which acts as a lubricant, lowering the shear resistance. Here we investigate the high speed sliding behavior of other important crustal rocks and assess the role that silica content plays in 'silica gel' formation. Tests were conducted on gabbro ($\sim$50 wt.% SiO$_{2}$), Tanco albite (68.6 wt.% SiO$_{2}$) and Westerly granite (69.2 wt.% SiO$_{2}$) in a 1-atm rotary shear apparatus. In each test, an annulus 54 mm in outer diameter and 45 mm in inner diameter was rotated against a flat circular plate of the same rock, at a constant normal stress of 5 MPa. Tests were begun by sliding at 10 $\mu$m/s for several mm of slip, followed by sliding at a constant velocity in the range 1 mm/s to 0.2 m/s for a displacement of $\sim$4 m. The value of the friction coefficient for all three rocks is $>$0.7 at 10 $\mu$m/s, in agreement with previous studies. A large value of $\mu$$\sim$0.8 is observed for gabbro at all velocities, whereas $\mu$ decreases dramatically with increasing velocity for granite and Tanco albite above 1 mm/s. At a given velocity, friction for granite and Tanco albite decreases rapidly over the first $\sim$0.2 to 1 m of slip (similar in magnitude to the slip weakening distances inferred from seismological data) followed by 'steady state' shear resistance. Trends of 'steady state' $\mu$ versus log velocity for granite and Tanco albite extrapolate to values of $\sim$0.3 and $\sim$0.35, respectively, at a seismic slip rate of 1 m/s. The friction coefficients for gabbro, granite, feldspar and quartz rocks decrease systematically with increasing silica content (for a given velocity and cumulative slip), suggesting that silica content plays a primary role in gel formation. Our results suggest that silica gel lubrication may be an important dynamic fault weakening mechanism during many crustal earthquakes.
DE: 8010 Fractures and faults
DE: 8100 TECTONOPHYSICS
DE: 8164 Stresses--crust and lithosphere
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting