HR: 12:05h
AN: S12B-08    [Abstracts]
TI: Experimental observations of fault zone compaction during stick-slip sliding: implications for fault strength and stability
AU: * deMartin, B J
EM: Brian_deMartin@Brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02912, United States
AU: Tullis, T E
EM: Terry_Tullis@brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02912, United States
AU: Beeler, N M
EM: nbeeler@usgs.gov
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Goldsby, D L
EM: David_Goldsby@brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02912, United States
AB: The conditions favoring stable (aseismic) or unstable (seismic) sliding on a fault depend on the interaction between the frictional properties of the sliding surface and the loading conditions of the surrounding rock. Rapid volumetric changes within a fault zone during unstable sliding can affect loading conditions, enhancing or diminishing instabilities if pore fluids are present. For example, if dilatancy occurs during rapid slip within a fault zone containing pore fluid pressure and fluid pressure cannot be maintained, then pore pressure will drop and the effective normal stress will increase (dilatancy hardening). Fault zone dilatancy will thus act to limit the magnitude of the instability. Previous experiments have shown that higher slip velocity promotes dilatancy, whereas a reduction in shear stress promotes compaction. Evaluating which of these competing effects dominates the volume change of the fault zone during unstable slip is important in understanding the magnitude of the instability that may occur. In order to evaluate these competing effects, we measured the mechanical behavior of 2 mm thick layers of quartz gouge and bare-surface Fontainebleau sandstone samples during large-displacement experiments in our rotary shear apparatus. The experiments were conducted at 25°C, normal stress on the fault surface ranging from 25 to 89 MPa, confining pressures ranging from 24 to 88 MPa, and pore fluid pressures ranging from 0.1 MPa to 79 MPa. We studied the interplay between volumetric strain, shear stress, sliding velocity, and effective normal stress during stable and unstable sliding. At the start of the experiments, prior to the onset of localization and stick-slip behavior, increases in sliding velocity are accompanied by dilatancy as found by previous workers. As the experiments proceed and stick-slip sliding is initiated, we observe the opposite phenomena: net compaction during rapid slip and stress drop. During unstable sliding it thus appears that the reduction in shear stress that causes compaction overcomes the tendency for increases in slip velocity to cause dilatancy. These results suggest that dilatancy-hardening effects may play a secondary role during unstable slip and earthquake nucleation.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8168 Stresses: general
SC: Seismology [S]
MN: 2007 Fall Meeting