HR: 0800h
AN: T21B-0483    [Abstracts]
TI: Premonitory Acoustic Emissions and Slip Nucleation during Stick Slip Experiments in Naturally and Smooth (Saw-cut) Faulted Westerly Granite
AU: Young, R P
EM: paul.young@utoronto.ca
AF: Lassonde Inst, University of Toronto, 170 College St., Toronto, ON M5S3E3 Canada
AU: * Thompson, B D
EM: b.thompson@liv.ac.uk
AF: Lassonde Inst, University of Toronto, 170 College St., Toronto, ON M5S3E3 Canada
AU: * Thompson, B D
EM: b.thompson@liv.ac.uk
AF: Dept. Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. MS/977, Menlo Park, CA 94025 United States
AB: We compare premonitory microcracking and stick slip events for two Westerly granite samples containing a natural and a saw-cut (smooth) fault. The samples were 190.5 mm in length, and 76.2 mm in diameter. Stick slip events were induced by triaxial loading at 150 MPa confining stress, at a constant strain rate of 5 x10-6 s-1. One sample contained a polished saw-cut fracture (roughened using 220 grit), at 30 degrees to the sample axis. The second contained a natural fracture that had been previously (quasi-statically) induced by triaxial loading. The experiments were monitored using a novel continuous Acoustic Emission (AE) recorder. Three stick slip events were induced on the saw-cut sample, accompanied by fewer than 100 located AEs. Frictional coefficients were 0.48, 0.51, and 0.59 respectively. The first motion of each stick slip was recorded as a large-amplitude AE signal. These located on the saw-cut fault plane, and represent nucleation sites of the stick-slip events. Nucleation location varied between events, probably being controlled by stress heterogeneities or surface conditions on the fault. Prior to the first slip, AE located at the fault tips, with a small amount of premonitory creep. (Approximately 1 cm of each fault tip was removed when the fault was cut and these areas were packed with sand.) AE Source mechanisms were calculated using Moment Tensor inversion, and correspond to compressive sources, consistent with crushing of infill material about the fault tips. Prior to the second and third slips, no premonitory creep was observed. AE locations concentrate on a small patch within the fault plane. Source mechanisms are consistent with double couple events, orientated in agreement with the macro-scale fault plane geometry. For the natural fault, one stick slip event was induced, with a frictional coefficient of 0.72. Over 3000 AE were located. Again, the stick slip event can be located on the fault plane, in an area that had previously been acoustically quiet, presumably locked due to fault topography. Premonitory AE located mainly on the lower portion of the fault plane, and we did not observe an increase in AE rate immediately prior to rupture. Significant post-slip activity was located. b-values are constant at approximately 1.2 in the 100 seconds before slip. For the post-slip sequence there is an instant decrease (0.6) and subsequent recovery (1.0) in b-value. The surface topography generated considerable differences in premonitory, and post slip AE activity. However the ultrasonic signature of the slip events appears similar for both the saw-cut and natural fault. From an analysis of AE location error residuals, the initial nucleation size is estimated to be less than 3 mm in both experiments. However it is unclear whether we are resolving the size of the AE, or just its first particle motion; this depends on whether these AE can be modeled as dynamic or kinematic cracks (the latter refers to instantaneous motion of the whole source).
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics (1242)
DE: 8005 Folds and folding
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: Fall Meeting 2005