HR: 0800h
AN: S41A-0925    [Abstracts]
TI: An Analysis of Fault Nucleation and Rupture in Westerly Granite Using Continuous Acoustic Emission Monitoring
AU: * Thompson, B D
EM: B.Thompson@liv.ac.uk
AF: Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow St, Liverpool, L69 3GP United Kingdom
AU: Young, R P
EM: paul.young@utoronto.ca
AF: Department of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow St, Liverpool, L69 3GP United Kingdom
AU: Young, R P
EM: paul.young@utoronto.ca
AF: Lassonde Institute, University of Toronto, 170 College St, Toronto, ON M5S 3E3 Canada
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: Determining the mechanics by which intact rocks fracture has significance in earthquake studies, in terms of understanding the failure of asperities or barriers on a fault. In order to study the transition from fracture nucleation to dynamic rupture, we performed triaxial compression tests on Westerly granite cores, using continuous Acoustic Emission (AE) monitoring. Previously, AE has been used to demonstrate the initiation and evolution of quasi-static fracture growth, and more recently, fast AE acquisition systems have provided information during nucleation stages of fracture in intact and pre-fractured samples. However triggered AE systems are limited by the inherent `mask' time, and the necessity of setting trigger levels. We report results from a newly developed Continuous Ultrasonic Waveform Acquisition System, named the Giga RAM Recorder. This system continuously streams 16 channels of 14-bit waveform data onto a 40 GB circular RAM buffer, which can be frozen at any point to capture the preceding 268 seconds of activity (assuming 5 MHz sampling frequency). Discrete AE events are then `harvested' from the continuous record following the experiment. In addition to continuous data, the system concurrently records conventionally triggered AE data throughout testing. Intact granite cylinders of diameter 76.2 mm and length 190.5 mm were loaded at a confining stress of 50 MPa. Two tests were conducted using constant displacement rates, one `fast' (10 x 10-6 m/s) and one `slow' (0.5 x 10-6 m/s). The `fast' loaded sample reached a peak stress of 665 MPa, followed by a total stress drop. In the three seconds prior to the stress drop, AE locations show nucleation and the initiation of a small fracture plane as the applied stress was increasing. For the `slow' loaded sample, peak stress of 642 MPa was followed by a 28 second period of post peak deformation prior to a complete stress drop. In this case, the AE locations show nucleation of the fault, which propagated over an area of approximately 70 mm x 20 mm, before dynamic rupture ensued. In a third test, we loaded a sample under highly sensitive AE feedback control, to initiate and quasi-statically propagate a fracture at increasing rates, over a period of 15 hours. In addition to AE locations, we have used Moment Tensor inversion to determine a sample of AE source mechanisms and have calculated b-values to demonstrate characteristics of fracture nucleation and propagation under these three loading regimes.
DE: 8010 Fractures and faults
DE: 5102 Acoustic properties
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting