Benson, P. M.; Pozzi, G.; Guerin-Marthe, S.; De Paola, N.; Nielsen, S. B.; Tomas, R.
We present initial pilot data from a newly developed apparatus designed to measure Acoustic Emissions (AE) during the shear of fault gouges to 25 MPa normal stress and up to a maximum speed of 1 m/s, simulating dynamic earthquake processes. The sample assembly consists of a titanium-vanadium alloy (Ti-alloy, Ti90Al6V4) anvil fitted with 6 ports on the lower (stationary) section for AE sensors that record the activity of the shearing occurring in the gouge layer above. AE data are amplified from between 6 to 70 dB and recorded to disk continuously at a sampling rate of 10 MHz; calibration tests with Teflon shims confirm that the machine noise is negligible. Gouge thicknesses of approximately 2 mm are used, confined with a Teflon ring. Here we focus on Gypsum gouge from the Volterra region of Italy, sieved to give a constant gouge range of between 63 to 90 micrometers. Mechanical data show the onset of weakening after a slip of 1-3 cm for velocities of v = 100 to 1 cm s-1 respectively. Microstructural observations reveal a shear zone bounded by sharp mirror surfaces, and the development of a dehydration front, which is likely to have produced small pockets of water. We also record a characteristic `pulsing' AE signal generated after shearing is arrested, manifested as a series of energy spikes occurring at regular intervals. However, these signals are only generally seen for shear tests conducted on gypsum gouges (not in anhydrite) at 10cm per second or higher. Taken together, we interpret these observations as evidence that the initial shearing generated a thin slip zone that heats up rapidly, generating the dehydration front. Once motion ceases, pockets of trapped pressurized water combined with thermal stress generates distributed micro-fracturing detected as an initial swarm of high energy AE, and allows fluids to vent in pulses to the ambient atmosphere. An initial seismic -b value analysis of the continuous AE waveform also supports these initial findings.