Poster #187, Seismology

Change in seismic velocity during laboratory triaxial stick-slip experiments

Kiran Pandey, Thomas H. Goebel, Taka'aki Taira, & Georg Dresen
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Poster Presentation

2021 SCEC Annual Meeting, Poster #187, SCEC Contribution #11222 VIEW PDF
Seismic velocity changes resulting from crack opening and closure within fault damage zones may be indicative of stress state and proximity to failure. However, such velocity changes are commonly extremely small and thus hard to resolve in the natural system at high temporal and spatial resolution. Here, we analyze subtle changes in seismic velocities in triaxial laboratory compression tests on pre-faulted samples. We use both direct phase picks and the coda part of the waveform which samples a broader region within the highly-scattering fault damage zones. Using this property of the coda wave, we measured high-resolution seismic velocity changes during the laboratory experiments. We used ho...mogenous Westerly granite samples with a precut fault at an angle of 30 degree to the loading axis. Coda Wave Interferometry (CWI) is applied to waveforms of active laboratory source pulses that were generated and recorded by an array of 8 pairs of piezoceramic transducers. The sample was subjected to confining pressure increase from 2 to 120 MPa, followed by axial load and stick-slip. The results demonstrate that the coda waves are sensitive to the change in seismic velocities that were not sensed by the direct phase arrivals. When comparing consecutives waveforms, lag times were observed to be non-uniform and increased as we progress towards the coda portion of the seismic wave. The coda part shows that the velocity change is highly-correlated with the applied confining stress and it samples a broader volume of the medium that is more sensitive to bulk confining pressure increase. The use of CWI in laboratory experiments may help quantify other frictional and fracture processes associated with labquakes.