Characterization of acoustic emissions from laboratory stick-slip events in simulated fault gouge

It is now widely recognized that earthquake faults exhibit a variety of slip behaviors ranging from near instantaneous and devastating rupture to slow slip events that last weeks to months. Simulating these events in the laboratory, in the form of stick-slip shear experiments, provides an opportunity to probe fault slip in a well-controlled environment usually unavailable in nature. We report on a suite of laboratory earthquakes, consisting of stick-slip cycles that are probed using a dense array of P-polarized piezoelectric transducers (PZT) to detect acoustic emissions (AE). The stick-slip experiments are performed on layers of glass beads, used as simulated fault gouge and which exhibit reproducible, earthquake-like dynamic rupture and frictional instability. The experiments were performed in a servo controlled biaxial testing apparatus in a double direct shear configuration. The AEs are treated as laboratory earthquake proxies and analyzed to gather information about their frequency, magnitude, energy content and location. We monitor these source characteristics and report on their behavior during periodic and aperiodic stick-slips cycles. We also report on AE during stable, aseismic shearing. We find that grain scale roughness could explain the presence of periodicity in unstable sliding behavior through observations of AE nucleation frequency, spectral content and AE energy.