Radiated Wavefields Emitted from Lab-Generated Stick-Slip and Acoustic Emissions in Simulated Faults

In laboratory frictional sliding experiments, both stick-slip and acoustic emissions (AEs) are recorded which are useful analogues for earthquakes and microseismic behaviour observed on natural faults. Moreover, in nature, a range of slip behaviour, from aseismic creep to earthquake slip, is seen but is sometimes difficult to related to particular fault properties. Laboratory studies are advantageous in this regard to investigate how changing fault properties such as mineralogical composition can impact slip and AE behaviour, as conditions can be carefully controlled and quantified.

In this study we conduct direct shear tests on geological analogue materials involving bare surfaces of PMMA (poly methyl methacrylate), as well as rate-strengthening clay and rate-weakening quartz gouges at 50 MPa effective stress. We investigate the role of these different materials in producing different slip behaviour that includes stick-slips and stable sliding. In particular the PMMA produces a rich variety of behaviour that may be related to the low elasticity of the material that allows events to fully nucleate within the sample volume. Additionally, we record the waveforms produced by both stick slip (if present) as well as AE. These are measured using piezoelectric crystals on both ends of samples, held within a sample assembly. Following absolute acoustic sensor calibration to remove instrument and path effects from measured waveforms, the frequency content of events is investigated. For stick-slip events, the source properties, including seismic moment and stress drop, are can be obtained.

Preliminary results on the frequency content of AE suggests that velocity weakening material have different frequency spectra than velocity strengthening gouges. Future work aims to constrain further the relationship between mineralogical composition and radiated wavefields from AEs and consider the implications at larger scales in natural fault zones.