PRE-SEISMIC FAULT CREEP AND THE EVOLUTION OF ELASTIC WAVE PROPERTIES FOR SLOW AND FAST LABORATORY EARTHQUAKES (Invited Presentation)

Pre and post-seismic variations in elastic wave velocities and amplitudes have been documented in a limited number of laboratory experiments and, in a few rare instances, in nature. However, the physical controls for these velocity variations continue to be poorly understood. We report on double-direct shear friction experiments conducted on fine layers (<200 µm) of quartz gouge sandwiched between rough Westerly granite faults. We instrument our fault-zones with active ultrasonic monitoring, transmitting 500 kHz pulses through the faults, to study the evolution of elastic wave amplitudes and velocities throughout the laboratory seismic cycle for a spectrum of slip modes from slow to fast. We generate this spectrum of fault slip behaviors at a constant normal stress of 10 MPa by modifying the loading stiffness of the apparatus. We document a clear pre- and co-seismic reduction in elastic wave amplitudes and velocities during the lab seismic cycle. Post-seismically, we observe a logarithmic-with-time recovery of the elastic wave amplitudes and velocities which we attribute to frictional healing and contact-area growth, in agreement with previous works. We also document a pre-seismic reduction in the elastic wave amplitudes that precedes the precursory velocity reduction. By carefully measuring the fault slip, we conclude that this pre-seismic amplitude reduction is likely due to the onset of slow preseismic creep during the nucleation phase. In addition, we use dynamic acoustoelastic testing to independently study the effect of shear loading on pre-seismic velocity changes. We demonstrate that the pre-seismic velocity reduction is likely controlled by a combination of background shear loading and preseismic creep. Our results indicate that elastic wave amplitudes are sensitive to tiny changes in asperity distribution and fault state. On the other hand, elastic velocities are additionally sensitive to variations in the bulk dynamic elastic moduli. Our results illuminate the mechanisms that control the evolution of elastic wave properties throughout the laboratory seismic cycle. Additionally, we demonstrate the utility of controlled-source seismic experiments as a viable method for real-time monitoring of crustal fault zones.