Frictional State Evolution During Normal Stress Perturbations Probed With Ultrasonic Waves

Fault normal stress changes dynamically during earthquake rupture; however, the impact of these changes on dynamic frictional strength is poorly understood. Here we report on a laboratory study to investigate the effect of normal stress perturbations on the friction of westerly granite surfaces sheared under normal stresses of 1-25 MPa. We measure changes in surface friction and elastic properties, using acoustic waves, for step changes in normal stress of 1–50% and shearing velocities of 1-100 μm/s. We demonstrate that transmitted elastic wave amplitude is a reliable proxy for the real contact area at the fault interface at steady state. For step increases in normal stress, wave amplitude increases immediately and then continues to increase during elastic shear loading to a peak value from which it decreases as fault slip rate increases. Friction changes in a similar fashion, showing an inelastic increase over a characteristic shear displacement that is independent of loading rate. Perturbations in normal stress during shear cause excursions in the frictional slip rate that must be accounted for in order to accurately predict the evolution of fault strength and elastic properties. Our work improves understanding of induced seismicity and triggered earthquakes with particular focus on simulating static triggering and stress transfer phenomena using rate-and-state frictional formulations in earthquake rupture models.