The Effect of Normal Stress Oscillations on Fault Slip Behavior Near the Stability Transition From Stable to Unstable Motion

Tectonic fault zones are subject to normal stress variations with a wide range of spatio-temporal scales. Stress perturbations cover a wide range of frequencies and amplitudes from high frequency seismic waves generated by earthquakes to low frequency transients associated with solid Earth tides. These perturbations can reactivate critically stressed faults and trigger earthquakes. Here, we describe lab experiments to illuminate the physics of such changes in friction and the mechanics of earthquake triggering and fault reactivation. Friction tests were done in a double direct shear configuration for conditions near the stability transition from stable to unstable motion. We studied simulated fault gouge composed of quartz powder and conducted experiments at reference normal stress from 10 to 13.5 MPa. After shearing to steady state sliding, we applied sinusoidal normal stress oscillations of amplitude 0.5-2 MPa, and period of 0.5-50 s. We performed numerical simulations using measured values of rate and state friction (RSF) parameters to assess our data. Our results show that low frequency stress oscillations cause a Coulomb-like response of shear strength that transitions from stable slip to slow lab earthquakes as frequency increases. At the critical frequency predicted by RSF we observe periodic stick-slip behavior. Perturbations of high amplitude and short period weaken the fault, while lower amplitudes strengthen the fault. We find that a modified RSF formulation is able to accurately match our laboratory data. Our findings highlight the complex effects of stress perturbations on fault strength and the mode of fault slip. Tectonic fault zones are subjected to transient stress perturbations that propagate in the Earth's crust. These stress changes have the potential to reactivate fault zones and can occur at different frequencies and amplitudes, ranging from seismic waves generated by earthquakes to smaller perturbations associated with Earth tides. To investigate this phenomenon, we conducted laboratory experiments using quartz powder to simulate a fault zone. During the experiment, we subjected the fault to sinusoidal stress oscillations with various amplitudes (0.5-2 MPa) and periods (0.5-50 s). Our results show that faults, under critical conditions, exhibit a variety of slip behaviors depending on the oscillation amplitude and frequency. We found that stress perturbations with high amplitude and short period are able to weaken the fault and generate fast instabilities, while lower amplitudes strengthen the fault. To accurately explain our laboratory findings, we developed a rate and state friction formulation, which successfully matched our experimental results. In conclusion, our study sheds light on the complex effects of stress perturbations on fault strength and the mode of fault slip. By understanding these dynamics, we enhance our understanding of earthquake triggering and fault reactivation, which has important implications for earthquake hazard assessment. Normal stress perturbations modulate fault strength and slip behavior High frequency, high amplitude oscillations generate dynamic weakening Rate and state friction laws predict the laboratory results