Stress triggering and the spectrum of fault slip behaviors

Tectonic fault zones are subject to normal stress variations with a wide range of spatiotemporal scales, resulting in stress field alteration. These perturbations can spread over a wide range of frequencies and amplitudes from the high frequency passage of seismic waves generated by earthquakes, to the low frequency of solid earth tides and underground fluid injection cycles. As a result of these normal stress perturbations, critically stressed faults can be reactivated. The resulting slip mode is then controlled by fault friction and elastic properties of the surrounding rock. Existing works show that complex behaviors may arise from the interplay between friction changes with slip and slip rate and stress perturbations.

To shed light on the mechanics of fault dynamic triggering we performed experiments in a Biaxial Apparatus in a Double Direct Shear configuration under critically stable stiffness conditions (K/Kc~1). We used powdered quartz gouge (Min-U-Sil 40) as starting material, and conducted experiments at reference normal stress of σ_n = 10-13.5 MPa. After shearing the material and reaching a steady state sliding, normal stress oscillations were applied with various amplitudes, varying from A = 0.5-2 MPa, and periods, T = 0.5-50 s. In addition, we used the laboratory derived friction parameters as input for forward modeling using Rate-and-State friction laws in order to assess if these laws can explain our data. Our results show that creeping faults, under critical stiffness conditions, are sensitive to normal stress perturbations showing a variety of slip behaviors depending on amplitude and frequency of the oscillations:

• Oscillation frequency has a major effect on fault stability. Low and high frequencies cause a Coulomb-like response of the shear stress, that is accompanied by a complex frictional response with slow events and period doubling. At the critical frequency predicted by the Rate-and-State friction, we observe dynamic weakening resulting in regular stick-slip events.
• Oscillation amplitude also plays a role with the main effect depending on the magnitude of the perturbation.
• Using a modified Rate-and-State equation (Linker and Dieterich, 1992), we are able to accurately model the laboratory data.

Our results show that normal stress perturbation on a laboratory creeping fault, at critical stiffness condition, can reproduce the entire spectrum of fault slip behavior depending on the oscillation properties.