Abstract
Slow slip events on tectonic faults, sliding instabilities that never accelerate to inertially limited ruptures or earthquakes, are one of the most enigmatic phenomena in frictional sliding. While observations of slow slip events continue to mount, a plausible mechanism that permits instability while simultaneously limiting slip speed remains elusive. Rate-and-state friction has been successful in describing most aspects of rock friction, faulting, and earthquakes; current explanations of slow slip events appeal to rate-weakening friction to induce instabilities, which are then stalled by additional stabilizing processes like dilatancy or a transition to rate-strengthening friction at high slip rates. However, the temperatures and/or clay-rich compositions at slow slip locations are almost ubiquitously associated with rate-strengthening friction. In this study, we propose a fundamentally different instability mechanism that may reconcile this contradiction, demonstrating how slow slip events can nucleate with mildly rate-strengthening friction. We identify two destabilizing mechanisms, both reducing frictional shear strength through reductions in effective normal stress, that counteract the stabilizing effects of rate-strengthening friction. The instability develops into slow slip pulses. We quantify parameter controls on pulse length, propagation speed, and other characteristics, and demonstrate broad consistency with observations of tectonic slow slip events as well as laboratory tribology experiments.
Highlights
Frictional instabilities are intrinsically linked with shear fracturing and material failure[1].Earthquakes are notable examples of such instabilities, featuring explosive, inertially limited rupture growth on faults following gradual development of instability
Frictional sliding on the slip surface is governed by rate-and-state friction, which provides a relation between shear strength τ, effective normal stress σ 0 = σ −p, and friction coefficient f that depends on sliding velocity V and state variable Ψ
We have investigated spontaneously occurring sliding instabilities that occur with mildly rate-strengthening friction due to the coupling of slip and effective normal stress via two different mechanisms
Summary
Frictional instabilities are intrinsically linked with shear fracturing and material failure[1]. The fault shear strength τ = f (σ−p), the product of friction coefficient f and effective normal stress, the difference between compressive total stress σ and pore fluid pressure p It is widely thought[2] that instabilities during sliding require rate-weakening friction, in which f decreases with increasing V (following a transient rate-strengthening response that stabilizes short-wavelength perturbations). We show that slow slip arises naturally from instabilities with mildly rate-strengthening friction These instabilities arise from two distinct mechanisms, both involving configurations where slip couples to changes in effective normal stress σ − p. We identify and characterize the sliding instabilities through 1.) linear stability analysis of perturbations about steady sliding, and 2.) numerical simulations of nucleation and propagation of slip pulses with fully nonlinear rate-and-state friction. We do not distinguish between the damage zone and host rock in this study
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