Slow Earthquakes and the Spectrum of Fault Slip Modes: A View From the Lab

A central goal of this work is to understand the extent to which fault friction vs. pore fluids and other factors is the cause of slow earthquakes and the spectrum of fault slip behaviors. Slow earthquakes and quasi-dynamic modes of fault slip such as tremor and LFEs have now been observed in essentially every tectonic setting, which suggests that the underlying mechanism(s) are generic rather than specific to a particular fault setting, rock type, or tectonic regime. Here, I discuss lab data that illuminate the mechanics of slow slip. I focus on frictional stick-slip failure events, the lab equivalent of earthquakes, that reproduce slow slip and the full range of slip behaviors observed on tectonic faults.  These studies document repetitive slip events and the complete lab seismic cycle for the full spectrum of slip behaviors from aseismic creep to slow slip and aperiodic elastodynamic failure. Working with data for repetitive slip events is critical for understanding the underlying mechanics. In the lab, we also document the rate of frictional weakening with slip k_c = σ_n (b-a)/D_c  ––the so-called critical stiffness–– where σ_n is fault normal stress, (b-a) is the friction rate parameter and D_c is the critical slip distance. We measure k_c for the same conditions of the slow slip events by altering the machine loading stiffness. These works assess directly frictional instability theory, which predicts the slip stability transition when the elastic stiffness of the fault zone k equals the frictional weakening rate k_c. The lab work confirms friction theory in relation to the transition from stable to unstable slip but it also reveals additional complexity showing that k_c varies with slip rate. Several works now document the velocity dependence of k_c(V) and its role in dictating slow slip in the lab. These works show complex behavior near k/k_c ≈ 1 including slow slip, aperiodic failure and chaotic motion. I discuss these results in the context of basic questions that remain regarding how slow ruptures can propagate quasi-dynamically, at speeds far below the Rayleigh wave speed, and how tectonic faults can host both slow slip and dynamic earthquake rupture.