Quasi-dynamic Stick-Slip Frictional Sliding and The Mechanics of Slow Earthquakes

Our understanding of earthquake physics and frictional slip on tectonic faults has been challenged by recent discoveries of seismic tremor, low frequency earthquakes and other modes of fault slip known collectively as slow earthquakes. These phenomena represent modes of failure that were thought to be theoretically impossible. Despite the growing number of observations of slow earthquakes and the fact that they can trigger catastrophic large earthquakes their origin remains unresolved. Basic questions remain regarding how slow ruptures can propagate quasi-dynamically, at speeds below the Rayleigh wave speed, and how tectonic faults can host both slow and dynamic earthquake rupture. I focus on laboratory results that illuminate the transition from stable sliding to repetitive, slow stick-slip and provide clues about the mechanics of slow earthquakes. Slow slip occurs near the threshold between stable and unstable failure, controlled by the interplay of frictional properties, including rate dependence of the critical rheologic friction parameter Kc, and elastic stiffness of the surrounding rock. The results suggest that slow stick-slip and transient, quasi-dynamic acceleration can arise from the same governing frictional dynamics as typical, fast stick-slip motions. As applied to tectonic faults, they imply that slow earthquakes may share similar mechanics with typical earthquakes governed by elastodynamics, contrary to other suggested models. However, the micromechanical origin of slip rate modulation during frictional stick-slip is still in question. I show that quasi-dynamic motion can be attributed to a dependence of Kc on slip rate, such that acceleration is quashed above a threshold slip rate, and discuss other possible mechanisms for slow earthquakes.