The spectrum of fault slip modes from elastodynamic rupture to slow earthquakes

Earthquake science is in the midst of a revolution. Our understanding of tectonic faulting has been shaken to the core by the discovery of seismic tremor, low frequency earthquakes, slow slip events, and other modes of fault slip. These phenomena represent modes of failure that were thought to be non-existent and theoretically impossible only a few years ago. 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 far below the Rayleigh wave speed, and how tectonic faults can host both slow slip and dynamic earthquake rupture. Here, I summarize results from laboratory experiments showing repetitive slow slip, describe friction laws for slow earthquakes, and discuss implications of the work for earthquake scaling laws. The lab results suggest that slow earthquakes occur for conditions near the stability boundary defined by the critical fault rheologic weakening rate Kc and that the spectrum of fault slip behaviors can be described with a single frictional mechanism. Other processes may contribute to the origin of slow earthquakes but the work summarized here shows that slow and quasi-dynamic fault slip can occur entirely as a result of frictional processes and fault zone heterogeneity.