A MOSTLY SEISMOLOGICAL PERSPECTIVE OF THE CONTINUUM OF FAULT SLIP MODES

The phenomena of seismic wave radiation arising from rapid fault slip (i.e., earthquakes) and the destructive potential of seismic waves perhaps explains why until just a decade or so ago the study of earthquakes was considered mostly a seismologic endeavor. While slower fault slip poses little direct hazard, understanding earthquakes also requires understanding not just fast, seismic fault slip, but also slow, quiet or silent, aseismic slip. The enormous advances in the ability to measure deformation on time-scales of seconds to years have shown us that the entire spectrum of slip modes contributes significantly to the slip budget, in differing proportions in different tectonic environments. The growing body of studies of both seismic and geodetic signatures of fault slip suggests a continuum of slip modes exists, rather than a few discrete, fast or slow modes. While the latter might seem to imply simpler underlying physical processes, commonalities have emerged among the large diversity of observations and inferences, which may be understood with reference to a simple framework. In this, patchworks of varying frictional properties comprise fault surfaces, with the diversity of behaviors reflecting different proportions and patterns of locked and continuously or episodically creeping patches. (Undoubtedly seismologist and geodesists must turn to petrologists, geochemists, and others to explain the processes controlling the macroscopic properties of friction.) I will illustrate this perspective with mostly seismologic observations and models from regions where aseismic slip dominates, triggering failures on locked patches that radiate seismic energy. For example, on shallow crustal faults, the seismicity can come from ordinary earthquake swarms when the slow slip is transient. Alternatively, repeating earthquakes can appear when the shallow slow slip is steadier. At depths of tens of km, tremor and other, more exotic, ‘low frequency’ seismic signals accompany slow slip. Some striking similarities exist among these classes of seismic sources, suggesting shared underlying phenomenology. I will also note some of the observational challenges, and how they hinder our interpretations.