Earthquakes are created by slip along faults kilometers beneath the surface, but not all faults produce earthquakes. We have long known that faults may be seismogenic or slowly, steadily creeping, and physical models explain these behaviors well. Over the past two decades, advancements in seismic and geodetic monitoring technologies led to the discovery that faults regularly slip at rates intermediate between steady creep and co-seismic. These newly discovered phenomena are called "slow slip" and "tremor", and they challenge our understanding of how faults slip.

Slow slip and tremor are primarily observed along subduction plate boundary megathrusts (Figure 1), leading to interest in how they may influence seismic hazards and what they may reveal about the physical conditions of the plate boundary at depth. Despite the fact that slow slip and tremor characteristics differ between subduction zones and along a single megathrust, there are consistent patterns emerging on where and under what conditions they are observed. For instance, slow slip occurs over a wide range of temperatures and pressure, but is primarily observed near spatial transitions between seismogenic and creeping parts of the fault zone. There is also reason to believe that one or more of the rock types that exist along the subduction plate boundary may have material properties that result in these "intermediate" slip modes (Figure 2). This is because these slip phenomena are most commonly observed in subduction zones, but have also been observed along some strike slip fault segments that were formerly ancient subduction plate boundaries. Finally, there is a growing body of evidence that pore fluid pressures are high near the megathrust where slow slip and tremor occur.

Whether a fault slips destructively, aseismically, or slowly is, in part, a function of the micro-scale properties of the rocks within the fault zone. My group uses high pressure deformation experiments to reproduce the micro-scale processes that occur within fault zones, and micro-mechanical models to relate these processes to geophysically observed slip. We are particularly interested in how the physical properties of megathrust rocks and extrinsic variables like fluid pressure and temperature together control how fault rock deforms, and thus how it slips.

In this presentation, I will review what we know about the phenomena of slow slip and tremor, including where they occur and how they have challenged our old models of faulting. I will discuss hypotheses for what may cause these modes of slip and what their occurrence might tell us about the conditions along the plate boundary at depths up to 50 km. Finally, I will show how deformation experiments in my group are being conducted to test and refine hypotheses for causes of slow slip and tremor.