About the research
One of the major geophysical discoveries of the past decade has been that of “episodic slow slip and tremor” in many of the world’s subduction zones. Most known faults either slip steadily at the plate tectonic rate, or spend most of the time “locked” and slip only during brief earthquakes, with slip speeds of order m/sec and propagation speeds of order km/sec (the speed of sound in rock). Slow slip events, on the other hand, have average slip speeds of only ~0.1 µm/s and propagate up to 300 km along strike at remarkably consistent rates of ~10 km/day. In terms of energy release they are comparable to magnitude 6–7 earthquakes, but they last for weeks rather than seconds. They have the additional remarkable property that they recur quasi-periodically, at intervals ranging from several months to a few years, depending upon location.
Coincident in time and space with the geodetically-observed slow slip is a new (to us) seismic signal termed “tectonic tremor.” Unlike typical earthquakes, which have impulsive P-wave and S-wave arrivals, tremor is a low-amplitude signal that can last for minutes to hours and that most often lacks clearly identifiable wave arrivals. Tremor is thought to consist of myriad “low frequency earthquakes” on the plate interface, comparable to magnitude 1–1.5 earthquakes in size but with durations roughly 10 times longer. The cause of this longer duration is unknown. Tremor has since been discovered on the deep extension of the San Andreas fault, about 10–15 km below the depth range of typical earthquakes. It thus provides an opportunity to probe the behavior of the deepest portions of plate boundaries.
In addition to representing a previously unrecognized style of fault slip, episodic slow slip and tremor are relevant to seismic hazards assessment. Slow slip increases the stressing rate on the locked portions of faults capable of producing magnitude 9 earthquakes. The up-dip extent of slow slip has also been interpreted by some as indicative of the expected down-dip extent of future major subduction-zone earthquakes. If correct, this would bring strong ground shaking considerably closer to downtown Seattle than had previously been thought. When slow slip was first discovered, the question for theoreticians was “How is it that slip over such a large region can accelerate but then decelerate without leading to an earthquake?” Now the problem is in some sense reversed; there are more proposed underlying mechanisms for slow slip than there are observational constraints to choose between them. My group works to obtain such observational constraints, and to conduct the numerical simulations necessary to narrow the range of plausible physical models of slow slip.