Department of Geology
University of Illinois, Urbana-Champaign
1301 W. Green St., 245 NHB
Urbana, IL 61801 USA
The lack of regularity in earthquake cycles continues to be a confounding issue in earthquake science. Lately, observations of non-volcanic tremors and episodic slips along a small number of well-instrumented tectonic plate boundaries are intriguing: some of these features seem to recur with predictable time-intervals. This paper gives an overview of episodic tremors and slips (ETS), aiming to pique interests in the community of complex systems research.
When propagation effects such as dispersion and attenuation are accounted for, seismic waves generated by earthquakes are generally impulsive in nature (broadband in the frequency domain, from mHz to tens of Hz) and have clear onsets. In contrast, seismic tremors are narrow-band bursts of ground vibrations that last for tens of minutes in duration. Most tremors are volcanic in origin, a result of resonance in (liquid) magma pockets surrounded by (solid) rocks. Recently, non-volcanic tremors, with a central frequency of a few Hz, have been observed in regions where no active volcanism exists. At the moment, the origin of non-volcanic tremors is unknown.
Meanwhile, slip events, slow but discernable motion of the ground with amplitude of a few mm and duration of about two months, seem to accompany some non-volcanic tremors. The slip events are too gradual in nature to cause seismic vibrations but nonetheless also seem to be caused by displacement across geologic faults at depth. Beneath the Island of Vancouver, slip events have recurred at regular intervals of about every 14 months since 1996 (when precise GPS-based measurements began) and large bursts of non-volcanic tremors correlate with each episode of slip event.
During the past 100 year’s of instrumental data, no significant earthquake has occurred along this subduction zone where the Juan de Fuca plate, a small counterpart of the Pacific plate is thrusting under the North American plate; but geologic data suggest that immense earthquakes may have occurred in the past millennium. Since slip during these “silent” events constantly raises the level of shear stress in “locked” (no-slip) portions of the same fault, the challenge is to understand how regular recurrence of slip events and tremors relate to highly sporadic large earthquakes.