Earthquakes tend to strike with a chaotic unpredictability that has stumped seismologists and sometimes caught cities off guard. Now, however, a team of geophysicists reports that so-called slow earthquakes stir deep below the Pacific Northwest about every 14 months. This regularity could shed light on the workings of tectonic plate boundaries called subduction zones, and it might even turn out to herald a season of heightened risk for larger quakes.
Typical earthquakes occur when a jammed and stressed fault suddenly breaks loose. In the Pacific Northwest, the stress arises from the Juan de Fuca plate trying to ram its way beneath the edge of North America. Toward the surface, the plates are locked tight. Further down the fault, where the rocks are hotter and more plastic, the plates are thought to slip slowly and continuously. Yet in 1999, a slow earthquake struck this part of the fault. There was no shaking, but over several weeks, Global Positioning System (GPS) stations reversed their usual direction and moved 2 to 4 mm to the southwest, as Herb Dragert of the Geological Survey of Canada in Sidney, British Columbia, and colleagues reported last year (Science, 25 May 2001, p. 1525).
Intrigued, a team of geophysicists led by Meghan Miller at Central Washington University (CWU) in Ellensburg went back and analyzed 10 years of data from these and other GPS stations. They found a total of eight such slow earthquakes in the same general vicinity. Most remarkably, the slow quakes started every 14.5 months, give or take 1 month, the team reports in the 29 March issue of Science. "That's incredibly exciting," says co-author Tim Melbourne, also at CWU. "If you can find a fault that's regularly but intermittently creeping, maybe you can make sense of this in a deterministic way."
It's too early to tell what effect the slow earthquakes might have on the risk of a blockbuster shakeup. But Dragert warns that the slow earthquakes might one day rupture the locked zone if it's close to a critical threshold. This suggests that a major event may be more likely during slow-earthquake season. But the CWU team points out that many slow temblors may take place in the 500 or so years between great quakes; this makes it unlikely that any single slow earthquake is going to be a meaningful precursor. "I'm not convinced that they're a harbinger of disaster," Miller says.