Feb. 4, 2009 — A bizarre form of earthquake, which happens over the course of two to three weeks but makes barely a rumble, are lending important clues to the Cascadia subduction zone in the Pacific northwest, one of the most dangerous fault zones on Earth.
For the last decade, slow-slip earthquakes have been measured in fault zones all over the world, baffling scientists. Though the ‘quakes’ release as much energy as a normal earthquake between magnitude 6.0 and 6.5, they produce almost no shaking.
But researchers have measured separate, small tremors at around the same time as the silent quakes. And in a new study in the journal Science, a team of seismologists show the two events are really one in the same.
Using a combination of seismic sensors and Global Positioning System (GPS) measurements of ground movements in northwestern Washington and British Columbia, the team has pinpointed the tremors as coming from between 30 and 45 kilometers deep in the crust. There, the fault is heavily lubricated with water, and the Juan de Fuca tectonic plate slides peacefully beneath the North American plate.
Near the surface, things are less placid. Every 500 years or so the Cascadia megathrust fault unleashes a hellish earthquake in excess of magnitude 9.0. Geologic records tell of tsunamis similar in size to the 2004 Indian Ocean wave that killed a quarter million people.
By connecting the slow-slip events and tremors directly to the deeper parts of the fault, seismologists can begin to unravel mysteries that could affect millions of people.
“This has two advantages,” Kenneth Creager of the University of Washington, a co-author of the study said. “We can locate the tremor more quickly, and we can infer the amount of slip taking place. That allows us to estimate how much stress there’s going to be on the plate.”
The probability of a devastating quake also increases slightly with each slow-slip event, which scientists call an episodic tremor and slip (ETS) event. On average, one occurs every 15 months.
“Every day there’s a probability that a magnitude 9 earthquake will occur,” Creager said. “The probability goes up during one of these events.”
But this should not be cause for alarm, Herb Dragert of the Geological Survey of Canada said.
“The amount of stress transferred is miniscule,” he said. “The only time this becomes important is as you start to approach critical stress along the fault.”
Since no one knows what critical stress for Cascadia is, a devastating earthquake can’t be predicted.
Geologic records indicate the last major quake along the fault was in 1700. Mega-quakes have recurred in as little as 200 years, or as long as 700 years, though, so it could still be centuries before the next one hits.
“We should not start worrying or anguishing about these ETS events, but it’s something we should take into consideration,” Dragert said. “If in a couple of hundred years I’m still alive, though, I’m going to start holding my breath every time we get one of them.”