Quiet quakes near Whidbey
June 25, 2008 · Updated 2:58 PM
Please don't panic, but there's a good chance we're having an earthquake, now, right this very moment.
Feel anything? No? Well, don't be fooled. As with the proverbial tree that topples in the unoccupied forest, just because you don't sense an earthquake doesn't mean it's not happening.
That's right: Add to your list of seismic worries the newly discovered "silent quake," a temblor that originates somewhere near Whidbey Island, approximately 20 miles underground and radiatess outward so slowly along the Cascadia subduction zone as to be almost undetectable.
The quakes can last for hours, days, weeks. Maybe even years. One kicked in -- silently, slowly -- on Feb. 7, and only just stopped a few days back. Nobody noticed this event, neither beginning nor end, except, of course, for a cluster of eager scientists monitoring even the slightest tectonic movement of the earth's upper crust via a network of strategic ground stations linked up with Global Positioning System satellites in outer space. Such gee-whiz technology allows measurements in millimeters.
According to Meghan Miller, a geophysicist at Central Washington University in Ellensburg, these periodic slow quakes have hit the Puget Sound region approximately every 14 months over the last decade. Miller calls such periodicity "stunning," in that all these brand-new observations and hypotheses of so-called slow earthquakes can be tested directly, in the here and now.
However, due to the fact that, geologically speaking, 10 years is but a tiny drop in a whole ocean of time, it remains to be seen whether slow quakes have predictive value. This means their relationship, either as harbinger or trigger, to great big quakes like the deep-down Nisqually shaker of February 2001 remains to be seen. Also, it's impossible to say whether their current periodicity is the norm.
"If they're happening as frequently as we think they are, they don't have predictive value," Miller said in an interview this week.
This is because their relation to larger quakes, which occur at intervals of, say, 500 years, becomes a moot point.
"We can't go into high alert every 14 months," she said.
Still, the six-month-old discovery of this unprecedented type of earthquake is tantamount to a genuine scientific breakthrough. Using technology that coordinates and bounces measurements one off the other allows Miller and her colleagues in the geology department to calibrate spatial movements most people couldn't spot at 10 paces.
"We're talking movements of 5, 6 millimeters," she said "We couldn't do this before."
The slow earthquakes occur along a part of the plate fault known as the transition zone. To get an image of this zone, picture the earth's crust as a cross-section of triple-decker cake. Let a dark, vertical line running from top to bottom represent the fault. The icing is the ground, with houses and mountains and trees and other surface details.
At the top layer of the cake, then, is where the tectonic plates are locked along the fault. When they release, they do so violently and quickly, causing great quakes. The lowest layer of the cake, on the other hand, slips with comparative ease along the fault.
"In between the fully locked and the fully slipping zone is the transition zone," Miller said.
She describes this middle layer as "meta-stable," meaning the plates stay stuck until a critical threshold is reached, and then they move. Miller calls this "stick-slip" motion. When it does slip, it doesn't rupture catastrophically or release seismic energy. It breaks slowly.
The motion may occur over a few days or a few weeks rather than instantaneously. But it also moves through the length of the fault. At any one place the slip may last a few weeks. These slipping zones appear to start near Whidbey Island, then head toward the coast, and then north and south.
Currently, Miller and her colleagues at CWU are utilizing GPS data in an attempt to give shape to the earthquakes, to trace their occurrence through the weave of time and space. They are looking specifically at the last five events.
"We're trying to make detailed maps of how these things spread out in time," Miller says.
Frequency, she said, has been her contribution to this new field of study.
Miller gives an answer of "yes and no" when asked whether these slow earthquakes could ever jump start a larger quake, or maybe cause Mount Rainier to blow its top.
"There are cases where events like this have been thought to trigger large earthquakes," she says. "But you have other faults to worry about."