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Earth quakes silently near Whidbey
Please, dont panic, but theres a darn good chance were having an earthquake, now, right this very moment.
Feel anything? No? Well, dont be fooled. As with the proverbial tree that topples in the unoccupied forest, just because you dont sense an earthquake doesnt mean its not happening.
Thats right: Add to your current list of seismic worries the newly-discovered silent quake, a tremblor that originates somewhere near Whidbey Island, approximately 20-miles deep underground and radiating outward so slowly along the Cascadia subduction zone as to be almost undetectable.
They can last for hours, days, weeks. Maybe even years. One kicked in silently, slowly on February 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 earths upper crust via a network of strategic ground stations linked up with Global Positioning System (GPS) satellites in outer space. Such gee-whiz technology allows measurements in millimeters.
According to Dr. 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, ten years is but a tiny drop in a whole ocean of time, it remains to be seen whether slow quakes have predictive value. Meaning their relation, either as harbinger or trigger, to great big quakes like the deep-down Nisqually shaker of February 01 remains to be seen. Also, its impossible to say whether their current 1.2 year periodicity is the norm.
If theyre happening as frequently as we think they are, they dont have predictive value, Miller says during an interview Thursday. This is because their relation to larger quakes, which occur at intervals of, say, 500 years, becomes a moot point. We cant go into high alert every 14 months, she adds.
Still, the six-month-old discovery of this unprecedented type of earthquake is tantamount to a genuine scientific breakthrough. The precision of all that amalgamated technology beaming coordinates and bouncing measurements one off the other allows Miller and her colleagues in the geology department to calibrate spatial movements most people couldnt spot at ten paces. Were talking movements of 5, 6 millimeters. We couldnt do this before, Miller says. Id say its pretty big.
These slow earthquakes occur along a part of the plate fault known as the transition zone. To get an image of this zone, picture the earths strata 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 stuff.
At the top layer of the cake, then, is where the tectonic plates are totally locked up along the fault; when they release, they do so violently and relatively 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 explains. She describes this middle layer as meta-stable, meaning the plates stay pretty much stuck until a critical threshold is reached, a end-point of tension, and then they move. Miller calls this stick-slip motion. However, she adds, when it slips, it doesnt rupture catastrophically. It doesnt release seismic energy. It breaks slowly. That 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. They appear to start near Whidbey and break out towards 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. Were trying to make detailed maps of how these things spread out in time, Miller says. Frequency, she adds, has been her contribution to this new field of study.
Miller gives the old yes and no when asked whether these silent, outward-radiating 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. Then she adds, a bit mischievously: But you have other faults to worry about.
Meaning the North Whidbey and South Whidbey Faults. But thats a different story.
You can learn more about periodic slow earthquakes at www.geodesy.cwu.edu. Or check out Millers article (co-written with fellow scientists Tim Melbourne, Daniel Johnson and William Sumner) entitled Periodic Slow Earthquakes form the Cascadia Subduction Zone in the March 29 issue of Science Magazine.