March 28, 2002
Contact: Meghan Miller (509-963-2825/fax 509-963-1109/e-mail: meghan@cwu.edu)
ELLENSBURG, Wash. - “There is an earthquake going on right now, yet no one feels it,” says Dr. M. Meghan Miller, Central Washington University geophysicist.
The temblor, or so-called slow earthquake, began on Feb. 7. An article by Miller and
her colleagues, titled “Periodic Slow Earthquakes from the Cascadia Subduction Zone,” will appear in tomorrow’s (March 29) issue of Science magazine — a weekly publication of the American Association for the Advancement of Science.
According to Miller, silent earthquakes periodically occur along the Cascadia subduction zone, off the coast of Washington, Oregon and northern California. These slow quakes have produced energy releases of magnitude 6 or more at least eight times in the past decade. They seem to initiate in Puget Sound near Whidbey Island and spread out from there, she points out.
By using ground stations that monitor Global Positioning System (GPS) satellites for precise daily positions, the CWU team observed the earthquakes that move so slowly that they are not detected by seismometers.
The new technology reveals that stuck faults release energy accompanied by seismic shaking during earthquakes, but also in other ways, such as these “silent” earthquakes that slip too slowly to generate seismic shaking.
“If slow earthquakes release energy that doesn’t generate seismic shaking, they are an important part of the seismic hazard puzzle,” Miller adds.
Slow earthquakes occur along a part of the plate boundary fault known as the transition zone. It’s below where the tectonic plates are stuck and release strain during great earthquakes, and above the portion where the fault slips more continuously.
“These areas seem to be ‘meta-stable’ — stuck enough that they don’t move until a critical threshold is reached and then they slip, but don’t rupture catastrophically,” Miller says. “The slip can take place over the course of hours, days, weeks — maybe years in some places.”
Miller and her CWU collaborators have reviewed a decade’s worth of GPS data, determining eight slow earthquakes took place in the same general vicinity over a 10-year period, all about 14 months apart.
“Until our recent observations, earthquake scientists didn’t have any direct evidence about periodic slow earthquakes. What’s exciting about the periodicity is that it’s 1.2 years,” she says, citing the opportunity to test this observation.
“Many times when scientists have recognized periodicity in solid earth behavior, the next event has not happened on schedule,” she adds with a laugh. “Certainly over the past 10 years these events have been highly periodic. Whether that holds for the entire cycle between great earthquakes on the Cascadia subduction zone is an open question.”
It’s also not yet known whether slow earthquakes can actually herald or trigger large earthquakes, or are related to those such as the Nisqually earthquake in February 2001.
Even though Miller calls the regularity and the frequency of the slow earthquakes “stunning,” in geological time 10 years is so small that she is not willing to speculate that they are the norm.
Miller was among the first scientists to use GPS technology to study earthquakes. Her most recent study was supported by grants from the National Science Foundation and the National Aeronautics and Space Administration. Miller’s co-authors include fellow CWU scientists Tim Melbourne, Daniel Johnson and William Sumner.
Further information is available at: www.geodesy.cwu.edu
