Mar. 29, 2002 The most recent evidence indicates there is an earthquake going on right now on the West Coast, yet no one feels it. The temblor, a so-called slow earthquake, has been ongoing since about Feb. 7, according to geologist Meghan Miller of Central Washington University in Ellensburg. She was among the first scientists to use GPS (global positioning system) technology to study earthquakes.
An article written by Miller and colleagues, "Periodic Slow Earthquakes from the Cascadia Subduction Zone," is published in this week’s (March 29) edition of the journal Science. The cited research was funded by the National Science Foundation (NSF).
"The only way we can observe these slow earthquakes is through this instrumentation," Miller says. "Until we had GPS geodesy, we regarded earthquake deformation in two main ways: long-term, steady state faculty motions, and the elastic strain where faults are stuck and let go during earthquakes. If it turns out that a major mechanism for releasing this elastic strain is through slow earthquakes that don't generate seismic shaking, then they become very important to understand."
Adds Jim Whitcomb of NSF’s division of earth sciences, which funded the research, “Understanding these ‘silent earthquakes’ that we have been missing all these years will have a profound effect on our ability to predict hazards from volcanoes and earthquakes.”
These quakes occur in an area of the plate boundary fault known as the transition zone, below where the tectonic plates are stuck and release strain during earthquakes, and above the portion where the fault slips continuously.
"These areas seem to be 'meta-stable' - stuck enough that they don't move until a critical threshold is reached and they slip, but don't rupture catastrophically," Miller states. "They can take place over the course of hours, days, weeks - maybe years."
These slow earthquakes seem to initiate in Puget Sound near Whidbey Island and spread out from there, she adds. Miller and her colleagues have reviewed a decade's worth of GPS data, determining that eight slow earthquakes took place in the same general vicinity over that period, all about 14 months apart.
“It means that we could recognize this, speculate when an event may happen and then test that hypothesis,” Miller points out. Even though Miller calls the regularity and the frequency of the slow earthquakes "stunning," 10 years is so small in geological time that she is not willing to speculate that they are the norm.
"Most times when we've recognized periodicity in solid earth behavior, we've been wrong," she says. "And we can still be wrong here. But, certainly over the past 10 years it's been highly periodic. Whether that holds for the entire inter-seismic cycle between great earthquakes is in question."
Miller adds it's not yet known whether slow earthquakes can actually herald - or trigger - larger ones.
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