Embargoed until 2 p.m. EST
NSF PR 02-21 - March 28, 2002
West Coast Earthquakes Ongoing, Scientists Discover
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
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
"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.