An earthquake is occurring right now along the U.S. West Coast, yet no one feels it, according to a new study just released by the National Science Foundation.

Discovered by geologists using the satellite-based GPS (global positioning system) system, the phenomenon now called a "slow earthquake," began about Feb. 7 and has been ongoing ever since.

Geologists at Central Washington University in Ellensburg, Washington were among the first to observe the unfelt, yet persistent temblors while conducting research funded by a National Science Foundation (NSF) grant. Their findings are detailed in a report titled, "Periodic Slow Earthquakes from the Cascadia Subduction Zone," published in the March 29, 2002 edition of "Science Magazine." [Registration required to view articles.] 

According to Central Washington University geologist Meghan Miller, the slow quakes may help relieve the strain between faults without producing surface damage. "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," said Miller.

Jim Whitcomb of NSF's division of earth sciences added, "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."

Researchers say the quakes occur in an area of the Earth's plate boundary fault known as the transition zone, below where the plates are stuck and release strain during earthquakes, and above the portion where the fault moves continuously. The area of science dealing with earthquakes and changes in the Earth's surface over time is called plate tectonics. 

"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."

Slow earthquakes seem to initiate in Puget Sound, Washington, 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.

It is not yet known whether slow earthquakes can actually herald - or trigger - larger ones, added Miller. "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." 

Not So Slow Earthquakes
The most devastating earthquake in U.S. history took place on Good Friday, March 28, 1964, in Prince William Sound, Alaska. The 9.2 magnitude earthquake and resulting tsunami killed 125 people and caused about $311 million in property loss. Today, the Earthquake Hazards Program of the U.S. Geologic Survey (USGS) provides the public with a wealth of information on our still very active Earth. Here are just a few of their fascinating resources:

• Recent Earthquake Activity in the USA -- California/Nevada -- Worldwide
• Real-time Seismograph Displays 
• The 15 Largest Earthquakes in the United States
• Earthquake Questions and Answers
• Did You Feel It? Report an earthquake