March 13, 2003
Parts of Washington, British Columbia in the midst of a ‘silent earthquake’
At this moment, parts of Washington and British Columbia are having an earthquake, but it is a slow-moving temblor that can’t be felt and won’t cause any injuries or damage. Still, by the end of the event, which already has lasted more than two weeks, it is likely to have released about as much energy as the Nisqually earthquake did in February 2001.
The movement is occurring deep beneath the Strait of Juan de Fuca and parts of Vancouver Island in the area where the Juan de Fuca tectonic plate slides beneath, or subducts, the North American plate. The event began on Feb. 26 and could continue for another two or three weeks.
This “silent earthquake” could provide scientists with insights on the next big subduction zone earthquake. Such quakes can exceed magnitude 8 or 9 and do tremendous damage, cause changes in land elevation and trigger tsunamis in the Pacific Ocean. But scientists in the United States and Canada say this event is not the beginning of a major subduction zone earthquake.
Scientists at the Geological Survey of Canada, Central Washington University in Ellensburg and the University of Washington have documented at least eight such earthquakes since 1992. A similar event in 1999 measured magnitude 6.7, compared to the 6.8 of the Nisqually quake. Such quakes occur on average every 14 months, though the current event seems to have come about a month early, said Anthony Qamar, a UW research associate professor in Earth and space sciences.
“This is deep, like the Nisqually earthquake, but it’s so slow that it doesn’t produce vibrations that people can feel,” said Qamar, who also is the state seismologist. “The question is what does the release of stress do to the locked zone just to the west, where a major earthquake could occur.”
The Juan de Fuca and North American plates meet just off the Pacific Coast in the Cascadia subduction fault. The “locked zone” is mostly offshore, where the Juan de Fuca plate dives deep into the Earth. This is where the brittle rocks of the two plates are locked together and cannot pass each other easily. Movement occurs when the rocks break, resulting in a major subduction earthquake, which happens in this region on average every 600 years. The last was in 1700.
The current event is deeper and further east from the locked zone, in an area where the rocks are much hotter and have become somewhat viscous and so can pass each other more easily and not cause the sudden, relatively brief jolt of a typical earthquake.
Previous slow earthquakes have been documented by data from Global Positioning System stations throughout the region. Normally, the North American Plate is being pushed to the northeast at about one-half inch a year along the coast and about one-quarter inch a year farther inland. But in a slow earthquake, the direction is reversed for a few weeks, and the Pacific Northwest Geodetic Array based in Ellensburg detects the GPS stations’ gradual move to the southwest.
The current event has shown up most sharply in data from GPS stations in the San Juan Islands and southern Vancouver Island. Canadian researchers have set up 20 temporary seismographs and three temporary GPS stations on southern Vancouver Island to improve monitoring.
However, unlike previous slow earthquakes, this one was first detected from tiny high-frequency vibrations recorded by seismographs at the UW and the Geological Survey of Canada.
“There is evidence of deep tremor, like a gurgling sound, that we normally associate with volcanoes, and it’s a deep vibration that is detectable by seismographs,” Qamar said.
A silent earthquake, or what Canadian researchers call a “slip event,” might be adding stress that some day could trigger a subduction zone earthquake.
Herb Dragert of the Canadian geological survey was lead author on a paper published in Science last year that suggested the subduction fault’s locked zone is slightly closer to failure after a silent earthquake. The paper also said silent quakes are probably part of a long cycle that builds up subduction zone stress over hundreds of years.
Qamar believes information gleaned from silent earthquakes will help scientists develop a greater understanding of the region’s seismic hazards.
“I guess in the end, no one knows what is the final straw that breaks the camel’s back and starts a major earthquake,” he said. “It could be one of these slow events. It could be something else.”
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For more information, contact Qamar at (206) 685-7563, qamar@ess.washington.edu; or Dragert at (250) 363-6447, dragert@pgc.nrcan.gc.ca, or Meghan Miller at Central Washington University, (509) 963-2825 or meghan@geology.cwu.edu
