By Sid Perkins
Once every 14 months or so, portions of coastal British Columbia and northwestern Washington State experience a slow ground motion that, if released all at once, would generate an earthquake measuring more than 6 on the Richter scale.
That’s what data from a network of Global Positioning System (GPS) equipment spanning the region show, say scientists who presented their findings at last week’s annual meeting of the Seismological Society of America, held this year in Victoria, British Columbia.
Scientists discovered the region’s so-called silent earthquakes when they analyzed data from the summer of 1999 (SN: 5/12/01,
p. 301: Pacific Northwest stirred, not shaken). GPS stations in the area, which lies at the western edge of the North American tectonic plate, typically move northeast at about 8 millimeters–less than a finger’s width–per year, says Herb Dragert of the Geological Survey of Canada in Sidney, British Columbia. But for 6 to 15 days in August and September 1999, each of seven GPS sites backtracked toward the Pacific before resuming its slow creep inland.
Now, analysis of 10 years of GPS data suggests that this event was just one of a series of silent earthquakes, a type of motion that hasn’t been observed elsewhere in the absence of a major temblor. The four most recent episodes began in July 1998, August 1999, December 2000, and February 2002. The ground motion occurred along a boomerang-shaped arc above the Cascadia subduction zone, where the offshore Juan de Fuca tectonic plate dives beneath the North American plate.
The 1998 event was larger than any since then, says Dragert. In that silent quake, a 60-kilometer-by-300-km area about 30 km or more beneath Vancouver Island and Puget Sound slipped about 30 mm over a period of 12 days or so. If that slippage had happened all at once, seismic instruments would have measured a magnitude-6.8 quake. As it was, there was no detectable vibration, making the slide invisible to seismometers.
The driving force for the silent quakes is the convergence of tectonic plates along the Cascadia zone. The shallow sections of that zone, which let loose and produce major earthquakes once every 5 centuries or so, are currently locked, says Dragert. Scientists don’t fully understand how stresses accumulate along the deeper portions, where the silent earthquakes seem to occur.
The silent quakes that scientists have identified so far have been spaced, on average, 440 days apart, says William Q. Sumner, a geophysicist at Central Washington University in Ellensburg. There’s no way to tell if the recent episodes are representative of the region’s long-term seismic behavior. “We’ve analyzed only about 10 years of data from processes that play out over hundreds of years,” says Sumner.
