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ScienceDaily (Feb. 2, 2009) — In the last decade, scientists have recorded regular episodes of tectonic plates slowly, quietly slipping past each other in western Washington and British Columbia over periods of two weeks or more, releasing as much energy as a magnitude 6 earthquake.
The slip events coincide with regular occurrences of what scientists call nonvolcanic tremor, which showed up clearly on seismometers but for which the origins were uncertain.
Now researchers from Italy and the University of Washington have concluded that both phenomena are signs of the same processes taking place about 25 miles deep at what is believed to be the interface between the Juan de Fuca and North American tectonic plates.
"We are now more confident that the tremor and the slip are both products of the same slip process," said Kenneth Creager, a UW professor of Earth and space sciences and a co-author of a paper describing the research being published Jan. 30 in Science.
The findings could have major implications for megathrust earthquakes in the Cascadia subduction zone, an area along the West Coast from northern California to southern British Columbia. Megathrust events are huge earthquakes, often in the range of magnitude 9, that occur in areas where one tectonic plate is forced beneath another.
The slow slip events appear to be building stress on the megathrust fault, where the Juan de Fuca plate is sliding beneath the North American plate, with the two locked together most of the time. That pressure is relieved when the plates slip during megathrust earthquakes such as one determined to have occurred off the coast of Washington on Jan. 26, 1700, estimated at magnitude 9.2. That quake was similar to the great Sumatra-Andaman Islands earthquake the day after Christmas in 2004, which also measured 9.2 and triggered a devastating Indian Ocean tsunami.
In such events, the plates are locked together for hundreds of years and then slip past each other by sliding 50 feet or more during a megathrust earthquake.