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ShareNov. 1, 1997 — Stumps of long-dead western red cedar trees are revealing new details of a cataclysmic earthquake along North America's west coast more than 100 years before the arrival of the first European occupants.
Two University of Washington researchers believe that evidence in the dead wood confirms that in the year 1700 a great earthquake struck the Pacific Northwest coast and set off a tsunami, a train of massive ocean waves, that flooded coastal Japan.
The scientists are reporting in the Oct. 30 issue of Nature magazine that they have dated the demise of six trees, along 60 miles of the Washington coast, by "reading" the annual ring patterns in the trunks and roots of the stumps. They found that each of the trees produced its final ring in the 1699 growing season. No further rings were evident, indicating that the trees were dead by the spring of 1700.
"We are saying this huge earthquake really happened," says David Yamaguchi, a UW dendrochronologist, or tree-ring analyst. Last year Japanese scientists reported that a tsunami that hit Honshu island on January 27, 1700, was probably caused by an earthquake on the Cascadia subduction zone, a 600-mile coastal fault stretching from British Columbia to northern California.
Based on the size of the tsunami, the Japanese researchers estimated the earthquake at magnitude 9, even though no Cascadia fault earthquake of magnitude 5 or above has ever been recorded by seismologists. The Nature article concludes that the tree dates "mean that the northwestern United States and adjacent Canada are plausibly subject to earthquakes of magnitude 9."
The second author of the paper, Brian Atwater of the U.S. Geological Survey and an affiliate UW professor, says the new dates clinch the argument that massive earthquakes can occur where the Juan de Fuca plate, an Oregon-sized slab of crust, collides with a large block of continental crust called the North America plate. The Cascadia fault forms the boundary between the two plates.
Over the past decade, Atwater and other researchers have discovered geologic evidence of repeated magnitude 8 or larger earthquakes along the Cascadia fault. However, the size of the 1700 earthquake remains a subject of much debate, which the tree-ring dates do not resolve, says Atwater. Some scientists have proposed a ceiling of magnitude 8; others have suggested a maximum magnitude of 9.5.
Only a handful of magnitude 9 earthquakes have occurred globally this century. The largest earthquake in the Pacific Northwest in historic times was a magnitude 7.4 in the north Cascade mountains in 1872. This was not on the Cascadia fault.
Yamaguchi first "read" the cedar stumps to show that a great earthquake happened some time after 1690. UW researcher Minze Stuiver then pinned the earthquake to between 1695 and 1710 with precise radiocarbon dating. This led the Japanese scientists to propose the Cascadia fault as the likely origin for the 1700 tsunami.
To test the Japanese theory, the UW researchers dug up the roots of a dozen cedar stumps from the Copalis River south to the Columbia River. In half of these stumps, the dating was inconclusive, but in six they found evidence that the trees had died before the start of the 1700 growing season.
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