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Linking Trees to Tsunamis

Scientists have strong new evidence suggesting that a major earthquake shook the Northwest coast of the United States around 1700, causing a tidal wave that hit the Japanese island of Honshu on 26 January 1700.

Many experts have believed since the late '80s that signs of earth subsidence point to a big earthquake or series of quakes occurring around the end of the 17th century at the plate boundary for the Cascadia subduction zone. This area extends some 1000 kilometers from Vancouver to California, where the ocean floor is sliding underneath Earth's mantle toward North America. Others, however, have argued that the ocean crust is too young and hot for the fault to stick and store the energy needed for a quake. Then Japanese scientists uncovered the record of the tsunami of 1700 and postulated it had to have been generated by a big one--of at least magnitude 8--in Cascadia. (Science, 23 September 1994, p. 1802).

The latest studies bolster this scenario. One team, led by Gordon Jacoby of the Lamont-Doherty Earth Observatory in Palisades, New York, took core samples from 33 sitka spruce trees, each at least 300 years old, that stand along a 100-kilometer stretch of Washington and Oregon coastline. In the November issue of Geology, the researchers report finding signs of waterlogging or trauma that had disrupted many of the trees' rings around 1699. These signs include changes in ring width; the presence of "traumatic resin canals" (sap-conducting tubes formed by altered cells); and "reaction wood," (dense cells formed in response to tilting).

In the other study, David Yamaguchi of the University of Washington, Seattle, and Brian Atwater of the U.S. Geological Survey, found that dead cedar trees still standing in what are now salt marshes breathed their last in the growing season of 1699. The analysis, reported in the 30 October Nature, was done by comparing the ring patterns in the cedar trunks and roots with those in a reference sample of old trees.

Until now, says Atwater, scientists knew about the quake "only from geological inference." But with these studies, the link with the tsunami records "is so strong that those village records become written proof that the earthquake happened."