for National Geographic News
An international team of scientists says that a great earthquake of magnitude 9 struck the Pacific Northwest in 1700, and created a tsunami that caused flooding and damage on the Pacific coast of Japan.
The study, published in the Journal of Geophysical ResearchSolid Earth, shows how a great earthquake along the Cascade subduction zone, which runs for 1,100 kilometers (680 miles) down the Pacific coast of North America, warped the ocean floor and briefly changed the shape of the overlying sea surface, sending large waves crashing onto Japanese shores only 10 to 20 hours later.
The study not only highlights the trans-Pacific connection between earthquakes and tsunamis, but also shows that earthquakes occurring along North America's west coast can be more powerful than previously thought.
"The findings leave little room to doubt that an earthquake in the Pacific Northwest occasionally attains magnitude 9," said Brian Atwater, a geologist with the United States Geological Survey based at the University of Washington in Seattle, and one of the study's authors.
A Massive Rupture
The relatively obscure Cascadia subduction zone, which extends from southern British Columbia in Canada to northern California, is a huge fault trace located under the Pacific Ocean, between two tectonic plates.
Great earthquakes occur on a shallow part of the fault boundary between the downward-moving oceanic plate, called the Juan de Fuca plate, and the overriding continental plate, known as the North America plate.
Until 20 years ago, the Cascadia fault was believed to be benign by most scientists. But then several discoveries in North America showed that the fault produces earthquakes of magnitude 8 or larger at irregular intervals, averaging about 500 years.
Radiocarbon evidence suggested that at least 900 kilometers (560 miles) along the fault ruptured between 1690 and 1720. However, no one knew the magnitude of the rupture.
Then, in 1996, Japanese researchers stunned their North American colleagues by linking a tsunami that struck Honshu Island in 1700 to geologic reports at the Cascadia subduction zone. Tree-ring dating in North America further suggested that an earthquake struck between August 1699 and May 1700.
Japan has a documented history of tsunamis, including trans-Pacific events, dating back to the 1500s. From the tsunami's time of arrival in Japan, the researchers concluded the Cascadia earthquake must have occurred in the evening of Tuesday, January 26, 1700.
The latest research shows the 1700 tsunami cresting as much as five meters (16 feet) in Japan. Scientists used computer simulations of trans-Pacific tsunamis to correlate the Cascadia earthquake size with the tsunami's heights in Japan.
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