Last "Big One" shook the Pacific Northwest 316 years ago Tuesday

Magnitude 6.8 Earthquake Hits Southern Alaska

This story is adapted from information provided by the National Weather Service Pacific Tsunami Warning Center

Just before midnight on January 27, 1700, a tsunami struck the coast of Japan without warning.

No one in Japan felt the earthquake that caused it.

Nearly 300 years later, scientists and historians in Japan and the United States solved the mystery of what caused this "orphan tsunami".

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The discovery came through careful analysis of historical records in Japan as well as oral histories of Native Americans, sediment deposits, and ghost forests of drowned trees in the Pacific Northwest of North America, a region also known as Cascadia.

The conclusion: this geologically active region, the Cascadia Subduction Zone, not only hosts erupting volcanoes but also produces megathrust earthquakes capable of generating devastating, ocean-crossing tsunamis.

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By comparing the tree rings of dead trees with those still living, they could tell when the last of these great earthquakes struck the region.

The trees all died in the winter of 1699-1700 when the coasts of northern California, Oregon, and Washington suddenly dropped 3 to 6 feet, flooding them with seawater.

That much motion over such a large area requires a very large earthquake to explain it - perhaps as large as 9.2 magnitude, comparable to the Great Alaska Earthquake of 1964.

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Such an earthquake would have ruptured the earth along the entire length of the 600-mile long fault of the Cascadia Subduction Zone.

The severe shaking could have lasted for 5 minutes or longer.

The resulting tsunami would cross the Pacific Ocean and reach Japan in about 9 hours, so the earthquake must have occurred around 9 o"clock at night in Cascadia on January 26, 1700.

The Pacific Tsunami Warning Center created an animation of this historical tsunami using the same technology they use for determining tsunami hazard in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model.

The RIFT model takes earthquake information as input and calculates how the waves move through the world"s oceans, predicting their speed, wavelength, and amplitude.

Source: http://kval.com/outdoors/last-big-one-shook-the-pacific-northwest-316-years-ago-tuesday