Scientists untangle events of mega-earthquake that hit Washington more than 300 years ago

DNR Chief Hazards Geologist Tim Walsh explains to a National Geographic documentary crew how these red cedars along the Copalis River were killed by a flood of seawater more than 300 years ago after a magnitude 9 earthquake..

DGER Chief Hazards Geologist Tim Walsh recently explained to a National Geographic television crew how scientists used radiocarbon dating and tree ring analysis to correlate the sudden coastal subsidence that drowned millions of trees in the Pacific Northwest 300 years ago and a tsunami documented in Japan in the same time period.

An analysis of these red cedar trunks indicated that they were submerged sometime between August 1699 and May 1700. This time frame coincides with numerous accounts of a tsunami of unknown origin — also known as the Orphan Tsunami of 1700 — that reached Japanese shores on January 26, 1700.

Apparently, a megathrust earthquake on the Cascadia Subduction Zone triggered land subsidence and seawater inundation that submerged coastal forests in the Pacific Northwest. Multiple records kept by Native American peoples of the Pacific Northwest also suggested violent shaking and flooding in the same time period as the Orphan Tsunami.

The earthquake of 1700 is the type of Cascadia megathrust earthquake that geologists say can happen again in the Pacific Northwest at any time. It rivaled the magnitude and character of the Tohoku earthquake that devastated northern Japan on March 11, 2011.

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