Thursday marks 323rd anniversary of Pacific Northwest’s last 9.0 magnitude mega-quake

SEATTLE — Thursday, Jan. 26, marks the 323rd anniversary of the last magnitude 9.0 Cascadia Megathrust earthquake, which hit the Pacific Northwest in 1700.

Its massive, 700-mile fault from mid-Vancouver Island to Cape Mendocino, Calif. may have ruptured along its length, according to the Pacific Northwest Seismic Network.

“This earthquake generated a tsunami that smashed into the Pacific coastline and areas throughout the Pacific Ocean. It is the most recent Cascadia Subduction Zone earthquake to have occurred among the 19 that are believed to be full-length events in the last 10,000 years,” PNSN tweeted Thursday.

Though it happened centuries ago, there is geological and historical evidence of the huge quake and tsunami that followed.

Indigenous tribes living on the coast of Vancouver Island passed down oral histories which describe an earthquake and tsunami that crushed homes and caused landslides. They described an extended shaking that was so strong that people were unable to stand.

The tsunami also damaged the Pacific coast of Japan. In the village of Kuwagasaki, northeast of Tokyo, the tsunami is believed to have crested at about 10 feet. It destroyed 13 houses and started a fire that devastated other homes.

Dead trees in “ghost” forests are another piece of evidence of the tsunami. The trees are believed to have been killed instantly by saltwater flooding when the coasts of northern California, Oregon and Washington suddenly dropped 1 to 2 meters.

“By comparing the tree rings of dead trees with those still living, researchers were able to give the 1700 earthquake a place in written history even though it predates Cascadia’s earliest documents by nearly a century,” PNSN said.

There are also obvious geological signs such as the shrinking of coastal regions and the drowning of marshlands.

“These signatures are identified multiple times, indicating that events like these have repeated at irregular intervals of hundreds of years,” PNSN said.

PNSN said the anniversary is a reminder to stay up on what to do during and after an earthquake.

For more information about how to prepare for an earthquake, visit https://ready.gov/earthquakes.

For more information about tsunami evacuation, visit the Oregon Department of Emergency and the Washington State Department of Natural Resources.