Cascadia last quaked 316 years ago; how do you prepare for the next one?

It was a dark and stormy night when the earth last served Northwesterners a catastrophic reminder that it is always in motion.

The Cascadia subduction zone 316 years ago tonight produced a Magnitude 9 megathrust earthquake that ripped a 1,000 km tear just off the North American coast, shaking and flooding land from British Columbia to California.

Oral traditions from the Quileute and Hoh tribes described the night the Thunderbird and Whale fought, shaking mountains, uprooting trees and covering the land with ocean water.

Geologists say the event was the result of the Juan de Fuca tectonic plate pushing under the larger North American plate. The violent subduction resulted in the quake that dropped the coast as much as 6 feet and produced a tsunami that reached almost 1,000 feet inland.

But it was the Cascadia quake’s impacts some 4,700 miles across the Pacific Ocean that allowed scientists to properly date and time the geologic event to around 9 p.m. Jan. 26, 1700.

Records in Japan told of the Orphan Tsunami of 1700. That documented tsunami, combined with analysis of red cedar trunks by scientists like Brian Atwater of the United States Geologic Survey revealed land subsidence and seawater inundation that submerged coastal forests.

What about the next one?

So that’s how we know the damage produced by the last Cascadia quake. But what about the next one? The geologic record shows the Cascadia subduction zone produces megathrust quakes every 300 to 600 years, after all.

Aberdeen Seismic Scenario
Shades of red indicate the number of buildings that could be damaged in the Grays Harbor area from another M9 Cascadia subduction zone earthquake.

DNR holds scenarios developed to show how seismic forces could impact all of Washington’s communities, from Aberdeen to Zillah. One of those scenarios shows the toll another Magnitude 9 Cascadia quake could inflict.

You can view that scenario, and 19 others on our seismic hazard catalog.

In addition to showing how ground would shake and even liquefy, the scenario catalog models show where water supplies might be disrupted; damage to hospitals and schools; disruptions to the electrical grid; damaged roadways; and potential injuries and deaths.

Though these models are hypothetical and incomplete, they are made to provide citizens and communities the best scientific information they can use in drawing up personal and emergency response plans.

You can find out more about the risks Washington’s active geology present to you and your loved ones with the scenario catalog or by the hundreds of reports on Washington geology filed in the Washington Geology Library. Those reports can also be accessed online through our new publications catalog.

For more tips on how to be best prepared for the next disaster, check with Washington’s Emergency Management Division.

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