#OnThisDay 321 years ago, Jan. 26, 1700 (at 9pm to be exact), the Pacific Northwest was rocked by the last great Cascadia Subduction Zone earthquake that sent tsunami waves all the way across the Pacific Ocean to the shores of Japan.

The magnitude is estimated to have been between Mw 8.7-9.2. 1000km of fault line slipped an average of 20m along its length. Coastal areas dropped by 1-2m, submerging the land below sea level.

How do we know all this information if there were no seismometers or written record from the PNW in 1700? It's an amazing story that brings together Indigenous knowledge, geo-forensics, and written history from across the world.

In this video, courtesy of @waEMD, Hoh Indian Tribe elder Viola Riebe tells the story of Thunderbird and Whale and of a specific tsunami occurring around 1700. A large earthquake must have triggered this event, but more info was needed to gather details.

"Ghost forests" of dead trees and tsunami deposits can be found far from the shore along the coast. Tree rings tells us they died between 1699-1700, and were killed when salty ocean water inundated the land. This video from @IRIS_EPO shows the process:

The final piece of the puzzle comes from across the Pacific. On January 27, 1700, written records in Japan tell of an "orphan tsunami" that caused minor flooding along the coast, but there was no earthquake. The wave must have come from an earthquake elsewhere in the Pacific.

Using all this evidence, Brian Atwater and other scientists conclusively determined the time and magnitude of this earthquake. Further evidence suggests such quakes occur about every 500 years, and Mw 8 events where only part of the fault ruptures occur every 200-300 years.

Today serves as a great reminder that we live in a seismically active region, and it is important to be prepared for earthquakes of any size. You can learn more about the 1700 quake and how to prepare for the Big One here: https://pnsn.org/blog/2020/01/27/getting-ready-for-the-next-great-cascadia-subduction-zone-earthquake