Puget Sound Marks 323 Years Since The Last 'Big One'

SEATTLE — Pacific Northwest seismologists on Thursday reflected on the last "Big One," a massive earthquake along the Cascadia subduction zone that shook the region for several minutes and sent a tsunami across the Pacific Ocean at the dawn of the 18th century.

In a social media thread, the Pacific Northwest Seismic Network (PNSN) chronicled the 323rd anniversary of the quake, including how scientists confirmed an event that occurred more than 100 years before Lewis & Clark stepped foot in what is now Washington state.

According to the PNSN, Indigenous Native American and First Nations peoples on Vancouver Island passed down oral histories of an earthquake and tsunami, which destroyed homes and triggered multiple landslides. Tribal members recalled violent tremors that leveled settlements and villages close to the waterline, while accounts from Japan documented a tsunami destroying more than a dozen homes northeast of Tokyo.

Other evidence of a major tsunami comes from "ghost forests," which researchers believe were instantly killed by saltwater floods in Washington, Oregon and California.

"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 wrote Thursday. "The event also left unmistakable signatures in the geological record (eg. subsidence of coastal regions, drowning of marshlands). These signatures are identified multiple times, indicating that events like these have repeated at irregular intervals for hundreds of years."

Looking back at the 1700 earthquake and 18 other "full-length events" along the Cascadia subduction zone over the last 10,000 years, seismologists said it's a good reminder for residents to prepare for the possibility of the next "Big One."

Researchers estimate a magnitude 9 earthquake has struck along the Cascadia subduction zone roughly once every 550 years and say there is a 15 percent chance could hit within the next 50 years.