323 years on: A look back at the Cascadia Fault earthquake of 1700

323 years on: A look back at the Cascadia Fault earthquake of 1700
Royal BC Museum and Archives.
Indigenous art depicting Earthquake Foot, artist A-nii-sa-put (Tim Paul), Hesquiaht, 1977, RBCM 15247a.

More than three centuries ago around this time, a catastrophic 9.0-magnitude earthquake struck Vancouver Island at 9 p.m. The megathrust is forever etched in history as one of the world’s worst natural disasters.

The “Great Shake” that struck Vancouver Island on Jan. 26, 1700 is well-documented, even though it happened 323 years ago.

Oral histories passed down by Indigenous communities, tidal records kept by Japan and modern scientific research have allowed researchers to determine not only the exact day the earthquake occurred, but even the hour it took place.

According to various retellings of Nuu-Chah-nulth stories, one recounts of a guest who while dancing, accidentally kicked one of their drums and got “earthquake foot.”

Another from the Quileute Tribe of modern-day Washington State tells of a battle between Thunderbird and Whale that uprooted trees and shook mountains.

Anthropologists from the University of Washington popularized some of these stories amongst scholars, and began to compare these stories, with one scientist in particular making a notable connection.

When an earthquake occurs, they often cause tsunamis. These powerful waves can propagate across the globe.

“An earthquake in Alaska can produce tsunami waves that propagate across the Pacific to reach the coasts of British Columbia, Japan, or even South America,” Michael Bostock, a seismology  professor at the University of British Columbia explained.

In the case of the great shake of 1700, scientists cross-reference the oral histories of Indigenous people from North America with the extensive tidal records maintained by the Japanese.

“Kenji Satake, a famous seismologist, realized that Japan has this extensive history of its tides, and there might be a record of a tsunami there,” Bostock recounted.

The Japanese records showed a tsunami with no shaking (indicating it wasn’t local, as shaking only occurs near the earthquake) hit their shores, Jan. 27. Bostock explained that the waves from such a displacement of water propagate across the ocean floor at roughly the same speed as a modern day airplane.

“It makes it possible to correct for its travel time and figure out the precise moment it had occurred: 9 p.m. PST.”

The other indicator scientists use to date earthquakes involves submarine sediment deposits from coastal marches and submarine landslides.

“Examinations of the sediments in coastal marshes and the sediments that were leftover revealed that if you dig down you’d find a repetition of different layers repeating and as you dig into the marshes we can identify what were likely tsunami deposits vs regular deposits,” said Bostock. Similar repeating deposits are observed in ocean sediment cores.

Using various geological dating methods, scientists can sift through layers of dirt and uncover the cyclical nature of these ‘megathrust’ earthquakes, similar to the one that occurred in 1700, and estimate their average recurrence interval at roughly 550 years.
“That’s the average time between events,” Bostock stressed. “The standard deviation is about 175 years which represents the variability. In some cases, there are periods of 700 years between megathrusts, and other times only 300 years.”
Every year Vancouver Island slips further westward by about the width of a pencil, placing increasing stress on the megathrust fault. We can’t predict precisely when the next megathrust earthquake will strike, but Bostock advises that its never to early to prepare for the eventuality. As they say: it’s not a matter of if, but when.

The Government of B.C. has a thorough guide on how to prepare for an earthquake on its website.

Roger CollinsRoger Collins

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