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About fifty miles off our coast, one slab of the Earth is slowly sliding under another. Every few hundred years it slips all at once and makes one of the largest earthquakes on the planet. The last time was the winter of 1700 — and the story of how we know the exact night is one of the great detective cases in all of science.
The Cascadia Subduction Zone runs offshore from Vancouver Island all the way to Northern California. There, a piece of the sea floor is diving beneath the edge of North America. The two plates snag and lock together, and for centuries the land quietly squeezes and bends — storing up strain like a stick you slowly bend in your hands.
When the snag finally lets go, the whole thing lurches at once. That kind of quake — a “megathrust” — can reach magnitude 9, among the biggest earthquakes Earth makes, and it can send a tsunami racing at both the coast nearby and shores an ocean away.
In late January 1700, a tsunami flooded villages in Japan — but no one there had felt a quake. Careful Japanese records noted the day and hour of this mysterious “orphan” wave.
Along our coast stand groves of gray, dead trees in the salt marsh. When the 1700 quake struck, the land dropped several feet in an instant and seawater drowned the forest. Tree rings date their death to the winter of 1699–1700 — including a famous ghost forest on the Copalis River, right on the Grays Harbor coast.
Coastal tribes carried oral histories of a terrible night when the ground shook and the sea rushed in — passed down for generations, and matching the very same event.
Line up the tsunami’s nine-hour trip across the Pacific with the drowned forests and the tribal accounts, and three totally different kinds of evidence point to the same answer: the evening of January 26, 1700, a magnitude-9 earthquake. Detective work, done with tree rings and old letters.
Be the detective: open each clue and see if you can crack how we know the date.
Open all three clues to crack the case.
Cascadia has made about 19 of these great quakes in the last 10,000 years. No one can say when the next one comes — but being ready is something we cancontrol, and it’s not scary once you know the plan:
Knowing the science is the first step to not being afraid of it. See the district’s safety & emergency information →
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