A geologist noticed an ocean bay on the coast of Washington with a bunch of dead trees in the water. Then he started noticing other bays with dead trees.
He surmised (correctly) that they were there due to landslides. He contacted a tree-ring expert to see what dates the landslides had occurred. (The trees died when they sank into the salt water).
The trees all died around the same time (sometime between 1699 and 1701). In landslides all up and down the coast indicating a HUGE earthquake.
Of course this was pre-history in America. But in Japan they had been keeping records of earthquakes and tsunamis for thousands(?) of years.
Somebody else went through the historical records in Japan. They found a something called a “ghost”(?) tsunamis. A large tsunami but with no earthquake felt in Japan. They looked at the size of the wave in Japan and it fit with the amount of shift in the landslides in Washington.
Based on the speed of the tsunami in water, they ended up calculating that the Cascadia Earthquake happened at about 2100 hours on January 26, 1700!
Further studies have shown that the quake has occurred periodically over time. On average about every 500 years and greater than Magnitude 8. (So - any day now!).
They also looked at the various Indian tribe stories - they also talk about the earthquake and tsunami.
An Alaskan tribe has a story about the Snow King - rapid glaciation a LONG time ago. And now understood as not just a fable, but real life based on archaeology. But that will be saved for another day.
There’s a lot of information about the length of the history of quakes in Japan, but maybe Duck-duck-go isn’t the best search engine to use to get information about the instruments used to measure them. One article said that the Han Dynasty Era information from China was what inspired the Japanese to keep records. Do you have a search engine recommendation?