The mega earthquake took place at about 9:oopm Pacific Time on January 26, 1700 (NS). Although there are no written records for the region from the time, the timing of the earthquake has been inferred from Japanese records of a tsunami that does not correlate with any other Pacific Rim quake..
The most important clue linking the tsunami in Japan and the earthquake in the Pacific Northwest comes from studies of tree rings which show that several "ghost forests" of red cedar trees in Oregon and Washington, killed by lowering of coastal forests into the tidal zone by the earthquake, have outermost growth rings that formed in 1699, the last growing season before the tsunami. This includes both inland stands of trees, such as one on the Copalis River in Washington, and pockets of tree stumps that are now under the ocean surface and become exposed only at low tide.
Sediment layers in these locations demonstrate a pattern consistent with seismic and tsunami events around this time. Core samples from the ocean floor, as well as debris samples from some earthquake-induced landslides in the Pacific Northwest, also support this timing of the event. Archaeological research in the region has uncovered evidence of several coastal villages having been flooded and abandoned around 1700
On average the Cascadia Subduction Zone will rupture about every 240 years +/-. That being said, we are 78 years past due according to historical and scientific date. The next "big" one has the potential of being a full rip. Meaning it will go the full 600 + miles from Crescent City, CA to Victoria, British Columbia. Shaking will be the worst along the coastal areas with an estimated shake time of 3 to 5 minutes. This will be felt all the way inland to Bend, OR (definitely not as severe). Following this will be the massive tsunami, the likes we have never seen... even greater than Japans in 2011. Your only warning for the tsunami is the severe shaking, that is your sign to GET TO HIGH GROUND immediately if you are anywhere along the coast.