How scientists know when the last big Cascadia earthquake happened

This is actually a NORTHCOM planning priority.

American researchers used carbon dating on the spruce, peat and fossilized plants. Sitka spruce as far apart as southern Washington and Northern California died from the high waters during the same few decades — sometime between 1695 and 1720. Their rings were wide right up through the last ring, proving that a sudden event rather than slow sea level rise was the culprit.

If a large Tsunami (20-30ft high) comes ashore near the mouth of the Columbia River after significant movement from the Cascadia Fault, it's goodbye for Warrenton, Gearhart, Seaside, Cannon Beach, Tolavana Park and Arch Cape on the Oregon Coast. The only saving grace for Warrenton will be the mile of 30ft high sand dunes between them and the water which should take some of the steam out of the wave yet flooding will still be extreme, but the rest of those cities are right on the beach. Downtown Astoria might be underwater as well.