Farther south, rice paddies and storehouses were flooded. Using a computer simulation to work backwards, scientists figured out that to make a six-foot tsunami in Japan, the Cascadia earthquake of 1700 had to be magnitude 9.0 or larger. Stories passed down for centuries by Native Americans in the region speak of shaking and flooding that came on a winter's night, consistent with the historical evidence from Japan. The geologists have also found clues to the size of the tsunamis that attacked the Pacific Northwest. The thickness of the sand deposits they left behind is one kind of clue.
Thicker deposits mean larger waves carrying more sediment scoured up from the seafloor.
By mapping the location of tsunami sands, it's possible to estimate the minimum height of the waves.