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Then the tsunamis came. Slicing into the peat with shovels and chain saws, geologists found a layer cake of peat, sand, and mud laid down by the inrushing tsunamis. The waves flattened plants and then entombed them in a layer of sand. The parts of the coast that ended up below the tide mark because of the earthquake were then covered by muddy sediment, and were colonized by salt-tolerant plants. The story doesn't stop on the Pacific Northwest coast. Across the ocean, in Japan, meticulous government records speak of flooding in several coastal towns on the evening of January 27/28, 1700. In northern Japan, 20 homes were damaged in the town of Miyako by waves up to 10 feet high. 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 comparing the historical deposits to those left behind by more recent events -- for example, the tsunami that attacked the California coast after the great Alaska quake of 1964 -- geologists can estimate the size of the past waves. Another clue comes from topography: By mapping the location of tsunami sands, it's possible to estimate the minimum height of the waves. The presence of a tsunami sand deposit east of a 10-foot-high ridge, for instance, means the tsunami that carried the sand there must have been at least 10 feet high. This and other evidence suggests that the waves were up to 30 feet or higher -- high enough to devastate low-lying coastal communities.
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