Studies of past subduction earthquakes are inexact things, based on finding their geologic signs: sudden changes of elevation that drown coastal forests, disturbances in ancient tree rings, buried beds of beach sand washed far inland and so on. Twenty-five years of research has determined that Big Ones affect Cascadia, or large parts of it, every few centuries. Times between events range from 200 to about 1000 years, and the average is around 500 years. The most recent Big One is rather well dated, although no one in Cascadia at the time could write. It occurred around 9 p.m. on 26 January 1700. We know this because the tsunami it generated struck the shores of Japan the next day, where the authorities recorded the signs and damages. In Cascadia, tree rings, oral traditions of the local people and geologic evidence support this story.
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Geodetic-leveling data indicate that a large patch of the interpolate decollement (the main fault between the North American Plate and the subduction oceanic plate) off southern Oregon is locked. This and other evidence has led to proposals that an earthquake with a magnitude as large as 9 could someday devastate the
Geodetic-leveling data indicate that a large patch of the interpolate decollement (the main fault between the North American Plate and the subduction oceanic plate) off southern Oregon is locked. This and other evidence has led to proposals that an earthquake with a magnitude as large as 9 could someday devastate the