10,000 years of Cascadia earthquakes The chart shows all 40 major earthquakes in the Cascadia Subduction Zone that geologists estimate have occurred since 9845 B.C. Scientists estimated the magnitude and timing of each quake by examining soil samples at more than 50 undersea sites between Washington, Oregon and California.

Core sample sites 1999-2009

U.S. Geological Survey scientists studied undersea core samples of soil looking for turbidites -- deposits of sediments that flow along the ocean floor during large earthquakes. The samples were gathered from more than 50 sites during cruises in 1999, 2002 and 2009.