Great Earthquakes of Variable Magnitude at the Cascadia Subduction Zone

Radiocarbon age ranges for evidence of great plate-boundary earthquakes and accompanying tsunamis at seven coastal paleoseismic sites, combined with relative amounts of coseismic subsidence and heights of tsunamis, show a 5000-year history of great earthquakes of variable magnitude at the Cascadia subduction zone. Evidence for rupture of most of the plate boundary (greater than 620 km) is only conclusive for the earthquake of 26 January 1700, but the earthquake about 1.6 ka also had a long rupture, substantial subsidence, and a high tsunami consistent with a magnitude 9 earthquake. Correlations suggest that earthquakes about 1.35 ka, 2.5 ka, and 3.4 ka also may have been close to magnitude 9. A rupture about 0.8 ka was limited to the northern and central parts of the subduction zone, and a northern rupture about 2.9 ka may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1.1 ka, 1.7 ka, 3.2 ka, 4.2 ka, 4.6 ka, and 4.7 ka. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Southernmost Cascadia is typified by short as well as long ruptures during great earthquakes of variable magnitude and more irregular recurrence than in northern Cascadia.