Seven great earthquakes, or earthquake series, probably ruptured the southern Washington part of the Cascadia subduction zone in the past 3,500 years. Each earthquake was probably of magnitude 8 or larger. The earthquakes define six recurrence intervals that average about 500 years. The longest interval, about 700-1300 years, was followed by two of the shortest, which together lasted less than 800 years. Another long interval, 600-1000 years, ended with an earthquake 300 years ago.
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USGS Numbered Series
Recurrence intervals for great earthquakes of the past 3,500 years at northeastern Willapa Bay, Washington