The last known great earthquake in the northwest was the 1700 Cascadia earthquake . Geological evidence indicates that great earthquakes ( > magnitude 8.0 ) may have occurred sporadically at least seven times in the last 3,500 years , suggesting a return time of about 500 years . Seafloor core evidence indicates that there have been forty-one subduction zone earthquakes on the Cascadia subduction zone in the past 10,000 years , suggesting a general average earthquake recurrence interval of only 243 years . Of these 41 , nineteen have produced a " full margin rupture , " wherein the entire fault opens up . By comparison , similar subduction zones in the world usually have such earthquakes every 100 to 200 years ; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip
1700 Cascadia earthquake
1700 Cascadia earthquake
Cascadia subduction zone