Great earthquakes on the Cascadia subduction zone have an estimated average recurrence interval of about 500 years. This estimate comes from geological evidence for at least seven magnitude-8-or-greater earthquakes during the past 3,500 years.
Cascadia stretches from northern California through Oregon and Washington to Vancouver Island. The Juan de Fuca Plate is being forced beneath the North American Plate, allowing stress to accumulate along a locked offshore boundary. When that boundary ruptures, it can produce a very large megathrust earthquake and a destructive tsunami.
The last known full-margin Cascadia earthquake occurred on January 26, 1700. Japanese historical records describe a damaging “orphan tsunami” on that date, while Pacific Northwest coastal sediments preserve evidence of the same event. These independent clues helped scientists identify its timing and size.
The figure can be misunderstood because geological studies identify more earthquakes than just the largest full-margin events. Some research estimates roughly 243 years for the broader set of subduction-zone earthquakes, but approximately 500 years is the standard rounded estimate for the great events highlighted in this question.