In the mid-1980ís, perception of earthquake hazard in Oregon and the Pacific Northwest abruptly changed from one in which earthquakes were not considered a significant hazard to one of extreme hazard due to the recognition that the largest possible type of earthquake has rocked the region in the past.
Compelling data indicate that the Cascadia subduction zone, the interface across which the Juan de Fuca tectonic plate offshore to the west dives beneath the North American plate onshore in the east, is one gigantic earthquake fault that is the source of very large earthquakes. These data point to January 26, 1700, at 9 p.m. as the date and time of the last earthquake. Evidence of this event includes an instantaneous 6-foot subsidence of coastal estuaries and bays, Native American oral history, widespread death of trees in the winter growing season, and a phantom tsunami in Japan. A region extending from Vancouver Island to northern California was shaken intensely by the 1700 earthquake.
That earthquakes of this type recur every 500 to 600 years is suggested by records of periodic coastal subsidence extending back at least 3000 years and a 10,000 year-record of intense shaking of the continental margin contained within sediments on the ocean floor to the west.
That this type of earthquake is likely to happen again is suggested by Global Positioning System measurements of displacement of the Earthís surface resulting from the accumulation strain energy across the fault that will be released in the next great earthquake.
Today, the past occurrence and future inevitability of a Cascadia Subduction zone earthquakes has gained widespread acceptance by Earth scientists, public officials, and emergency planning organizations. This fundamental shift in the perception of earthquake hazard has profound physical, social, and economic implications for the Pacific Northwest. For the next Cascadia earthquake, those implications include: that most of the physical infrastructure may not perform well (schools, hospitals, fire stations, bridges, utilities, etc.); that relief efforts will be hampered by physical isolation of the Northwest; and that economic losses are likely to be in the 10ís of billions of dollars.