seismic sources adopted in the most recent U.S. Geological Survey seismic-hazard assessment for Alaska.5 A geophysical speculation has such an earthquake recurring at intervals of 750 years or more.
The Homer and Seldovia maps6 show two scenarios: a repeat of the 1964 earthquake, and a hypothetical break on a local fault believed inactive in the past 2 million years. Recurrence intervals for 1964-style earthquakes have averaged about 600 years during the past 5,000 years.7
Far-field tsunamis pose the most expectable source of tsunami hazards in Cannon Beach and Seaside, Oregon. However, as in Alaska, it is the near-field tsunamis that dominate the hazard in terms of tsunami size.
Worst-case inundation extends more than twice as far inland for a near-field (Cascadia) tsunami than for a far-field (eastern Aleutian) tsunami, according to an inundation map prepared by the State of Oregon in 2008 for the tourist town of Cannon Beach.8 A similar contrast is apparent in maps, of the nearby town of Seaside, that were prepared a few years earlier by a group of federal, state, and local scientists.9 The Seaside maps show contrasting observations (inundation limits and sedimentary deposits of the 1964 Alaskan tsunami, versus sedimentary deposits of the 1700 Cascadia tsunami), as well as tsunami heights depicted in terms of probabilities that are tied to estimated recurrence intervals.
Far-field tsunamis are the most expectable in Oregon because they can beam toward that state from multiple parts of the Pacific Rim. Thus, in aggregate, they happen more often than do tsunamis from Cascadia sources alone. The Seaside mapping accordingly shows far-field tsunamis as the dominant source of hazard for flooding that would lap onto the edges of town. Only at lower probabilities, commensurate with Cascadia recurrence intervals that average about 500 years, do the waters cover the entire town.
A Cascadia rupture that includes the California part of the subduction zone produces a simulated tsunami that, at Crescent City, runs inland for double the inundation distance of the 1964 Alaskan tsunami. This Cascadia tsunami, moreover, begins with a positive (leading elevation) wave that arrives in less than a half hour.10
1. National Science and Technology Council. 2005. Tsunami Risk Reduction for the United States: A Framework for Action. Subcommittee on Disaster Reduction and United States Group on Earth Observations, National Science and Technology Council, Washington, DC.
2. Dunbar, P.K. and C.S. Weaver. 2007. U.S. States and Territories National Tsunami Hazard Assessment: Historical Record and Sources for Waves. National Tsunami Hazard Mitigation Program, National Oceanic and Atmospheric Administration, Silver Spring, Maryland.