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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 maps 6 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 inter-
vals for 1964-style earthquakes have averaged about 600 years during the past 5,000 years. 7
OREGON
Far-ield tsunamis pose the most expectable source of tsunami hazards in Cannon Beach
and Seaside, Oregon. However, as in Alaska, it is the near-ield tsunamis that dominate the
hazard in terms of tsunami size.
Worst-case inundation extends more than twice as far inland for a near-ield (Cascadia)
tsunami than for a far-ield (eastern Aleutian) tsunami, according to an inundation map pre-
pared 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 prob-
abilities that are tied to estimated recurrence intervals.
Far-ield tsunamis are the most expectable in Oregon because they can beam toward that
state from multiple parts of the Paciic Rim. Thus, in aggregate, they happen more often than
do tsunamis from Cascadia sources alone. The Seaside mapping accordingly shows far-ield
tsunamis as the dominant source of hazard for looding 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.
CALIFORNIA
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 eleva-
tion) wave that arrives in less than a half hour. 10
REFERENCES
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 Tech-
nology 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.
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