Geoscience Reference
In-Depth Information
Alaska (Figure 2.2, Appendix A). The next six paragraphs illustrate unknowns that are spelled
out more fully as questions in Appendix A.
Far-ield heights of tsunamis from circum-Paciic sources
. For hazard and evacuation mapping,
simulations of far-ield tsunami heights on U.S. Paciic shores require estimates about tsunami
sources at subduction zones on the Ring of Fire (Figure 2.2). The simulations are sensitive to
earthquake size because far-ield tsunami height increases substantially with earthquake
magnitude (Abe, 1979). For example, to make worst-case simulations for Pearl Harbor, a NOAA
group used 18 tsunami sources, together spanning all subduction zones on the Paciic Rim
(Tang et al., 2006). For each source, the group used an earthquake of magnitude 9.3 (McCaffrey,
2008). For some of the subduction zones, the assumed magnitude 9.3 differs from the largest
known earthquake magnitude of the past, which falls short of 8.0 or 8.5 (blue or brown, respec-
tively, in Figure 2.2). This difference in assumed source magnitude results in the correspond-
ing worst-case tsunami on distant U.S. shores to be larger by a factor of about 10 from the
tsunamis of the past.
Far-ield hazards from Alaskan sources
. Earthquake-generated tsunamis from sources along
the Aleutian-Alaskan subduction zone pose far-ield tsunami hazards in Washington, Oregon,
California, and Hawaii. If a hazard map is to give each of these sources appropriate weight, it is
necessary to estimate how large the earthquakes can get and how often they happen (Geist
and Parsons, 2006; González et al., 2006). For tsunamis from most of the Aleutian-Alaskan
subduction zone, currently there is almost no basis for such weighting except for geophysical
estimates of how much plate motion gets spent on earthquakes of various sizes (González et
al., 2006; Wesson et al., 2008). Little is known about how often the tsunamis actually recur ex-
cept for the source of the oceanwide 1964 Alaskan tsunami (Carver and Plafker, 2008). Even for
the 1964 source, it is unclear whether the next large tsunami is expected to recur sooner than
average because the 1964 earthquake ended a recurrence interval, which was 300 years longer
than the previous 600-year average.
Near-ield hazards from Cascadia tsunamis
. Although documented geological histories of great
Cascadia earthquakes extend thousands of years into the past, persistent uncertainties about
them have yielded wide-ranging estimates of tsunami hazards. A probabilistic analysis of
tsunami hazards in Seaside, Oregon, showed the hazard to be sensitive to variability in earth-
quake size and recurrence (González et al., 2006). A subsequent hazard map for nearby Cannon
Beach doubled the maximum tsunami height relative to previous estimates (Priest et al., 2009).
Caribbean hazards
.
Islands of the Caribbean are threatened by tsunamis mainly from local
tectonic sources and the subsequent potential for submarine landslide. One tsunami source,
near Puerto Rico, may even threaten coastlines as far away as Massachusetts. Local tsunami
sources caused loss of life in the Virgin Islands in 1867 and in western Puerto Rico in 1918
(O'Loughlin and Lander, 2003). The 1867 tsunami began during an earthquake (Reid and
Taber, 1920; McCann, 1985), as did the 1918 tsunami (Reid and Taber, 1919). However, the 1918
