Geoscience Reference
In-Depth Information
Relative Hazards of Near- and Far-ield
Tsunami Sources
The 2005 National Science and Technology Council report describes the nation's mix of
tsunami threats in terms of sources termed “local” and “distant”:
United States coastal communities are threatened by tsunamis generated by both
local sources and distant sources. Local tsunamis give residents only a few minutes
to seek safety. Tsunamis of distant origins give residents more time to evacuate the
threatened coastal areas, but require timely and accurate tsunami forecasts of the haz-
ard to avoid costly false alarms. Of the two, local tsunamis pose a greater threat to life
because of the short time between generation and impact. The challenge is to design
a tsunami hazard mitigation program to protect life and property from two very differ-
ent types of tsunami events.
1
The relative tsunami hazard of local and distant sources varies with the region according
to a nationwide assessment prepared a few years ago for the National Tsunami Hazard Mitiga-
tion Program (NTHMP).
2
Distant sources account for most of the tsunami hazard in Hawaii,
while local sources predominate in Alaska. Washington, Oregon, California, and the Caribbean
face a mix of local and distant. So does the U.S. Atlantic seaboard, from its nearby landslides
and its exposure to hypothetical tsunamis from the Puerto Rico Trench.
3
Below are summaries of tsunami hazard studies in Alaska, Oregon, and California that allow
direct comparison between local and distant.
ALASKA
Nearby tsunami sources dominate the hazard depicted on tsunami inundation maps of
Kodiak, Homer, and Seldovia—communities in the vicinity of the rupture area of the giant 1964
Alaska earthquake.
The Kodiak maps
4
depict seven scenarios: four of them for partial or complete breakage
of the 1964 rupture area, one for surface rupture of a thrust fault that extends offshore from
Kodiak Island, and two tsunamis of distant origin. The scenarios with the greatest inundation
result from repetition of 1964-style earthquakes, and the scenarios with the least inundation
result from distant earthquakes off the Aleutians and at Cascadia.
The smallest of the modeled inundations in the Kodiak area corresponds to a distant
earthquake on the Aleutian-Alaska subduction zone west of Kodiak Island. The starting as-
sumption here is a break that extends across the so-called Shumagin seismic gap and includes
rupture areas of earthquakes in 1938 and 1946. Such a hypothetical earthquake is among the
