Geoscience Reference
In-Depth Information
Examples of Tsunami Sources
That Threaten the United States
Estimates of tsunami losses and heights are from the NOAA tsunami database.
1
Source Tsunamis Unknowns
FAULTS
—Seismic slip on faults generates tsunamis directly by displacing the loors of water bodies.
The slip can also generate tsunamis indirectly through shaking that triggers slides; a category of source
treated separately below. Tsunamis most commonly result from slip on the subduction-zone faults that
convey one tectonic plate beneath another.
Aleutian-
Alaskan
subduction
zone
—Along
about 2,500 km
of its length,
ruptured almost
completely
in a series of
earthquakes
between 1938
and 1965.
2
The zone's largest 20th-century tsunamis,
both on nearby coasts and on distant
ones, were generated during the Aleutian
earthquake of 1946 and the Alaskan
earthquake of 1964. The far-ield part of the
1946 tsunami, chiely generated directly
by faulting,
3
caused most of Hawaii's
recorded tsunami deaths. Similarly, the
greatest tsunami in Washington, Oregon,
and California written history originated off
Alaska with tectonic displacement during
the 1964 earthquake. Judging from geologic
records of predecessors to the 1964
earthquake during the last 6,000 years (Fig.
3-3c),
4
ocean-wide tsunamis from the 1964
source recur at irregular intervals averaging
close to 600 years.
How often do Aleutian sources spawn
tsunamis comparable in far-ield size
to the tsunamis of 1946 and 1964?
How much are recurrence intervals
lengthened by aseismic slip in the
fault-rupture areas? Will the next large
tsunami from the 1964 source recur
sooner than average because the
1964 earthquake ended a recurrence
interval close to 900 years, about
300 years longer than average? How
persistent are the lateral limits of
Aleutian-Alaskan fault ruptures of
the 20th century as boundaries that
deine individual tsunami source
areas?
5
Cascadia
subduction
zone
—1,100
km long.
Conirmed as a
tsunami hazard
by geophysical
and geological
research in
the 1980s and
1990s.
6
The main nearby tsunami source for
Washington, Oregon, and northern
California. Also among the main distant
sources for Hawaii.
7
Intervals between the
zone's great earthquakes (of estimated
magnitude 8.0 or 9.0) average close to
500 years and range from a few centuries
to a millennium (Fig. 3-3d).
8, 9
The most
recent of Cascadia's great earthquakes, of
estimated magnitude 8.7-9.2,
10
spawned an
ocean-wide tsunami in A.D. 1700 (Fig. 2f ).
What proportion of Cascadia's great
earthquakes produce unusually
large tsunamis by attaining
magnitude 9.0?
9, 11, 12
How do those
proportions vary along the length
of the subduction zone? What
partial-length ruptures should be
assumed by tsunami modelers?
13
What parts of the zone are likely to
augment tsunamis on nearby shores
by producing greater than average
deformation of the ocean loor?
12
