Geoscience Reference
In-Depth Information
Source
Tsunamis
Unknowns
Source of the
1755 Lisbon
tsunami
—
Offshore faulting
related to
collision of the
Nubian (African)
and Eurasian
plates
29
The 1755 tsunami was noted in the
Caribbean, from Barbados northwestward
to Cuba. Its maximum estimated Caribbean
height is 7 m. The tsunami is unknown
from ports along the U.S. Atlantic seaboard,
probably because of shielding by submarine
hills that directed the transatlantic waves
northwestward toward Newfoundland and
southwestward toward the Caribbean and
Brazil.
30
How did the 1755 tsunami affect
Puerto Rico and the Virgin Islands?
Does it account for overwash of
Anegada, in the British Virgin Islands
northeast of Puerto Rico (Fig. 3-3e)?
How often can tsunamis like the one
in 1755 be expected?
Seattle fault
—
One of several
faults capable
of displacing
waters of Puget
Sound.
31
Six-meter uplift along the Seattle fault
generated a tsunami in Puget Sound during
an earthquake about 1,100 years ago.
32
The same earthquake set off slides in Lake
Washington.
33
How often does the Seattle fault
produce earthquakes like the one
1,100 years ago? Do tsunamis result
from slip limited to the Seattle fault's
backthrusts, which have a post-glacial
history of repeated earthquakes?
34
SLIDES
—Most slides that set off tsunamis have been triggered by earthquake or, less commonly, by
volcanic eruption. Several grand examples:
•
Lituya Bay, Alaska, 1958—An earthquake-induced rockslide in 1958 set off a giant wave that trimmed
trees to an altitude of 525 m.
35
•
Sunda Strait, Indonesia, 1883—The explosion of Krakatau triggered a tsunami that killed an estimated
35,000 persons.
36
•
North Sea, 8,000 years ago—The Storegga slide displaced 2,400-3,200 km
3
of ocean-bottom materials
37
and generated waves known from tsunami deposits in Norway and Scotland.
38, 39
•
Big Island of Hawaii, 120,000 years ago—Flank collapse produced tsunami run-ups to heights of
hundreds of meters.
40
A catastrophic ancestor to the local Hawaiian tsunamis that killed 46 persons in
1846 and 2 in 1975.
41
Slide-generated tsunamis rarely amount to much on distant shores. Compared with the areas of ocean
loor displaced by faulting during great subduction zone earthquakes, their source areas are usually
compact. Slides therefore yield tsunami waves of short period that diminish rapidly with distance. This
decrease helps limit the hazards to the U.S. Atlantic coast from lank collapse in the Canary Islands, off
West Africa.
42
Alaskan slides
during the 1964
earthquake
—
Slides at
Chenaga,
43
Kenai
Lake,
44
Seward,
45
Valdez,
46
and
Whittier
47
The separate tsunamis from the Chenaga,
Seward, Valdez, and Whitter slides together
account for 79 of the 106 Alaskan deaths
from tsunamis that the 1964 earthquake
triggered (Fig. 3-2e). Most of the slides
resulted from shaking-induced failures of
deltas.
What do these slides imply for Puget
Sound deltas as potential tsunami
sources?
31
