Geoscience Reference
In-Depth Information
Hawaiian Islands will get more than 6-h warning of any
tsunami generated around the Pacific Rim, while the west
coast of United States receives more than 4-h notice of
tsunami originating from either Alaska or Chile. The real
concern is the potential warning time, or margin of safety, if
a tsunami originates near the edge of the continental shelf,
off the Hawaiian Islands or the continental shelf of Wash-
ington State. Of the 10 most destructive trans-oceanic tsu-
nami over the last 250 years, 84 % of fatalities occurred
within the first hour of generation; 12 % within the second
hour, and the remainder subsequently (Gusiakov
2008
).
There are two possible scenarios for locally generated tsu-
nami. In the first scenario, a tsunamigenic earthquake is
responsible for the tsunami. The earthquake can occur at the
shelf break or in deeper water offshore. In either case, once
the wave begins to cross the continental shelf, the depth of
water determines its velocity. Hence the slope and width of
the shelf dictate the tsunami's travel time. In the second
scenario, the earthquake generates a submarine landslide on
the shelf slope. In this case, the longer it takes a submarine
slide to develop, the further it has moved from shore and the
longer it takes for the resulting tsunami to propagate to the
coast.
A crude approximation of the time it takes tsunami
spawned by these processes to cross a shelf can be deter-
mined by dividing the shelf into segments, and calculating
the time it takes the wave to pass through each segment
using Eq. (
2.2
). The calculations are simplified if the shelf is
assumed to have a linear slope. These results are presented
in Fig.
10.12
for different shelf slopes and widths. These
relationships should be treated cautiously because they are
based upon simplified assumptions. For example, the Grand
Banks earthquake of November 18, 1929 occurred at the
edge of the continental shelf, 300 km south of the Burin
Peninsula of Newfoundland that was eventually struck by
the resulting tsunami. This tsunami arrived 2.5 h after the
earthquake—well within the 4 h indicated in Fig.
10.12
.
Tsunami induced by submarine slides may travel as fast as
1,500 km h
-1
—much faster than linear theory would sug-
gest. If anything, the margin of safety shown in Fig.
10.12
is
too lenient.
Figure
10.12
shows that there is a log-linear relationship
between travel time and the distance to the shelf break. This
relationship holds for shelf widths as narrow as 2 km and as
wide as 500 km. The figure also indicates that the travel
time for a tsunami asymptotically approaches 3.25 min for
the steepest shelf slopes. These relationships can be put into
a more familiar context using two examples. In the first
example—that of the east coast of the United States—the
shelf break lies more than 165 km from shore. Here, a
tsunami generated at the edge of the shelf would take over
135 min or 2.2 h to reach the closest point at shore. This
does not seem like much when compared to the time that
(a)
500
200
100
50
20
10
5
Margin of safety
2
1
0
0.5
1.0
1.5
2.0
Slope in degrees
(b)
500
200
100
50
20
10
Margin of safety
5
2
1
5
10
20
50
100
200
500
Distance (km) from shore to shelf
break at 130 m depth
Fig. 10.12
Travel time for tsunami moving across a continental
shelf. a For various slopes. b For various shelf widths
residents along the west coast of the United States have for
tsunami generated in Alaska or Chile, but it is more than
sufficient when compared to the second example, that off
Sydney, Australia, where the shelf is steep, being only
12-14 km wide. Unlike the east coast of the United States,
substantial evidence has been found along this coast for the
impact of tsunami. Here, a tsunami generated on the con-
tinental slope would take only 10-12 min to reach shore.
Within this time, one would be hard-pressed to reach safety
if sunbathing on a local beach, or even worse, surfing off
one of the headlands. At Wollongong, south of Sydney, the
seabed also shows geological evidence for a submarine
landslide measuring 20 km long and 10 km wide positioned
50 km offshore (Jenkins and Keene
1992
). A tsunami
generated by this slide would only take 40 min to reach
shore.
10.5
Where Should You Go if There Is
a Tsunami Warning?
While it may seem obvious from the previous sections,
there is more to this question than meets the eye. Obviously
one should not rush to cliffs, take to boats inside harbors, or
