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 Tsunamis
What is a tsunami?
Tsunamis are a potential hazard,
rather than a real major hazard,
in the Eastern Caribbean. In the recorded history of the region,
nobody has ever been killed by a tsunami in any of the islands
from the Anegada Passage to Trinidad and damage has been minor.
Tsunamis are the one geological hazard that are actually increasing
with time. This is because the submarine volcano, Kick `em Jenny,
in the southern Grenadines is gradually evolving into a condition
where it is more and more likely to generate a significant tsunami.
We now consider the probability that Kick `em Jenny will generate
a significant tsunami (amplitude more than ten meters at ten kilometers
from source) within the next 50 years to be greater than 50%.
Why is it called a tsunami?
The word tsunami is taken from two Japanese words which
mean harbor wave.
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What a tsunami is not!
A tsunami is often referred to as a tidal wave
but it is not a tidal wave since it is not caused by the tides
and is not related to the tides. Tides are caused by gravitational
influences of the moon, sun and planets while tsunamis are mainly
caused by earthquakes, landslides and volcanic eruptions. The
pictogram above is now the universally recognized symbol for
a tsunami but it is also misleading. Less than one fifth of
tsunami waves break in this way, like a surfer's dream. A much
more common pattern is that the sea first withdraws an abnormal
distance and then returns like a rapidly-rising tide flooding
low-lying areas. This may be repeated several times.
Understanding tsunami terminology
| Crest |
The
highest point of the wave. When far from land the amplitude
of a tsunami may only be a few metres but as it approaches
land, the amplitude increases rapidly to a height of
possibly several kilometres. |
| Wavelength |
The
distance from one crest of the tsunami to the next. For genuine
tsunamis the wavelength in deep water is very much greater
than the depth of the ocean. The mean depth of the ocean is
about 5 km so that the wavelength
of typical tsunamis is greater than 5 km. |
| Velocity |
The
speed at which the tsunami is approaching land. Far out in
the ocean, tsunamis travel at about 800km/hr (500 miles/hour)
but they slow down considerably as they approach land. |
| Run-up |
The
average distance traveled inland by a tsunami. |
| Draw down |
A noticeable sudden retreat of
the water. |
| Inundation |
The maximum land area covered
by water as a result of a tsunami. |
How are tsunamis different from other sea or ocean waves?
Tsunamis have long wave heights and long periods.
Typical wind-generated waves at the beach may have a period
of about 10 seconds and a wave height of 1.5 metres while a
tsunami can have a period of 1 hour with a wave height greater
than 10 metres.
How do earthquakes produce tsunamis?
The abrupt shifting of the sea floor can result
in the sudden displacement of water from its equilibrium position.
Quite often this movement is downward and results in a significant
depression of the sea surface. As the displaced mass of water
attempts to regain its equilibrium, waves are formed. Because
the first movement is usually downwards, the first wave is usually
a withdrawal of the sea rather than an influx. The greater the
vertical shift in the sea floor, the larger the waves will be.
Why do some earthquakes not generate tsunamis?
Only large earthquakes which occur underneath
or near the ocean and create movements in the sea floor generate
tsunamis. All oceans can experience tsunamis but there are more
large, destructive tsunamis in the Pacific Ocean because of
the many major earthquakes along the margins of the Pacific
Ocean and also because dip-slip earthquakes (which involve vertical
rather than lateral ground motion) are more common in the Pacific
than elsewhere.
How do volcanoes produce tsunamis?
Volcanoes can generate tsunamis in a number
of ways. J.H. Latter (1981) conducted a study of 69 historic
cases of volcano-generated tsunamis and produced a list of 10
known ways as shown in the Table below:
| Probable
cause of Volcanic Tsunami |
% of
Total |
| Earthquakes accompanying eruptions |
22 |
| Pyroclastic flows impacting on water |
20 |
| Submarine explosions |
19 |
| Caldera collapse or subsidence |
9 |
| Avalanches of cold rock |
7 |
| Base surges with accompanying shock waves |
7 |
| Avalanches of hot material |
6 |
| Air-waves from explosions |
4.5 |
| Lahars (mudflows) impacting on water |
4.5 |
| Lava avalanching into the sea |
1 |
What happens to a tsunami as it approaches land?
As a tsunami nears the shallower water
close to the shore, the viscous drag of the continental shelf
slows the front of the wave. The first sign of an approaching
tsunami is usually a significant retreat of the sea. As a result,
the trailing waves pile on top of the waves in front of them (like
a rug crumpled against a wall), thereby significantly increasing
the height of the wave before hitting the shore. Although a tsunami
advances much slower as it approaches land, its momentum is powerful
enough to flatten houses, buildings and trees and carry ships
far inland.
What are the effects when a tsunami hits land?
Tsunamis can devastate coastlines, causing
widespread property damage and loss of life. They strip beaches
of sand that may have taken years to accumulate, uproot trees
and other coastal vegetation and cause large-scale flooding.
Can tsunamis be predicted?
Scientists cannot predict when earthquakes
will occur and so, they cannot determine exactly when an earthquake-generated
tsunami will occur. Volcanically-generated tsunamis can be forecasted
if the volcano is carefully monitored.
Do tsunamis pose a threat to the Eastern Caribbean?
Tsunamis have affected the Eastern Caribbean
in the past. Some of these were generated by earthquakes as shown
in the Table below. The effects of these tsunamis were minor although
in 1918, a tsunami caused about 29 deaths in Puerto Rico.
Past earthquake-generated tsunamis in the Eastern Caribbean
| Year |
Description |
| 1690 and 1843 |
The Leeward Islands earthquakes of these
years generated small tsunamis
but the effects were confined to the Leeward Islands and were
very
small in comparison with the effects of the shaking. |
| 1755 |
The eastern Atlantic (Lisbon) earthquake
generated a tsunami which
crossed the Atlantic and was noticed in several islands. Unfortunately,
some recent accounts of this tsunami have been seriously exaggerated.
Contemporary accounts indicate that the amplitude (peak-to-peak)
in Antigua was less than two metres. |
| November 18, 1867 |
An earthquake-generated tsunami may have
killed up to 20 people in the Virgin Islands but in the historical
accounts there is some confusion between deaths caused by
the tsunami and those caused by a hurricane a few days before.
This tsunami was noticed as far south as Grenada but caused
no damage or casualties. |
Another source of tsunamis in the
Eastern Caribbean has been the Kick `em Jenny submarine volcano
which has generated at least 2 tsunamis since 1939.
Interesting facts on the tsunamigenic potential of
Kick `em Jenny
Did you know that...
-
The Kick `em Jenny eruption
of July 24, 1939, generated a tsunami with an amplitude
of 1-2 metres in the southern Grenadines and northern Grenada.
Run up in Barbados was sufficient to flood roads on the
west coast.
-
Another Kick `em Jenny eruption
on October 30, 1965, produced a minor tsunami. Since 1939,
Kick `em Jenny has erupted at least 11 times and more violent
eruptions are likely in the future. These can generate tsunamis
that can cause serious damage to the coastal areas of several
islands in the Eastern Caribbean.
- Tsunami travel times from Kick `em Jenny
to nearby islands are generally less than 10 minutes which
leaves very little warning time for the general public.
WHAT TO DO ?
If a Tsunami Warning is issued,
NEVER go down to the beach to watch the wave come in because
you will not live to tell the story! Remember that a tsunami
is a series of waves and the first wave is not necessarily the
biggest. Stay out of danger until an "all-clear" is issued by
the competent authority.
Before a tsunami
-
Find out if your home is in
a danger area.
-
If you live in a low-lying
area make yourself familiar with the quickest way to retreat
to high ground. Make sure all family members know the evacuation
plan.
-
If you are close to the sea
and the water retreats by an abnormal amount, move to
high ground at once. Do not stay to see what happens.
-
Listen to the radio for official
updates and instructions.
-
Have the telephone number for
your Disaster Response Agency at hand.
-
Gather disaster supplies:
Flashlight and extra
batteries
Portable, battery-operated
radio and extra batteries
First Aid kit and
manual
Emergency food and
water
- Develop an emergency plan in the event
that family members are separated (for e during the workday
when adults are at work and children are at school). Agree
c a close friend or relative that should be contacted if children
cannot reach their parents and vice-versa.
After a tsunami
-
Stay tuned to a battery-operated
radio for the latest emergency information.
-
Help injured or trapped persons
and persons requiring special assistance (infant;
elderly people and persons with disabilities.)
-
Do not move seriously injured
persons unless they are in immediate danger of furtl
injury. Call for medical assistance.
-
Stay out of damaged buildings.
Return home only when authorities say it is safe
-
Shovel mud while it is still
moist to give walls and floors an opportunity to dry.
-
Check for electrical shorts
and live wires. Never attempt to move live wires.
-
Check for gas leaks.
-
Check for damage to sewage
and water lines.
-
Check food supplies and have
tap water tested by the local health department.
-
Fresh food that has come in
contact with flood water may be contaminated and
should be thrown out.
USEFUL CONTACT INFORMATION
Seismic Research Unit
The University of the West Indies
St. Augustine, Trinidad. W.I.
Tel: (868) 662-4659
Fax: (868) 663-9293
Email: uwiseismic@uwiseismic.com
Website: http://www.uwiseismic.com
Caribbean Disaster Emergency Response
Agency
Building #1 Manor Lodge, Lodge Hill
St. Michael, Barbados
Tel: (246) 425-0386
Fax: (246) 425-8854
Email: cdera@caribsurf.com
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