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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.
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
||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.
||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.
||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.
||The average distance traveled inland by a tsunami.
||A noticeable sudden retreat of the water.
||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
|Pyroclastic flows impacting on water
|Caldera collapse or subsidence
|Avalanches of cold rock
|Base surges with accompanying shock waves
|Avalanches of hot material
|Air-waves from explosions
|Lahars (mudflows) impacting on water
|Lava avalanching into the sea
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
|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.
|| 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: [email protected]
Caribbean Disaster Emergency Response Agency
Building #1 Manor Lodge, Lodge Hill
St. Michael, Barbados
Tel: (246) 425-0386
Fax: (246) 425-8854
Email: [email protected]
CDEMA Annual Reports