Following is a Letter by Rafi Ahmad
to the Editor of The Daily Observer dated August 14, 1998 :
Kindly allow me space in the Daily Observer/ Sunday Observer to share with the readers some additional information and clarifications arising from the Sunday Feature : "Tidal wave threat" of 26 July 1998.
IS TSUNAMI A THREAT TO JAMAICA?
We would like to congratulate "Sunday Observer" for selecting "Tidal wave threat" as a Sunday Feature, July 26, 1998. It was well presented. However, following the printing of this feature there were questions and interested persons needed clarifications and more information.
One of the aims of Unit for Disaster Studies is to provide, as far as possible, accurate, uptodate and state-of-the-art information on natural hazards to all concerned. Also, we hope that presentation of facts and data on the physical environment in which we live shall stimulate young minds who may wish to select environmental topics for their school project work. We see the news media as an effective medium for timely public education and also an essential component in all the efforts towards protection against natural disasters.
Our first concern is that in describing natural hazards and disasters we should speak the same language. It is not uncommon to find situations where different phrases to describe the same phenomenon by the scientific community and media create confusion in people's minds. Moreover, from a practical point of view, and as is the practice in the medical profession, for example, curing of a specific ailment and future protection from it depends on a correct diagnosis. The process of diagnosis in turn depends on an accurate description of the problem and its history by the patient, confirmation by laboratory tests where necessary, and investigators overall familiarity with the disease in question. Similarly,in order to practice prevention and loss reduction from natural hazards we must know exactly what is it that we wish to correct.
We recommend to Office of Disaster Preparedness and Emergency Management (ODPEM) that a working group is established, as a matter of priority, to standardize the definitions and usage of various natural hazard related terms following the relevant international conventions.
What is a tsunami and how it originates?
Tsunami (note added later: the correct plural of tsunami is tsunami) are not "tidal waves" for they have nothing to do with the tides that are due to gravitational forces of the moon and sun. They are sea waves caused by various processes as explained below. Storm surges are an abnormal rise in sea water level associated with hurricanes and other storms at sea. These processes result in the flooding of the coastline termed as "coastal flooding". The effects of coastal flooding caused by tsunamis are the same as those of storm surges. Flooding associated with the runoff in streams is "riverine flooding". In this article we are concerned with tsunami only. A gravity wave may propagate in seawater from a large disturbance of the sea floor started by an earthquake, eruption of a submarine volcano, or a massive landslide. Such a wave is called a tsunami, following a Japanese word meaning, " harbour waves". In the English-language tsunami (soo-NAHM-ee) are termed seismic sea waves; in French, vagues sismique and raz de maree; and in Spanish, maremoto. Tsunami are relatively more common in the area of the Pacific Ocean and especially along the Pacific Rim.
Tsunami is a collective name for a series of travelling sea waves of extremely long length and period caused by a number of physical processes, which may be genetically linked. Among the various types of faults that cause earthquakes, it is the dip-slip faults with predominant vertical movements that are generally tsunamigenic. However, earthquakes may also trigger landslides both onshore and in the submarine environments that are responsible for many known destructive tsunami occurrences worldwide. The submarine landslides, variety lateral spreads, are believed to be responsible for many of the localized tsunami in Jamaica. The best example of this is the submarine landslide that took with it a part of Port Royal to the bottom of the Kingston Harbour on Tuesday, 7th June 1692 at about 11.40 a.m.
Tsunami may reach enormous dimensions and the energy associated with these waves enable them to travel across entire ocean basins. The velocity of a tsunami is equal to the square root of the product of the depth of water times acceleration due to gravity. Upon entering shallow waters, the wave height increases dramatically, inundating low-lying coastal areas. Harvey Blatt has estimated that a 4.8m high tsunami traveling at 643 km/hr (the speed of a jetliner) in the deep ocean water may hit a coastline at a speed of 48 km/hr with a wave about 30m high. In a real situation the height of a wave will vary from place to place depending upon a number of factors including the offshore and near-shore coastal zone features.
Tsunami rank among the most destructive phenomena and have been responsible for some of the worst disasters in human history. The damage potential of tsunami is measured on a "Scale of Tsunami Intensity" developed by UNESCO in 1978.
How to recognize a past tsunami event?
Since a sea wave is the cause of coastal flooding, it leaves behind its signature on the coastline in the form of sediments, called tsunami deposits. Identification and mapping of these sedimentary deposits and determination of their ages have enabled geologist to reconstruct paleoseismic histories (time period of significance to humans) of many areas worldwide. Mapping of tsunami deposits inland is the most reliable way to determine the extent of tsunami run up distances. Age determinations of tsunami deposits facilitates an extension of the seismic history of a region beyond historic times and forms a basis for determining tsunami return times with confidence. This type of research would be very rewarding in Jamaica.
Tsunami have often been associated with unusual phenomenon. It has been reported that sea glowed brilliantly at night in Japan during 1933 tsunami event, a result of stimulation of vast numbers of the luminescent organism Noctiluca miliaris. Also, fishermen in Japan have reported that sardines caught during tsunami events often have swollen stomachs, having swallowed bottom –living organisms which were raised to surface by disturbances on the sea floor.
The science of tsunami is a relatively new scientific discipline dating back to some 50 years.
Tsunami hazard on Jamaica: Analysis of the currently available data on various natural disasters in Jamaica suggests that tsunami hazard is not as significant as hazards due to landslides, riverine flooding, hurricane and earthquakes. However, Jamaica has a history of tsunami that have affected the coastline since 1688 causing loss of life and destruction of property. (Note: records are available dating back to 1688).
Jamaica's onshore and offshore geologic setting makes the coastline particularly vulnerable to tsunami hazard. The World Map of Natural Hazards (1988) prepared by Munich Re shows the entire coastline of Jamaica exposed to tsunami. Since most of the urban population in Jamaica resides in coastal areas any future tsunami is likely to have a disastrous impact because of the development in the coastal areas. It therefore makes sense to be aware of this hazard while determining the acceptable risks in the coastal settlements.
Lander has listed the following tsunami events in Jamaica:
Port Royal, March 1, 1688; Port Royal and eastern Jamaica, June 7, 1692; north coast, June 11, 1766; Savanna-La-Mar, October2, 1781, hurricane and earthquake may have occurred simultaneously; Montego Bay, October 27, 1787, a low validity report; November 11 or 12, 1812, details are not available; north coast and Kingston harbour, August 12, 1881; and Port Royal, Kingston and eastern Jamaica, January, 14, 1907. Accounts of some of these events are not comprehensive and more research is needed to validate their practical and site specific use.
The Caribbean region is also vulnerable to teletsunami, which may have their origin outside of the region. For example, James Lander has reported that the tsunami generated by the November 1, 1755 Great Lisbon Earthquake, Portugal
"… sent waves into the Caribbean with amplitudes of 7m at Saba, 3.6m at Antigua and Dominica, 4.5m at St.Martin, leaving a sloop anchored in 4.6m of water laying broadside on the dry bottom, and 1.5-1.8m at Barbados. There are no reports for their effects in other islands in the Caribbean, but with waves of these sizes, they probably affected most of the islands." We do not have data on the return time of this event. Lander has further stated that, " Such tsunami will have about eight hours of lead time before they arrive from the source of the Lisbon tsunami, and have long periods of up to an hour. The danger period may last for upto twelve hours".
Kick'Em Jenny is a submarine volcano, located between Grenada and Carriacou in the Lesser Antilles volcanic arc. It erupts regularly. A tsunami travel time chart prepared by Lander shows that a tsunami generated by this volcano would take about 150 minutes to reach Kingston.
Literature on tsunami in Jamaica: Tsunami occurrence and impact in Jamaica are available in " A Catalogue of Felt Earthquakes for Jamaica with reference to other islands in the Greater Antilles" by J.M. Tomblin and G.R. Robson, 1977. There are two other studies dealing with the regional analysis of tsunami occurrence. Firstly, reference is often made to a study conducted in the late 1970's by M.Thorm and C. Deane of St. Augustine Campus, UWI. (Readers, please contact the author if you have a copy of this report. Many thanks.). Secondly, a readily available recent study " Caribbean tsunami: an initial history" by James F. Lander" in Natural Hazards and Hazard Management in the Greater Caribbean and Latin America (Editor: R. Ahmad), Publication No. 3, Unit for Disaster Studies, UWI, Mona. Lander's paper includes a comprehensive bibliography on Caribbean tsunami.
Living with tsunami in Jamaica:
Tsunami is a natural phenomenon which can not be prevented, however, this does not imply that no one should live in coastal areas. Loss reduction from potential tsunami is possible through a continuous monitoring of tides on the coastlines, warning systems, determination of tsunami return periods, identification of historic and pre-historic tsunami signatures which may be preserved as tsunami deposits inland, tsunami runup distances, hazard mapping of population centres at scales relevant to human safety highlighting those areas which are relatively safe, maps displaying population and critical facilities at different levels of risk, and also evacuation routes to be followed in emergency situations. Tide gauge data are invaluable in issuing tsunami related warnings and it is important that tide gauge recorders be installed in all the critical areas islandwide.
Response to tsunami hazard:
Should it not be a responsibility of the citizens to learn about the hazards and protect their lives and property?
A person living in a coastal area should be interested in the following:
Is information on tsunami hazard available? How reliable it is? If a tsunami were to affect my town how bad it would be? What is my acceptable risk? Is it worthwhile to distribute potential losses by taking an insurance policy? Is there a tsunami warning system? Is there any guidance available on construction of houses? How do I protect myself? What are the escape routes? Do I need to have a personal hazard protection plan for the family?
However, in order for the citizenry to make informed decisions it is equally important that those scientific studies are supported which facilitate identification of physical constraints on the landuse.
It is important that we should, as a nation, carry our environmental protection concerns further than where we are right now, and show by way of examples that implementation and monitoring of hazard loss reduction programmes at all levels will pay rich dividends. Is there any success story? The bottom line is this- be informed and protect yourself and your family.
SOME TSUNAMI SAFETY RULES:
(From Tsunami Glossary, UNESCO, 1991)
All earthquakes do not cause tsunami, but many do. When you hear that an earthquake has occurred, stand by for a tsunami emergency.
An earthquake in your area is a natural tsunami warning. Do not stay in low-lying coastal areas after a local earthquake.
A tsunami is not a single wave, but a series of waves. Stay out of danger areas until local authorities issue an "all clear". A noticeable rise or fall (withdrawal) of coastal water sometimes heralds approaching tsunami. THIS IS NATURES WARNING AND SHOULD BE HEEDED. A small tsunami at one beach can be a giant a few miles away. Don't let the modest size of one make you lose respect for all.
All tsunami- like hurricanes- are potentially dangerous, even though they may not damage every coastline they strike.
Never go down to the beach to watch for a tsunami. When you see the wave you are too close to escape it.
During a tsunami emergency, give your fullest cooperation to the people who are trying to save your life.
Stay tuned to the radio or television during a tsunami emergency- bulletin issued by local authorities (ODPEM) can help save your life.
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