ALTHOUGH THE EPICENTRE of the November 29 earthquake was in the sea, just north of Martinique, most of the eastern and southern Caribbean shook. From Puerto Rico in the north to Guyana and Venezuela in the south, and the islands in between, felt the tremor.

The 7.3 intensity Richter scale quake was at a depth of 90 miles (145 kilometres). And mercifully this was so, because had it been not so deep a tsunami could have been generated. And this would have spelt disaster to many of Martinique's neighbours.

But just a few years ago, a team of American scientists made a frightening revelation. They predicted that there is a serious risk of a devastating tsunami occurring in the Caribbean Sea off the coasts of Puerto Rico, Haiti and the Dominican Republic. They based their forecast on historical records.

Undersea earth movements

Scientists Nancy Grindlay and Meghan Hearne, both of the University of North Carolina, and Paul Mann of the University of Texas, Austin, writing in Eos, the newspaper of the American Geophysical Union, in 2005, stated that destructive tsunamis have been generated in the past 500 years by undersea earth movements along the boundary between the Caribbean and the North American tectonic plates – two of the moving slabs of rock that cover the ocean floor.

That's an average of one significant tsunami every 50 years. The most recent occurred in 1946 – 61 years ago – when a magnitude 8.1 earthquake in the Dominican Republican triggered a giant wave that killed 1 800 people.

The scientists said the dates implied that another tsunami was overdue, but they couldn't predict when it might happen.

An earthquake in that northern part of the Caribbean could generate waves up to 40 feet (12 metres) high and threaten the lives of up to 35.5 million people living in coastal areas. Smaller waves could reach Florida, the Gulf of Mexico and as far north as New Jersey.

"The rapid increase in population in the northern Caribbean to its present level of 35.5 million people means that future tsunamis will be much more destructive than the historical ones," the scientists wrote.

George Pararas-Carayannis, former director of the International Tsunami Information Centre in Honolulu, Hawaii, said that since 1489, 88 tsunamis – most of them moderate – had been reported in that part of the Caribbean which was located on an earthquake fault and ringed by volcanoes.

"Several of these were generated by volcanic eruptions and by collateral volcanic flank failure, debris avalanches and landslides," he wrote in the Science of Tsunami Hazards, a professional journal.

At least six Caribbean tsunamis are known to have killed people: in 1692, 1781, 1842, 1867, 1918 and 1946. The total death toll is unknown but at least 2 000 persons perished.

The Eos article states that the northern Caribbean is capable of generating tsunamis of at least up to 40 feet (12 metres) high. It added that the effects of past tsunamis have extended up to 1 320 miles (2 124 kilometres).

"More sobering than the historical record of tsunamis is the presence of large-scale underwater landslide features that may have produced immense, prehistoric (before 1400 AD) tsunamis along the northern margin of Puerto Rico that were much larger than any of those known from 500 years of historical records," the report said.

Underwater landslides cause tsunamis by displacing large volumes of water, forcing it to surge upward in a powerful wave.

Martitia Tuttle, a tsunami expert in Georgetown, Maine, who is not part of the Eos team, said there's evidence of a major earthquake along the Caribbean Plate boundary about 800 years ago.

"Strain has been accumulating on that fault since then," she said. "Enough strain has accumulated to generate a quite large – 7 to 8 magnitude – earthquake, but when we can't say."

The Caribbean Plate encompasses Jamaica, Hispaniola, Puerto Rico, the Leeward and Windward Islands, Barbados, Trinidad and Tobago, northern Venezuela, northern Colombia, Central America and Mexico (just south of the Yucatan Peninsula).

Early warning systems

An earthquake fault within the Caribbean Plate is not too far south of Barbados. It stretches across northern Trinidad, across to Venezuela and Colombia.

The United States government has allotted US$3 million for the installation of the stations that will have early tsunami warning systems.These centres will be established in Grenada, Barbados, Antigua, Turks and Caicos Islands, the Dominican Republic, Jamaica, Guantanamo (Cuba), Honduras and Panama. Trinidad and Tobago already has such a station.

Jean Weaver, United States seismic coordinator for Central America, South America and the Caribbean, said the stations "will provide real time data, monitor and measure earthquakes greater than 6.5 on the Richter scale, which are usually tsunami-generating earthquakes." However, she pointed out, not all earthquakes greater than 6.5 generate tsunamis. (CMC)

Hartley Henry's column, Under The Microscope, will return next week.