Plate 1: House, in Lowlands, Tobago, damaged in the 22nd April 1997 Tobago earthquake (Photograph by Godfrey Almorales, April 23 1997)

Global Setting
On Boxing Day 2004, the devastation that a great earthquake and associated hazards can inflict was forcefully brought to the forefront of global consciousness. It is the first time in known history that individuals, from every corner of the Earth, were able to literally watch a disaster unfold on such a scale. It is with a large measure of disbelief that people wrestle with the loss of 200,000 human beings in an instant. It seems somehow surreal. However, this is hardly the first time that an earthquake has resulted in destruction of areas in the near vicinity, and reached out, via tsunami waves, hundreds of kilometres to extend the destruction. It will not be the last.

A strong earthquake, magnitude 6.1 and over, occurs somewhere in the world, on average, every two to three days, and a great earthquake, magnitude 8 and above, approximately every year. However, an earthquake does not have to be one of the biggest to cause a disaster. On 23rd December 1972, an earthquake of magnitude 5.5 devastated Managua City, Nicaragua. The earthquake occurred at shallow depth directly below the city. The damaged area extended 27 square kilometres, with 13 square kilometres of total destruction. However, beyond 15 km from central Managua, there was virtually no evidence of the earthquake.

In the closing decades of the 20th Century, so-called natural disasters appeared to be on the increase. Research suggests that human activity has contributed in large measure to the phenomena of global warming, the hole in the ozone layer, forest fires, flooding, all with their associated consequences. Although the geological hazards do not appear to be susceptible to human activity in the same way, how and where we choose to live can and do affect the scale of the effects from earthquakes and their associated hazards. For example, for the death toll to be as high as it is for this most recent Sumatran earthquake, and resulting tsunami, that number of people had to be in the affected areas at the time, absence of warning not withstanding. Choosing to introduce high-density population centres in coastal areas, going so far as to reclaim land to do so, in a region known to have significant seismic activity is, at least, imprudent.

Plate 2: Damage to Roman Catholic Church in Porstmouth, Dominica (photograph by R. Robertson, 2004)

Eastern Caribbean Setting
In the global setting, the Caribbean is a relatively small plate. However, the processes involved in plate motion make the islands and countries lying on the plate boundary vulnerable to geological hazards sufficiently serious that they should be taken into account in all long-term development plans. Eruptions at the volcanoes in Martinique, Mt. Pelée, and St. Vincent, Soufrière, in 1902 claimed over 30,000 lives. Since 1995 the on-going eruption of the Soufrière Hills volcano in Montserrat has made two-thirds of the island unsafe for human occupation. The effects are not only local to the particular island. Ash from an erupting volcano can affect the operation of jet engines, and cause them to stall, with obvious implications. Breathing ash-laden air aggravates respiratory ailments, and prolonged exposure may cause damage to healthy lungs. Displaced people may spill over to other islands in the region, and, in sufficient numbers, can strain small-island economies.

Earthquakes have been equally serious in their effects. In the 19th Century when the colonial powers settled the islands of the Caribbean, and constructed buildings of stone, they did so ignorant of the earthquake hazard and the vulnerability of that particular type of construction to collapse from earthquakes. Their ignorance did not serve as a shield. The great earthquake of 8th February 1843, east of the Lesser Antilles, between Guadeloupe and Antigua, was felt as far south as British Guiana. The high damage area had a radius of about 250 km, and extended from Saint Lucia to St. Martin. Close to the epicentral area, the city of Pointe-à-Pitre, in Guadeloupe, was destroyed. At least 4,000 people lost their lives. Table 1 shows the known, very significant earthquakes to have occurred in the Eastern Caribbean.

Table 1: Very significant Eastern Caribbean earthquakes

On average, the Eastern Caribbean generates an earthquake of magnitude greater than 7.0 approximately every 20 years and one of magnitude 8.0 and above every one hundred years. The most recent major earthquake was in 1974 and occurred north-west of Antigua. There was no great earthquake, magnitude 8.0 and above, in the Eastern Caribbean during the 20th Century.

Tsunamis and the Caribbean
Tsunamis are most prevalent in regions where the seas are bounded by tectonic regimes that can produce geologic events capable of displacing a huge volume of water. The two most common tsunami-generating events are volcanic activity and earthquakes. Tsunamis resulting from volcanic activity may be forecast and mitigated effectively if the volcano is closely monitored. On the other hand, earthquake-generated events are unpredictable, and are, therefore, more problematic. Some of the important characteristics favouring earthquake-generated tsunamis are the generation of large (>M6.5) earthquakes, occurring at shallow (<30 km) depth within the Earth’s crust with significant vertical displacement of the sea floor, and the presence of deep subduction trenches with steep and unstable slopes. Tsunamis propagate very efficiently in deep water, and in the open ocean, they travel at speeds of 800 k.p.h. (500 M.P.H.). Closer to shore, the speed is reduced, but still fast and, therefore, a tsunami generated by an earthquake located between Trinidad and Puerto Rico would reach the nearest islands within minutes and the most distant island in the region in less than an hour. The travel time to the Eastern Caribbean for externally generated tsunamis (‘tele-tsunamis’) is estimated to be 4-6 hours.

Tsunamis have affected the Caribbean in the past. Those affecting Trinidad and Tobago and the Eastern Caribbean may be generated by major earthquakes occurring along the Eastern segment of the Caribbean plate (Eastern Venezuela to Puerto Rico), large dome collapses at volcanoes in the Eastern Caribbean, eruptions of submarine volcanoes (only 1 known to be active) in the region, great earthquakes in Europe, mega-slides into the Caribbean Sea or Atlantic Ocean, and meteors striking the Earth in either the Caribbean Sea or Atlantic Ocean. Since the area was first settled regional earthquake sources have generated approximately 1 destructive tsunami per century. Most have occurred in the Greater Antilles and there is no record of significant tsunamis from any source impacting Trinidad and Tobago since its settlement. During the same period (~500 years) two tele-tsunamis have been recorded in the Caribbean. Earthquakes occurring in the Azores Fault Zone generated both tsunamis. Although inundation was reported in some Caribbean islands neither resulted in any casualties. Due to the relatively shallow water depths along the island chain the effects of events generated within the region rarely extend beyond 200 km. Notable exceptions are events that were channelled through the Grenada Basin. Table 2 lists the known tsunami generating earthquakes, which have occurred in the north-eastern Caribbean.

Table 2: Known north-eastern Caribbean tsunamigenic earthquakes (From NEIC web catalogue; Ramsey, 1953; Robson, 1964; Grases, 1990)

Kick ‘em Jenny
Based on current understanding of the processes involved, the probability of a tsunami from an eruption of the Kick ‘em Jenny submarine volcano (near northern Grenada) is very low. It will not become significant again until the volcano grows to within 130 meters of the surface of the sea. A recent (2003) survey revealed that the vent through which future eruptions will most probably take place was much deeper, at 264 meters. The more immediate threat posed by the Kick ‘em Jenny volcano is to shipping, as the volcano is constantly emitting gases, which lower the buoyancy of the surrounding water. This phenomenon is known to have caused vessels to sink. It is for this reason that there is a 1.5 km exclusion zone around the summit of the volcano.

Seismic Research Unit
The Seismic Research Unit has been operating in Trinidad and Tobago since 1952, originally with responsibility for monitoring activity in the English-speaking Eastern Caribbean, but in recent years has expanded to include the northern Dutch Antilles. Initially, the focus was on the volcanic hazard. However, as instrumentation increased, and the processes involved became better understood, the significance of the earthquake and associated hazards was recognised. The earthquakes are distributed such that there is a broad zone of shallow seismicity. Intermediate depth and deeper seismicity occur in better defined, overlapping bands. In general, the deepest events occur further west than other events. Fig. 2 displays the epicentres of all available earthquake locations for the period 1955-2000. Live volcanoes are also a feature of the islands and these are found from Grenada, in the south, to Saba, in the north. The observations are consistent with subduction activity resulting from the collision of Atlantic Ocean lithosphere, on the east, with the Caribbean Plate, on the west. The seismic monitoring equipment has grown from three stations in those early years to over 50 instruments. Data are recorded at about a dozen nodes in the network and then downloaded automatically to computers at the Unit via the Internet or directly by telephone connection.

Fig.2 Eastern Caribbean Seismicity 1955-2000

In the early 1950’s, it was a response to episodic volcanic earthquakes, in Nevis, which gave rise to the establishment of the organisation out of which the Seismic Research Unit grew. Today, however, seismicity is not the only indicator of volcanic unrest being monitored. Staff at the Unit also routinely collect and analyse the gases emitted by the volcanoes, and the movement of the flanks, inflation/deflation, in an attempt to gauge the status of the volcanoes. Other techniques are being tested with a view to expanding the capability to recognise incipient unrest. Earthquake prediction of time, place and magnitude has been more elusive. However, research into earthquake forecasting is being pursued with very encouraging results.

The more specialised nature of the techniques involved in earthquake and volcano monitoring requires a more specialised team. Expertise at the Unit includes instrumentation specialist, geologist, seismologist, volcanologist, information technologist, geochemist, geophysicist and public education and outreach officer. Nine technical, clerical and support staff complete the team.

The 1997 earthquake south of Tobago of magnitude 6.1 is the most recent, strong earthquake to have affected Trinidad and Tobago. Plate 1 shows the damage that was caused to a house in the Lowlands area.

The most recent strong earthquake, magnitude 6.3, to have affected the region occurred near the north coast of Dominica on 21st November 2004. There was significant damage in Guadeloupe, where one child was killed, and Dominica. Plate 2 is an example of the damage seen in Dominica.

Will we get it right?
Responding in a panic when nature demonstrates real power is less desirable than a well-thought out programme to address the hazards we face, both at the individual and national levels. The Earth on February 26, 2005 is the same Earth of December 26, 2004. The same processes are at work now as then. Earthquakes will continue to occur, every day, somewhere. One day, the somewhere of the next major or great earthquake may be close to one of our Eastern Caribbean islands. If we are truly wise in preparing for our future, this reality must factor into all our development plans. Everything we do now will count. This is no rehearsal. In general, every generation is called upon to deal with a potentially catastrophic event just once, and we must get it right the first time. We should not expect to fight Nature and win and ignoring the known hazards does not negate them. In any battle, as the events of December 26, 2004 powerfully demonstrates and the world has been reminded, nature always has the final say.