Low probability of tsunamis in Caribbean

IN THE aftermath of last month’s devastating Asian tsunami which has so far claimed approximately 155,000 lives in 11 countries, Prime Minister Patrick Manning appointed a high-powered ministerial team to put contingency plans in place in the event of such a major disaster affecting Trinidad and Tobago. Two scenarios were devised for the team to consider — a tsunami generated by a major earthquake off Trinidad’s east coast, or one triggered by an eruption of the underwater volcano off Grenada. However, according to regional experts, there is a very low probability of a tsunami occurring in the Caribbean and planning for such a rare event should not be a priority at this time.

In an interview with a regional newspaper, scientists at the Seismic Research Unit (SRU) at the University of the West Indies (UWI), St Augustine, recommended that more priority be placed on the dangers posed by immediate natural hazards in the region such as earthquakes, hurricanes and volcanic eruptions. This issue of tsunamis affecting the Caribbean has been a major talking point since the Boxing Day 9.0 earthquake off the west coast of  northern Sumatra and the resulting deadly tsunami. Asked whether an earthquake from outside the region can cause a tsunami that affects us, scientists said this is possible, acknowledging that a tsunami may be generated from earthquakes occurring within and outside  the region.

However, three conditions are necessary for the generation of a tsunami: the earthquake must occur beneath the ocean and be strong (at least of magnitude 6.5), the earthquake must occur at a shallow depth (less than 50 km below the surface of the Earth), and the earthquake must be dip-slip in nature (which means there is a vertical component, which displaces a large volume of the sea). There has been major concern about the possible tsunami threat posed by two volcanoes —  the underwater volcano Kick ’em Jenny and the  Cumbre Vieja volcano in the Canary Islands. With respect to Kick ’em Jenny, the scientists said the threat of a tsunami from an eruption of  that volcano is extremely low since a 2003 survey revealed that the summit of the volcano is quite deep. They said Kick ’em Jenny poses a grave danger to shipping as the volcano is constantly emitting gases which lower the buoyancy of the water and can cause vessels to sink.

As for the Cumbre Vieja volcano, located on the island of La Palma in the Canary Islands, off the west coast of Africa, the scientists said there is a school of  thought that if the western flank of the volcano were to collapse and drop 150 to 500 km of rock into the sea, massive tsunamis could be generated which could devastate islands in the Caribbean, as well as coastal areas in. According to the SRU scientists, while this may be possible, this is in fact a worst case scenario and the likelihood of such a catastrophic event is remote. On the issue of tsunami warning systems for the region, SRU officials say several factors should be seriously considered before assuming that such a system would be beneficial to the region. Acting Head of the SRU, Dr Richard Robertson, advises: “Before the region spends valuable resources on setting up new instruments for a tsunami early warning system, we need to strengthen our existing networks and focus on improving public education and communication activities.”

Two components are essential for any tsunami warning system. One is a network of instruments that allows scientists to rapidly determine when and where potentially tsunamigenic earthquakes occur and whether a tsunami has actually been generated and if so how big it is. To enable this to happen, an efficient seismograph network and an efficient tide gauge network, both with real time response capability, will be needed.  The second component of the system is an efficient public information and education system that allows agencies to issue tsunami warnings and ensures the public knows how to respond to these warnings.  This second component is particularly important since it makes little sense to issue a warning unless people know how to respond.

In terms of the Caribbean and most regions of the world, the first component of this system is more or less in place. This is because within the past ten years seismograph networks and communication systems have improved such that it is now possible to detect and locate earthquakes down to about magnitude 5.5 within a few minutes anywhere in the world.  This is particularly true for the Eastern Caribbean which has one of the densest seismograph networks in the world. It is possible, during regular working hours and with the existing network to recognise potential tsunamigenic earthquakes (magnitude 6.5 and above) within a few minutes.  The process could also be automated so that if such earthquakes occurs an alarm is sent to the duty scientists outside normal working hours.  This will increase the response time to at least 30 minutes.

Given the existing seismological networks in the Caribbean and the experience scientists have gleaned in operating them, setting up an organisation specifically dedicated to tsunami warning in the Caribbean is not currently expedient, Robertson said. If the SRU were to issue a tsunami warning every time there was a shallow earthquake of magnitude greater than 6.5, it would have issued two warnings in its 52-year existence (east of Dominica 1969 and Antigua 1974).  Both of these would have been false alarms since both earthquakes generated tsunamis with amplitudes less than one cm. 

If a tsunami is generated by a “local/internal” source it will reach the nearest islands within minutes and the whole region within less than an hour.  In order to have any real benefit from an early warning system we would need to respond within about 15 minutes and contact relevant disaster management officials within another 15 minutes.  This is not impossible but would require that the SRU and all of the disaster offices in the region be manned or readily accessible 24 hours a day.  Even then issuing a warning to vulnerable communities in the nearest islands would be nearly impossible. 

For externally-generated tsunamis (“tele-tsunamis”) there is another problem. The most likely source for the Caribbean is the Azores fracture zone in the Atlantic Ocean.  A tsunami generated in this region would reach the Caribbean within 4-6 hours so there would be ample time for a warning.  However, this is an Atlantic-wide problem which would best be handled by a European-based centre.