Cayman and tsunamis

 

Before a massive earthquake on December 26, 2004 generated a tsunami that swept across the Indian Ocean claiming a quarter of a million lives, for those in the western world “tsunami” was not really a household word. For many, it was probably the only tsunami that they had heard of in their lifetime. A little over six years later, another tsunami generated by a large earthquake devastated coastal areas in north east Japan.  

For people living on the coast, particularly in earthquake-prone areas, and indeed in regions where other types of natural disaster occur, it is only natural to wonder if and how a tsunami could affect them.  

A tsunami is defined as a series of waves caused by the displacement of a large volume of water. This displacement of water is most commonly caused by a large underwater earthquake, as was the case with the two recent tsunamis. They can however also be generated by volcanic eruptions, landslides, explosions or even the impact of asteroids and meteorites. 

Powerful earthquakes can and do occur in the Caribbean as last year’s earthquake in Haiti demonstrated. They have even occurred in the Cayman Islands. Simon Boxall, Communications Officer with Hazard Management Cayman Islands, recently unearthed a letter from Reverend James Elmslie dated 1849 in which he describes a large earthquake in the Cayman Islands, writing, “Very near this place (George Town), the earth opened up in more than 20 places, some of them wide and deep. Even at sea the shock was severely felt. One of the island vessels, when twenty miles from land, received such a shock that the crew thought the vessel was broken to pieces.” 

Tsunamis can potentially occur in the Caribbean: According to the National Science Foundation there have been 27 verified tsunamis in the Caribbean region in the past 500 years, the last of which was in 1946. When one compares this figure to the 195 recorded tsunamis in Japan alone, however, the probability for this region is clearly much lower. The topography of the Cayman Islands is such that landslides do not occur, eliminating one possible cause of tsunamis. The potential danger to the Cayman Islands comes from tsunamis generated elsewhere in the Caribbean, reaching these shores. But this too is unlikely: First, the fact that these islands rise so steeply from the ocean floor means there is virtually no continental shelf. Normally tsunami waves increase in size when they hit the shallow waters of such shelves and that is when they become dangerous. Diego Garcia Island in the Indian Ocean is a dormant volcano that rises sharply from the ocean floor. Although it lay in the path of the 2004 tsunami, the island was not affected because it lacked the sloping shelf of continental shorelines that cause such waves to rapidly amplify.  

Secondly, tsunamis usually occur in subduction zones – where one tectonic plate slides under another. The subduction zones in the Caribbean region are located far to the east of the Cayman Islands and it is thought that other landmasses would shield the Cayman Islands from the worst of a tsunami generated in that area.  

There is no historical evidence of tsunamis having affected the Cayman Islands in the past. As the islands’ history only really dates back to when the islands were first settled, 275 years ago, this cannot be taken to mean such events never have occurred, or never will.  

Indeed, the Cayman Islands recently took part in a regional tsunami exercise to assess the response capabilities and develop a communications protocol in the event of such an occurrence. HMCI has been looking into options for warning systems such as coastal sirens. However, Simon Boxall says, “We also have to look at things like the cost versus the actual level of risk.” A vulnerability analysis for Grand Cayman concluded that the threat of a tsunami is very small. In the unlikely event that a tsunami were threatening the island, the reality is that these waves travel at between 300 and 500 mph, so it is unlikely there would be time to issue and act on a tsunami warning.  

Furthermore, on such a small and low lying island there is no significantly higher ground to move to nor can one travel very far inland. The best one can do, advises HMCI, is to familiarise oneself with the warning signs, such as the sea retreating and the ground shaking severely. Do not wait for an official warning: move away from coastal areas and evacuate vertically to higher floors as quickly as possible. That said, HMCI stresses that the vast majority of earthquakes do not cause tsunamis and the particular underwater topography of the Cayman Islands make a destructive tsunami even less likely.  

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