Hazards

The other day at work a colleague mentioned in passing the possibility of earthquakes here in the Caymans. What ?!! was my reaction. But he asserted me we live in an area prone to volcanic activity. That made me ponder what kind natural hazards I was exposing myself and my family to, by living in this region of the world. Based on a look at various websites, I came up with the following four major hazards: earthquakes, volcanoes, tsunamis and hurricanes.

Earthquakes, volcanoes and tsunamis enjoy a common father: the movement of tectonic plates. Grand Cayman lies on the plate boundary between the North American and Caribbean plates (see image), next to the Cayman Trough (or Trench).

The tectonic plates in Cayman’s region are in continuous lateral movement against each other, with the Caribbean plate traveling east at approx. 20 mm a year. The lateral movement limits the size of earthquakes in this area. However, in December 2004 a quake of 6.8 magnitude rocked Grand Cayman - there was no major damage though.

The following image shows the Cayman Trough together with an indication of Caribbean plate boundaries. The Trough has a maximum depth of 7,686 meters.

The depth of the Trough is more easily seen in the image below. It provides a perspective view of the sea floor of the Atlantic Ocean and the Caribbean Sea. Florida is on the upper right. The purple sea floor at the bottom center of the view is the Puerto Rico trench, the deepest part of the Atlantic Ocean. At the top of the image and to the left of Cuba you can see the Cayman Trench and what looks like small mountains. The top of the mountains are the Cayman islands - the realisation that we are on top of massive underwater mountains in the middle of nowhere made me feel small and vulnerable.

Along the northern east part of the Caribbean plate, including areas in the vicinity of Jamaica and the Virgin Islands, moderate earthquakes of shallow depth are generated. In the Eastern Caribbean seismic events are common and principally associated with a subduction zone at the junction of the Caribbean Plate and the North American Plate. The North American Plate slides underneath the Caribbean Plate along a north-south line just east of the arc of Caribbean islands.

Several of the islands of the Eastern Caribbean are volcanic in origin. Grenada even has a submarine volcano called Kick 'em Jenny. Studies dating back to 1972 indicate that minor eruptions have been occurring on a fairly regular basis and that the summit of the volcano is growing at a rate of approximately 4 metres per annum.

To conclude, on a hazard rating of 1 to 10 for Cayman: volcanoes = 0, earthquakes = 2.

So what about the possibility of a tsunami? The majority of tsunamis are related to tectonic displacements associated with earthquakes at plate boundaries. However, tsunamis can also be generated by erupting volcanos, landslides or underwater explosions (and meteorites). For islands in the vicinity of Kick’em Jenny that is bad news.

Grand Cayman has large boulders on its south coast that geologists believe were moved from the seabed to land by a tsunami. Carbon-dating of dead sea-plants on the rocks put the event in 1662 plus-or-minus 25 years. That time-period comes close to including the 1692 earthquake and the event-generated tsunami and landslide that killed over 2,000 in Port Royal, Jamaica. However geologists believe that Grand Cayman's rocks were moved by an even greater event.

For us in the Cayman, I doubt tsunamis pose a significant threat. It seems to me that possible movements in the Cayman trench are at a depth and of a type (horizontal not vertical movement) that would be unlikely to create a tsunami. My hazard rating: tsunami = 1.

Finally, we have hurricanes which name is derived from the Mayan storm god Hunraken and the Arawak (Amerindians encountered by the Spanish explorers) word hurican, which meant the devil wind. As I noted in a previous post one of the greatest of all recorded hurricanes occurred in October 1780. Nearly 20,000 people perished. Warmer waters also increase their intensity.

Here in Grand Cayman, hurricane Ivan (two years ago) caused massive damage. Damage which is still evident today. Apart from strong winds and flying debris, one of the major contributors to damage was the surge of water, flooding almost all parts of the low-lying island. I would say the likelihood a direct hit by a hurricane is substantial. My hazard rating: hurricane = 8.

Writing about hurricanes reminds me of an article I read in the local newspaper a few weeks ago. It was on the (economic) cost of hurricanes. It is pretty obvious that people who live in hurricane prone areas must expect to incur additional costs to protect themselves. Building construction needs to be of a higher grade, insurance is more expensive, etc. However, one thing I had not thought about (which in retrospect seems obvious) is the cost to businesses of having emergency hurricane disaster plans in place and the cost of acting upon these plans in the event of a possible hurricane event. Think of staff evacuation, additional back-up options for data, standby office space off shore etc. When hurricane Dean passed south of Cayman with no consequence whatsoever a month ago, most firms engaged their pre-hurricane disaster plans. While some incurred little cost, some sent all staff off island at considerable expense.