Little risk for Cayman from Haitian fault slip

Although scientists believe pressure along the fault line that slipped near Port-au-Prince, Haiti this week will likely move westward toward Jamaica, there is little chance it could cause an earthquake that would affect the Cayman Islands.

The earthquake in Haiti was caused by a slippage of the Enriquillo-Plantain Garden fault zone, which runs from the Dominican Republic to eastern Jamaica.

Canadian geologist Murray Roed, author of the book Islands from the Sea; Geologic Stories of Cayman, said Thursday the Cayman Islands are not even part of the same Tectonic plate as Haiti and Jamaica.

‘The Cayman Islands belong to the North American Plate. Haiti is on the leading edge of the Caribbean plate,’ he said, noting that Jamaica is also part of the Caribbean plate.

Even if an earthquake was to affect Jamaica, it wouldn’t likely be felt much here, he said.

‘Jamaican is also on the eastern edge of the Caribbean plate. Cayman is quite a ways away from that.’

In addition, Mr. Roed said that the North American plate, of which Cayman is a part, has a much more stable subsurface.

Although Cayman might not have to fear an earthquake from slippage along the Enriquillo-Plantain Garden fault zone, Mr. Roed pointed out that there is another very active fault near Cayman. The Oriente Fracture Zone is just south of Grand Cayman, roughly along the northern edge of the Cayman Trough, also known as the Cayman Trench. The fracture zone is where the North American and Caribbean Tectonic plates meet, and thus a source of earthquakes.

It was along this fracture zone that a magnitude-6.8 earthquake occurred on 14 December, 2004, rattling Grand Cayman but causing little damage. Mr. Roed explained why a strong magnitude-6.8 earthquake – which could cause devastation elsewhere – could cause just minor damage in Cayman.

‘Most of the fault is along the Cayman Trench, which is pretty deep,’ he said, noting that the depth of the earthquake in Haiti was, in comparison, shallow.

‘That’s a big difference,’ he said.

Even though the Oriente Facture Zone is offshore, it poses some threat to Cayman since it’s a fairly active fault, moving about the same speed that a fingernail grows, Mr. Roed said.

Last month, Hazard Management Cayman Islands issued a press release on the fifth anniversary of the 14 December, 2004, earthquake here.

‘Since 1990, there have been four earthquakes that were magnitudes six or more in the general area of the Cayman Islands,’ the press release stated. ‘A recent vulnerability analysis conducted by Natural Disasters Assessment Consulting Group – published in June 2009 – for Grand Cayman shows that the estimated return period of a destructive earthquake ranging in magnitude from 7.2 to 7.5… is 180 to 500 years…’

In addition to the December 2004 earthquake, Grand Cayman felt the affect of a magnitude-6.2 earthquake that occurred 75 miles north-northeast of Montego Bay, Jamaica in February 2007. In March 2008, some residents reported feeling the 4.6-magnitude earthquake that occurred 65 miles east of George Town.

Since earthquakes cannot be predicted or tracked in advance like hurricanes, knowing what to do when an earthquake happens is key to preparations.

Deputy Director of Hazard Management Cayman Islands Omar Afflick said in last month’s press release that the earthquake response recommended by the US Federal Emergency Management Agency is the ‘duck, cover and hold’ procedure.

‘Basically, when you feel shaking, stay calm, move away from glass windows and duck under a heavy piece of furniture such as a desk or table,’ he said. ‘If there is nothing available for you to duck under, you should cover your head with your arms.

‘Do not run for the door and head outside, but if you are outdoors already, you should move to an area, which is open, where there are no trees, buildings or light poles that can potentially fall on you.’

Mr. Afflick said that if you are driving in a car, you should pull over to the side of the road.

Tsunamis

The active Oriente Fracture Zone poses a tsunami threat to Cayman.

Most tsunamis originate near subduction zones – areas where Tectonic plates are moving toward each other – when one plate slips under another one under water, causing the displacement of water. The closest subduction zones to Cayman are far away and unlikely to cause any effect.

However, tsunamis are also caused by other phenomena, including submarine landslides.

In his book, Mr. Roed wrote that a tsunami was generated from a large magnitude earthquake and subsequent submarine landslide near Jamaica in 1692. He noted that some scientist believe the Cayman Island were affected by that tsunami.

‘It would not be prudent to say that the Cayman Islands are not at risk from an earthquake-induced tsunami,’ Mr. Roed wrote in his book. ‘But return periods of such events worldwide for any particular locality are in the range of one every 500 to 800 years.

‘Needless to say, erosion of the Cayman Islands during such an event would be catastrophic for the people and the land.’

Speaking about the matter on Thursday, Mr. Roed said that along the Cayman Trench there are steep slopes that could slide as a result of an earthquake.

‘I think that a small displacement there could create a tsunami.’

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