Officials: Tsunami low risk to Cayman

Hazard Management officials say tsunamis pose a low risk to the Cayman Islands, following two earthquakes that were felt in Cayman on Wednesday.

Simon Boxall, spokesman for Hazard Management Cayman Islands, said although the thesis needs additional research, it is possible that the bathymetry, or shape of the underwater landscape, may mitigate against the impact of a tsunami wave in the Cayman Islands.

“Cayman is renowned for its steep underwater drop-offs close to the shoreline. It is thought that the absence of a large area of shallows or shelf areas around the islands could make the Cayman Islands less vulnerable to the impact of an approaching tsunami because the wave might displace around the islands and not run up on a shallow shelf area and impact the coast,” Mr. Boxall said.

“There are indications that the wave dynamics of ‘local’ tsunamis are different from distant or ‘tele-tsunami’ in that the wave of a local tsunami can be very high and focused, but travel much shorter distances. Almost all recorded tsunamis in the Caribbean Sea have been ‘local’ tsunamis caused by slips, slides, slumps and other types of submarine landslides.”

Mr. Boxall said, in a tsunami, the whole sea level would rise to a distance of 50 miles or more.

“A tsunami wave in deep water, like the Cayman Trench, travels at speeds in excess of 400 miles per hour. However, once the tsunami reaches shallow water, it slows down dramatically. So, for example, once the leading edge of the tsunami reaches water of a depth of 15 feet, friction slows the wave down to more like 50 miles per hour. This causes the water to rise up as the leading edge slows and trailing part of the wave continues to speed in from behind.”

According to the Cayman Islands Hazard Management, since 1492, 10 tsunamis have been recorded on the Caribbean North American Plate boundary line, an average of one tsunami every 52 years.

Hazard Management follows the Caribbean Tsunami Warning Program, where alerts would be sent via the National Oceanic and Atmospheric Administration. However, Mr. Boxall said that, in the event of a tsunami, there would be little time for warning, although people are asked to evacuate “vertically.”

“It is a very rare thing,” Mr. Boxall said. “But if there was a very violent earthquake, we would have to be aware of a tsunami threat.”

United States Geological Survey geophysicist John Bellini also said there is a very low tsunami risk in the Caribbean.

“There is a plate boundary just to the south of the Cayman Islands, so people do need to be aware that they can have earthquakes,” Mr. Bellini said.

“We really need a large earthquake to cause a tsunami, at least a seven but generally an eight or higher. Compared to the Pacific region, there’s a very low risk. They are very uncommon in the region.”

Earthquakes

Earthquakes, however, do pose a threat to Cayman because of the proximity of the Oriente Fracture Zone, which runs along the southeast coast of Cuba to an area just west of Cayman, roughly following the northern edge of the Cayman Trough.

On Wednesday, a 5.1 magnitude earthquake was recorded 22 miles south of George Town at a depth of 6.2 miles, around 5 a.m. A second earthquake measuring 5.2 magnitude was recorded at a depth of 6.2 miles, about 26 miles south of George Town, shortly after 6 p.m.

“It wasn’t an aftershock, they were two separate earthquakes,” Mr. Boxall said. “Sometimes they do tend to come in groups, and we have seen that in the past.”

Cayman’s most destructive earthquake was recorded at a magnitude of 6.8 on Dec. 14, 2004, just months after Hurricane Ivan. USGS records also reported a 5.9 magnitude earthquake in 2010, a 7.9 magnitude quake in 1900, while Missionary Notes of the United Church Council, and Antigua and the Antiguans, as well the Patriot newspaper recorded severe earthquakes in 1843 and 1849.

Mr. Boxall said it was rare for Cayman to record two earthquakes in the same day.

“We probably only get one felt earthquake a year. It’s not common to have a felt earthquake here,” he said. “But we live in an area close to the North American and Caribbean plates, so we have to know how to respond to an earthquake.”

Mr. Boxall said in the event of an earthquake, residents should “duck, cover and hold” and wait until the shaking stops before moving outside to an open area. He said while Cayman building code regulations ensure strong structures, people need to be aware of loose structures such as fans and air-conditioning units.

Before this week, the last quake recorded near the Cayman Islands was a 4.3 magnitude in August. There were two smaller earthquakes also recorded by the USGS in June.

Mr. Bellini said the survey does not record earthquakes below 4.5, so it is common for smaller earthquakes to go undetected and unreported.