Earthquakes

Scientists: Cuba quake unusual, but recent Caribbean activity fairly typical

 

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dsmiley@MiamiHerald.com

From earthquake-ravaged Haiti to the quiet northern coast of Cuba, the ground is moving.

Tremors moved across the Haitian capital of Port-au-Prince early Monday morning, causing a brief scare nearly four years to the day that a 7.0 magnitude shock devastated the region. The quake came just days after a major tremor rocked Puerto Rico and less than two weeks after normally quiet fault lines shook north of Cuba, rattling parts of Florida where the ground rarely rumbles.

But while the Jan. 9 earthquake east of Havana has puzzled scientists, experts say the two-week spate of tremors is for the most part business as usual. The ground is always moving, it’s just that South Floridians rarely take notice.

“The seismicity that’s happened in the Caribbean during the last week is just normal seismicity,” said John Bellini, a geophysicist with the U.S. Geological Survey. “There’s always been lots of earthquakes going on there, but when something makes the news and people feel it, it becomes more observed. That’s typical.”

In fact, it’s a daily occurrence in the Caribbean. About 50 tremors are recorded each day in the region, according to the website Earthquaketrack.com, which tracks seismic shocks using data from the United States Geological Survey. Most register below 3.0 on the Richter scale and are considered very minor.

The source of the frequent tremors in the region is the slow movement of the Caribbean plate, which moves about one inch each year along a boundary against the North American plate. It’s when the two plates catch along fault lines, build up pressure and then release that quakes happen, in a process sometimes called “stick-slip.”

Occasionally the quakes are more significant, such as the 6.4 magnitude tremors recorded off Puerto Rico on Jan. 13. Earthquakes that size may happen off Puerto Rico once every few years, Bellini said.

“I won’t say every year but they do happen,” he said.

When it comes to the islands of Hispaniola and Puerto Rico, two east-west fault lines cause much of the seismic activity, including the quakes seen during the last eight days. The Septentrional fault line, which runs across the northern parts of Haiti and the Dominican Republic and stretches out into the Caribbean Sea, was the cause of the 6.4 magnitude quake off Puerto Rico.

The quake, with an epicenter about 35 miles north of the island, was the strongest in the region this month, and caused government officials to shut down the Arecibo Observatory, the site of one of the world’s largest radio telescopes. No injuries were reported, but dozens of small aftershocks have followed.

A second fault, called the Enriquillo, runs along the south of Haiti near Port-au-Prince. It’s the fault that shook Monday, and the same line that shook in 2010, destroying entire sections of the nation’s capital and killing more than 100,000.

No damage was recorded Monday, according to Haiti’s disaster office. Nevertheless, the tremors were talked about across the country and were a reminder that Haiti still lacks a building code.

The quake that began the last 11 days of seismic activity, however, is more difficult to explain. University of South Florida professor Tim Dixon said Cuba often experiences earthquakes, but they’re almost always located near a Caribbean plate boundary toward the south of the country near Guantanamo Bay.

The Jan. 9 quake occurred to the north of Cuba, along the Nortecubana fault system of the Hicacos and Las Villas faults. Dixon, a professor in the university’s School of Geosciences, said those faults were believed to be inactive.

“It was thought until that earthquake happened that that area of Cuba did not have active faults, but the earthquake proved us wrong,” he said. “We get it right 90 percent of the time, but the other 10 percent we get surprised.”

Both Dixon and Bellini said it’s difficult to know which fault line quaked because research in the area is limited due to difficult US-Cuba relationships.

“We haven’t for political reasons had as much scientific exchange with Cuba,” said Dixon. “The only [seismic] map I’ve got of Cuba goes back 40 years and it’s in Russian.”

So can the normally quiet state of Florida expect any more quakes from the apparently active Nortecubana system?

So far, no aftershocks have been recorded, according to Bellini. But both geophysicists said it’s impossible to forecast earthquakes.

“When a bunch of little earthquakes go off,” said Dixon, “unfortunately it doesn’t tell us much about what will happen next.”

Herald staff writer Jacqueline Charles contributed to this report.

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