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Why is Puerto Rico having so many earthquakes?

                                People help shop owners remove supplies from Ely Mer Mar hardware store, which partially collapsed after an earthquake struck Guanica, Puerto Rico, today.
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People help shop owners remove supplies from Ely Mer Mar hardware store, which partially collapsed after an earthquake struck Guanica, Puerto Rico, today.

Puerto Rico has been trembling for more than a week.

An earthquake with a preliminary magnitude of 6.4 struck off the coast of the island at 4:24 a.m. local time today, according to the U.S. Geological Survey. That came after a week of smaller tremors clustered in the same area offshore.

Three strong aftershocks, with preliminary estimates of 5.6, 5.2 and 4.5 magnitude, followed today’s big quake. A bigger aftershock, of 5.8 magnitude, hit at 7:18 a.m. local time. The temblors knocked out power to much of the island, seriously damaging homes and buildings and leaving at least one person dead.

Today’s shaking followed a 5.8-magnitude quake and aftershocks that leveled homes Monday. There were dozens of smaller earthquakes today within a few miles along the southwestern coast of the island, according to the Geological Survey.

“Things are really very tense about what has been going on, because this is not normal — so many tremors,” Mayor Nelson Torres Yordán of Guayanilla said this week.

But the Caribbean, a region that faces the threat of hurricanes from the Atlantic in late summer and early fall, is also subject to seismic hazards like those striking Puerto Rico this week.

Why are there so many earthquakes in Puerto Rico right now?

At about three times the size of Rhode Island, Puerto Rico is squeezed between the border of the North American and Caribbean tectonic plates.

The Puerto Rico Trench, north of Puerto Rico and the U.S. Virgin Islands, is an undersea fault zone. The North American plate is sliding under the Caribbean plate there, creating the potential for earthquakes and undersea landslides that can set off tsunamis.

“We’re just as likely to have earthquakes as a place like California, Japan, New Zealand, Alaska,” said Elizabeth Vanacore, a seismologist with the Puerto Rico Seismic Network.

When the tectonic plates in the region slide past each other and squeeze together, energy and stress build up until one side of a fault pops up, unleashing an earthquake. The earthquakes redistribute stresses along the fault for a time, until those stresses build up again and new tremors occur.

The North American plate is edging south and sinking underneath the Caribbean plate at about 2 centimeters per year, according to Gavin Hayes, a research geophysicist at the Geological Survey. To the south, the Caribbean plate is moving in the opposite direction, northward, at a similar rate.

“They are converging at a relatively low rate in comparison to plate boundaries around the world,” he said. In South America, it can be as high as 6 centimeters per year, while in the southwest Pacific it is up to 10 centimeters per year. Slower convergence rates mean strain builds up more slowly, making earthquakes either smaller or more rare, he said.

Should residents be worried about a tsunami?

Mayita Meléndez, mayor of Ponce, said people living near beaches are desperate to get out of their homes for fear of tsunamis. “It’s not safe,” Meléndez said. “The earth is moving constantly.”

Early this morning, the U.S. National Tsunami Warning Center said there was no tsunami threat from that morning’s earthquake. The local authorities initially issued a tsunami watch before canceling it, according to Puerto Rico’s emergency management agency.

A tsunami happens when a quake on the seabed suddenly pushes water upward, producing a perilously towering wave. In 1918, an earthquake near the island’s northwestern coast triggered a tsunami that killed 118 people, according to the Geological Survey.

How rare are earthquakes in the region?

Puerto Rico has seen damaging quakes before, but major earthquakes in the southwestern part of the island are unusual in recent history.

The last significant temblors recorded in that area, in 1991 and 1999, had a magnitude of about 4.1, according to the Puerto Rico Seismic Network, whose data dates to 1986.

Most of the seismic activity has occurred in the underwater band to the north of the island, close to where the tectonic plate boundary is. The Geological Survey has recorded 61 earthquakes of 4.5 magnitude or within about 50 miles of the northern shoreline since 2009.

Major earthquakes have regularly devastated cities in the Caribbean, including Kingston, the capital of Jamaica, which was destroyed twice in three centuries. Ten years ago, a 7.0-magnitude earthquake struck Haiti on Jan. 12, killing more than 200,000 people and destroying tens of thousands of buildings.

“Puerto Ricans should be aware that they live in a tectonically active region,” Hayes said. “Earthquakes can be expected any time.” He emphasized the importance of building to seismic codes.

Is it safe to travel to Puerto Rico?

Seismologists said that more earthquakes are expected to be felt on the island in the next few days because the seismic activity is occurring close to land. But the frequency of aftershocks is expected to slow down over time, which means the chances of another intense earthquake will also fall.

Arrivals and departures at the international airport in San Juan, which was running on generators, were operating on or close to schedule this morning.

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