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Why Is Puerto Rico Having So Many Earthquakes?

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 on Tuesday, according to the United States 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 Tuesday’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.

Tuesday’s shaking followed a 5.8-magnitude quake and aftershocks that leveled homes on Monday.

“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.

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 United States 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.

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

Early this morning, the United States National Tsunami Warning Center said there was no tsunami threat from Tuesday 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 service.

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.

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.

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 Tuesday morning.

Alejandra Rosa and Patricia Mazzei contributed reporting.

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