SAN JUAN, Puerto Rico (AP) — A 4.5-magnitude earthquake hit Puerto Rico on Thursday in the latest of a rare string of quakes that has frightened many in the U.S. territory.

The most recent quake occurred eight miles (12 kilometers) south of Guayanilla at a shallow depth of four miles (seven kilometers) and was felt in the capital of San Juan and elsewhere in Puerto Rico, according to the U.S. Geological Survey. No immediate damage was reported.

“We haven’t stopped shaking,” said Ángel Vázquez, emergency management director for the southern coastal town of Ponce. “It’s the first time something like this happens.”

The flurry of quakes began the night of Dec. 28, with a 4.7-magnitude quake followed by a 5.1-magnitude one that hit near Puerto Rico’s south coast and sent dozens of panicked people into the streets. Goods fell off supermarket shelves, cracks in homes were reported in some coastal towns, and a large rock fell and blocked a road. No injuries were reported.

Since then, more than 1,100 earthquakes have occurred in that region. The majority have not been felt, except for the 4.2-magnitude one that hit Dec. 31 and the one on Thursday.

“I’ve spent 29 years with Puerto Rico’s Seismic Network, and it’s the first time I observe this kind of activity,” director Víctor Huérfano told The Associated Press. “There’s no way to predict when it’s going to end, or if it’s going to lead to a major event.”

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He said the flurry of quakes have been extremely superficial and have occurred along three faults in Puerto Rico’s southwest region: Lajas Valley, Montalva Point and the Guayanilla Canyon.

“In general, the force behind all of this is the North American plate and the Caribbean plate squeezing Puerto Rico,” he said.

A similar cluster of quakes occurred last year along Puerto Rico’s northwest coast following a 6.0-magnitude earthquake in late September that led to more than 1,200 quakes in that area, Huérfano said.

One of the largest and most damaging earthquakes to hit Puerto Rico occurred in October 1918, when a 7.3-magnitude quake struck near the island’s northwest coast, unleashing a tsunami and killing 116 people.