The Geological Survey said that residents should be ready for more aftershocks in the coming days. The island is almost certain to have dozens more quakes of magnitude 3 or higher, and there is a 63 percent chance of an earthquake of magnitude of 5 or higher.

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 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.