This seismic activity in the region isn't unusual. The Caribbean islands are prone to earthquakes, and seismologist Dr. Lucy Jones says it's because the plates under the islands are moving in multiple directions.
Puerto Rico, which lies on the Caribbean plate and is bordered by the North and South American plates, gets squeezed between faults, Jones explained.
"The primary motion is just offshore to the north of Puerto Rico, there's a very deep trench that's the deepest part of the Atlantic Ocean," Jones said. "But, there's also another trench to the south of Puerto Rico and the island itself is sort of squeezed between these faults, and it gets jostled around.
Puerto Rico has a long history of quakes because it sits on a complicated plate boundary. The small Caribbean plate pushes over the South American plate's oceanic crust east of PR. North of PR, the Caribbean plate slides past North America. Puerto Rico gets squeezed between. pic.twitter.com/iHenv2XL4O— Dr. Lucy Jones (@DrLucyJones) January 7, 2020
Jones said the current swarm of quakes is off the southwest coast, and adds that all the quakes since Dec. 29 can be considered one earthquake sequence. More temblors measuring 5.0 are likely, and another 6.0 quake is possible, she said.
Puerto Rico is caught between two subduction zones. The current quake swarm is off the southwest coast, past of the adjustment between the two big faults. Sort of like Northridge 1994 was on a small fault to adjust for the bend in the San Andreas fault. pic.twitter.com/AsJDAwZbLv— Dr. Lucy Jones (@DrLucyJones) January 7, 2020
Tuesday's quake was the strongest to hit Puerto Rico since October 1918, when a magnitude 7.3 quake struck near the island's northwest coast, unleashing a tsunami and killing 116 people.
More than 950 quakes and aftershocks have been recorded in the area of Tuesday's event since Dec. 31, though most were too weak to be felt, according to U.S. Geological Survey.
The Associated Press contributed to this report.