At least one person is dead and another eight injured in the quake, the Associated Press reported.
"Citizen security is a priority, so vulnerable areas are being inspected and all necessary measures will be taken to ensure the safety of all Puerto Ricans," Gov. Wanda Vázquez said in a statement.
At least 20 aftershocks have since been reported, with the largest of the quakes hitting early Tuesday around 4:24 a.m. local time. As of this writing, government offices and school facilities had been closed. Additionally, some hospitals in the southwestern area had also been closed due to the quake.
Power to the region was taken down by the 6.4 quake, with local authorities confirming Tuesday that two plants had suffered "light damage," with power expected to be brought back later that day. The main airport in the region is still operating thanks to generator-backed power.
Vázquez has signed a state of emergency declaration, which CNN notes will activate the National Guard of Puerto Rico for relief efforts. The quake is the biggest to hit Puerto Rico since a 6.4 quake hit an estimated 61 miles northeast of the island in 2014. Among the damaged structures from Tuesday's quake is the Punta Ventana rock formation, which reports say was "destroyed."
Tuesday's quake was first reported with a 6.6 magnitude, though that figure was later adjusted to 6.4.