It was the second significant earthquake on the island within a 24-hour period.
Officials said a 73-year-old Ponce man died when a wall in his home collapsed.
Authorities closed public offices and schools in Puerto Rico’s southwestern region and Gov. Wanda Vazquez, facing her first natural disaster since taking office in August, urged calm.
“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,” Vazquez tweeted.
A school in Agripina Seda collapsed during the quake, but education officials said others received less significant damage.
The power outage cut off water to about 300,000 who use electrical pumps to get water into their homes.
The quake Tuesday struck off the southern coast, 10 miles off Indios and about 60 miles southwest of San Juan, the U.S. Geological Survey said. It was the strongest of a series of quakes that have shaken Puerto Rico in the last two weeks.
“Strong to very strong shaking occurred across parts of southern Puerto Rico closest to the event, and moderate shaking occurred across the rest of the island,” the USGS said.
The USGS issued an orange alert for economic loss, estimating the damage could range as high as 1 percent of Puerto Rico’s economy. It noted that past orange-level events required a regional or national level response.
The event follows a magnitude-5.8 quake Monday, which hit just off the island’s southern coast and buckled an iconic natural stone formation arch in Guayanilla known as Punta Ventana, or Window Point.
The USGS said the quakes resulted from an oblique strike-slip fault at shallow depth where the North America plate converges with the Caribbean plate.
“Administration officials, including FEMA Administrator Pete Gaynor, have been in touch with [Vazquez] and her team today, and we will continue to monitor the effects and coordinate with Puerto Rico officials,” White House deputy press secretary Judd Deere said in a statement.