By comparison, the 1906 San Francisco earthquake is estimated at a magnitude of 7.8 to 8.25. The magnitude 6.7 Northridge quake in 1994 was much smaller but caused more than $40 billion in damage. Each change of 1 magnitude increases ground motion by 10 times and releases about 32 times more energy, according to the USGS.
The temblor Wednesday was centered in the Pacific about 90 miles southeast of Lima, at a depth of about 25 miles, authorities said.
The shaking was felt along a wide swath of coastal Peru, including the capital, where thousands of residents fled their houses Wednesday and early Thursday. Three fatalities were reported in the Lima area from heart attacks, authorities said.
"We don't know how long the aftershocks will continue, but it's sure that they will continue," Hernando Tavera, director of seismology at the Geophysical Institute of Peru, told the Andina press agency.
Authorities here said that the earthquake was the region's deadliest since a magnitude 7.9 quake struck north of Lima in 1970, causing more than 50,000 deaths.
Special correspondent Leon reported from Lima and Times staff writer McDonnell from Sao Paulo, Brazil. Andres D'Alessandro of The Times' Buenos Aires Bureau and staff writer Bob Drogin in Crawford, Texas, contributed to this report.