At least three people were killed Thursday after a 7.1 earthquake rocked northeast Japan – knocking out power to millions as the country seeks to rebuild and recover from last month's devastating quake.
Right away, it was clear this wasn't just another aftershock as buildings swayed violently for minutes. Four million people were immediately plunged into darkness when two power plants were knocked offline.
Authorities told people near the coastline to evacuate and head to higher ground as a small wave hit the coast 12:40 a.m. local time Friday. Thursday's quake, which was 25 miles deep, is the strongest aftershock since the 9.0 quake on March 11.
About 25,000 people are believed to be dead following the twin disasters.
But the greatest concern was for the Fukushima nuclear reactors, where seven workers were forced to evacuate.
After midnight in Japan power company officials sought to reassure the public. "We don't recognize any new leaks so far," a Tokyo Electric Power Company official said, adding that radiation levels remain steady.
The Fukushima Daiichi plant has been on the verge of a meltdown since the last month's quake damaged the complex and its cooling system.
For a battered Japanese public, it was another frightening reminder. "The problem is so large it's difficult to comprehend," a woman told ABC News.
At two other facilities, the Onagawa Nuclear Plant and the Higashidori nuclear plant, the quake caused power outages that forced both onto emergency generators to keep fuel rods safe. At Onagawa, water from spent fuel rods actually spilled onto the floor but was contained.
Experts monitoring the crisis worry that more strain is being put on reactors that are already overburdened.
"These reactors have been subjected to an enormous amount of stress and every shock adds to it," said nuclear policy expert Joe Cirincione.
ABC News' Russell Goldman, Akiko Fujita and the Associated Press contributed to this report.