The quake came a day after a 7.8-magnitude temblor struck near Tonga, prompting a tsunami warning to countries as far away as Fiji and New Zealand. The tsunami warning was canceled, but Tonga reported it never received a warning.
A tsunami warning was not issued Friday.
The U.S. Geological Survey said at least six aftershocks occurred near Tonga, while others were recorded near Fiji and Vanuatu. The strongest aftershock occurred at 12:25 a.m. local time at a depth of almost 24 miles.
It was located 85 miles east-northeast of Nuku'alofa and 1,310 miles northeast of Auckland, New Zealand.
Though a major tsunami never materialized after Thursday's quake, the communication failure raised troubling questions about the effectiveness of such alerts, which have come under global scrutiny since an earthquake-driven tsunami in the Indian Ocean nearly 18 months ago left at least 216,000 dead or missing.
The Pacific Tsunami Warning Center in Hawaii, which issues the alerts, said Tonga — a 170-island archipelago about halfway between Australia and Tahiti — failed to receive the warnings because of a power failure. The cause wasn't known. Gerard Fryer, the center's acting director, said changes may be necessary.
"That's something they're going to look at, and we're going to have to work out an additional messaging scheme for them," Fryer said.
There were no reports of injuries from the magnitude-7.9 temblor, which struck about 95 miles south of Neiafu, Tonga, and 1,340 miles north-northeast of Auckland, New Zealand. Authorities lifted the tsunami warnings within two hours.
Tonga, with a population of about 108,000, is alerted through the World Meteorological Office, one of about a dozen ways the Pacific Tsunami Warning Center issues alerts. If power failures are a concern, a satellite phone could be used to notify officials in the future, Fryer said.
The center, usually staffed by two scientists or a few at the most, said it doesn't have the manpower to contact Pacific nations individually to ensure its warning was received. Thursday's earthquake came just a week after the center upgraded to around-the-clock staffing.
"What we try to do is cover all the bases," said Barry Hirshorn, a geophysicist at the center. "The important base for us is to make sure our message gets out. If people don't get it, it's not worth anything, but we don't have people in every country who can help keep their sirens running and their power running. It's frustrating."
Any warning probably would have been too late for Tongans because the epicenter was so close. But Mali'u Takai, deputy director of Tonga's National Disaster Office, told The Associated Press in a telephone interview that the system that should have passed on an alert from the center had malfunctioned.
"Nobody got a warning through the emergency satellite system in our meteorological office," Takai said.
Within 16 minutes of the 4:26 a.m. earthquake, the Honolulu-based center began transmitting warnings that a possible tsunami could strike Fiji within two hours and then, an hour later, New Zealand.
The warning set off an alarm in the Fijan capital of Suva. But authorities apparently failed to inform citizens, many on tiny and remote islands with poor communications. At the Wakaya Club, a private luxury Fijian island resort where recent guests have included Rolling Stone guitarist Keith Richards, staff were alerted to the danger through satellite television news — but guests were never told.
In New Zealand, hundreds of residents on the country's east coast fled their homes after hearing media reports. But authorities did not issue a national civil defense warning, said Allen Walley, a spokesman for New Zealand's National Crisis Management Center, because "overseas media reports had incorrectly suggested a threat."
New Zealand coastal resident Philip Payne, manager of the Ocean Beach Motor Lodge, said he did not know about the threat until it had passed. But the lack of contact from Civil Defense has him worried because it would have been a major exercise to evacuate his 60 guests, along with his own family.
"We had no awareness whatsoever," Payne said. "I'm concerned. We certainly would have needed to be contacted by Civil Defense and told of where we could have evacuated our guests if necessary."
In Hawaii, 14 coastal schools in flood zones were closed as a precaution. Teachers and students were told to take the whole day off, but many didn't get word until they arrived at school.
In Tonga, Mary Fonua, a publisher in the capital, Nuku'alofa, said it was the most powerful quake she had felt in 27 years there. Shelves were seen overturned in bookstores. Power in the city was restored after two hours, but most phone lines were jammed by incoming calls.
The communication failure in Tonga lends greater urgency to a test of alert systems in 23 countries on both sides of the earthquake-prone Pacific that is scheduled to take place in two weeks. The Hawaii center, which falls under the National Oceanic and Atmospheric Administration, came under criticism for not reporting more aggressively on the tsunami in the Indian Ocean floor.
On Dec. 26, 2004, the most powerful earthquake in four decades — magnitude 9.0 — ripped apart the Indian Ocean floor off Indonesia's Sumatra island, displacing millions of tons of water and spawning giant waves that sped off in all directions.