A magnitude 7.3 earthquake struck offshore of Japan's crippled nuclear power plant early on Saturday, triggering small tsunamis but causing no damage.
An earthquake official with the Japan Meteorological Agency said the quake was an aftershock of the magnitude 9.0 earthquake and tsunami that struck the same area in 2011, killing about 19,000 people and devastating the Fukushima Dai-Ichi nuclear power plant.
There was no damage and only one minor injury was reported, the national Fire and Disaster Management Agency said. Tsunamis of up to 40 centimeters hit four areas along the coast, but a tsunami advisory was lifted less than two hours after the quake.
The epicentre of the 2.10am quake was about 290 kilometres off Fukushima, and it was felt in Tokyo, 480 kilometres away.
"It was fairly big, and rattled quite a bit, but nothing fell to the floor or broke. We've had quakes of this magnitude before," Satoshi Mizuno, an official with the Fukushima prefectural government's disaster management department, told reporters by phone. "Luckily, the quake's centre was very far off the coast."
Mizuno said the operator of the troubled Fukushima plant, Tokyo Electric Power, said no damage or abnormalities had been found.
Japan's meteorological agency issued a 1-metre tsunami advisory for a long stretch of Japan's northeastern coast. It put the quake's magnitude at 7.1, while the US Geological Survey recorded a magnitude of 7.3. The US Pacific Tsunami Warning Centre did not post warnings for the rest of the Pacific.
The meteorological agency reported tsunamis of 40 centimeters in Kuji city in Iwate prefecture and Soma city in Fukushima, as well as a 20-centimeter tsunami at Ofunato city in Iwate and a 30-centimeter tsunami at Ishinomaki in Miyagi prefecture.
All of Japan's 50 nuclear reactors remain offline as the government decides whether they meet more stringent requirements enacted after the 2011 quake, which triggered multiple meltdowns and massive radiation leaks at the Fukushima plant about 250 kilometres northeast of Tokyo.
A string of mishaps this year at the Fukushima plant has raised international concerns about the operator's ability to tackle the continuing crisis.
Nuclear Regulation Authority Chairman Shinichi Tanaka has scheduled a Monday meeting with Tokyo Electric's president to seek solutions to what he says appear to be fundamental problems.