Japanese authorities issued a tsunami alert for the northeast coast Friday after a powerful 7.3-magnitude undersea earthquake struck, setting buildings in Tokyo swaying violently.
Media reports said a one-metre-high wave could sweep ashore in an area badly hit by the March 2011 tsunami that devastated a large swathe of the northeast coast, killing thousands.
Residents of at least one town, Minamisanriku in Miyagi prefecture, were advised to evacuate to higher ground, reports said, suggesting other towns were also affected.
It was not possible immediately to confirm the reports, with telephone operator NTT saying the network was jammed with the weight of callers.
A presenter on state broadcaster NHK repeatedly told viewers to get to safety.
"Remember last year's quake and tsunami," he said. "Call on your neighbours and flee to higher ground now!"
NHK, quoting the national meteorological agency, said the tsunami was expected to hit the coast of Iwate at 5:40 pm (0840 GMT), Fukushima at 5:50 pm, and Aomori and Ibaraki at 6:00 pm.
The United States Geological Survey measured the quake's magnitude at 7.3. It said the tremor struck a relatively deep 36 kilometres (23 miles) under the Pacific.
The epicentre was 284 kilometres (176 miles) east of Sendai, or 459 kilometres (285 miles) northeast of Tokyo, according to the USGS.
NHK said the Japan Meteorological Agency had issued a tsunami warning, one notch lower than a tsunami alert, for the Pacific coast of Iwate, Fukushima, Aomori and Ibaraki prefectures.
There was no threat of a Pacific-wide tsunami, US monitors based in Hawaii said. Officials in both Indonesia and the Philippines south of Japan said there was no threat of a localised tsunami.
Nuclear plant operator Tokyo Electric Power told AFP there were no reports of any problems at the crippled Fukushima nuclear plant.
"No abnormalities have been recorded on instruments at Fukushima Daiichi nulcear plant's six reactors," a TEPCO spokesman said.
"All workers were ordered to take shelter inside buildings at the Fukushima plant.
"No abnormalities were confirmed with the radiation monitoring posts at the Fukushima plant. No abnormalities were seen with the water processing facilities."
Prime Minister Yoshihiko Noda was heading to his office where he would be monitoring the situation, Jiji Press said.
Japan Railways East temporarily suspended Shinkansen bullet train services to check any damage, Jiji said, while Haneda Airport near central Tokyo was reported to be operating normally.
