Update | Japan's Fukushima nuclear plant evacuated after tsunami warning

Workers at the crippled Fukushima nuclear plant were evacuated when a small tsunami hit Japan after a powerful undersea earthquake yesterday, highlighting the continued threat to the area devastated by the 2011 quake-tsunami.

The magnitude-7.1 earthquake struck at 2.10am off the coast of northeastern Japan and its epicentre was located off Fukushima prefecture at a depth of 10 kilometres, the Japan Meteorological Agency said.

The quake prompted the agency to issue a tsunami warning and thousands of residents were urged to evacuate in the prefectures of Iwate and Miyagi. Ships left a port south of the Fukushima nuclear power station as a precaution after the tremor. Buildings shook as far away as Tokyo, 230 kilometres south of the nuclear plant.

"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," said Satoshi Mizuno, a disaster management official with the Fukushima prefectural government. "Luckily, the quake's centre was very far off the coast."

Mizuno said the operator of the troubled Fukushima plant, Tokyo Electric Power Co (Tepco), reported that no damage or abnormalities had been found.

Two workers patrolling wells used to measure underground water at Fukushima sought higher ground after the tremor struck, a Tepco official said, adding that few workers were working on the waterfront at the time.

Another nuclear plant, at Onagawa, was the site of the largest wave recorded yesterday - 55cm - but there were no problems reported there. All of Japan's 50 nuclear reactors remain offline as the government decides whether they meet more stringent requirements enacted after the 2011 quake.