The biggest tsunami measured was 20 cm (7.9 inches) along the coasts of Fukushima and Miyagi prefectures, the agency said.

The US Geological Survey said the 7.0 magnitude tremor struck beneath the north Pacific Ocean, 77 miles (123 km) east northeast of Iwaki, Japan at 0239 GMT at a depth of 25 miles (40 km).

Nuclear power facilities in the area were unharmed and were continuing to operate as usual, said officials from Tokyo Electric Power Co and Tohoku Electric Power.

The quake was epicentred in the same area as a June 14 tremor which killed at least 10 and left as many again missing.

Earthquakes are common in Japan, one of the world's most seismically active areas. The country accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.

In October 2004, an earthquake with a magnitude of 6.8 struck the Niigata region in northern Japan, killing 65 people and injuring more than 3,000.

That was the deadliest quake since a magnitude 7.3 tremor hit the city of Kobe in 1995, killing more than 6,400. (Reporting by Yoko Kubota; Editing by Jeremy Laurence)