TOKYO - A strong earthquake with a preliminary magnitude of 7.2 hit off the coast of northeastern Japan on Wednesday, the Japanese Meteorological Agency (JMA) said.

A tsunami warning of up to 50 cm was issued for northeastern Japan, public broadcaster NHK reported.

The focus of the tremor was 10 km (6 miles) below the surface of the earth, off the coast of Aomori prefecture, NHK said.

Tohoku Electric Power said its Onagawa nuclear plant was operating normally after the quake. Tokyo Electric Power also said there was no impact on its power plants in the region.

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.