TOKYO (AP) -- A strong earthquake with a preliminary magnitude of 6.8 struck off the northern Japanese coast early Thursday but prompted no immediate reports of injuries or other damage, the nation's weather agency said.

The Meteorological Agency said there was no danger of a tsunami, or seismic waves, from the 12:26 a.m. (11:26 a.m. EDT Wednesday) quake, which occurred about 75 miles below the ocean's surface off the coast of Iwate.

The offshore earthquake caused strong shaking in northern Japan, but no tsunami alert was issued by the agency.

The quake was believed to have lasted about 30-40 seconds. Railway operators said that they had already stopped trains so that tracks could be inspected.

No abnormalities had been detected at the nearest nuclear power plant in the area, which continued controlled operations, according to local media reports.

Tomio Kudo, a grocery shop owner in the town of Hirono, where the shaking was most violently felt, told an interview with public broadcaster NHK.

"Everything has fallen off the shelves, scattered all over the floor," he said. "Even a big refrigerator has moved about 30 centimeters (1 foot)."

Koji Yoshida, an official for Morioka Fire Department in nearby Aomori prefecture (state), said the tremor was strong but nothing fell off shelves.

"So far we have not received any reports of damage or casualties," he said.

Japan is one of the world's most earthquake-prone countries.

Last month, a 7.2-magnitude earthquake struck sparsely populated rural areas in northern Japan, killing at least 12 people, leaving 10 others missing and injuring more than 300.