A massive earthquake struck northwestern Japan, prompting officials to issue a warning of a tsunami that could reach heights of more than 3 feet along parts of the coast.

The 6.8-magnitude quake struck Tuesday night off the western coast of Yamagata, about 30 miles southwest of the city of Sakata, according to the country’s meteorological agency.

The quake’s epicenter is fairly shallow — only about 6 miles below the sea’s surface. Shallow quakes tend to inflict more damage on the Earth’s surface.

Predictions by the agency indicate that the tsunami could be as high as a yard along the coast of the northwestern prefectures of Yamagata, Niigata and Ishikawa.

No abnormalities have been reported at any of the seven reactors at the Kashiwazaki-Kariwa nuclear plant, which were all off-line.

With Post wires