TOKYO (AP) _ A strong earthquake jolted Tokyo and a wide area of Japan early Tuesday. There were no immediate reports of injuries or damage.

The quake, centered far under the Pacific Ocean, registered a preliminary 7.1 on the Richter scale, according to the Central Meteorological Agency. The U.S. Geological Society in Menlo Park, Calif., estimated the magnitude at 6.6.

The quake, which struck at 12:55 a.m. (11:55 a.m. Monday EDT), shook Tokyo and neighboring Yokohama the strongest, but was felt as far away as Japan's northern island of Hokkaido, more than 500 miles to the north, the meteorological agency said.

The quake was centered about 270 miles southwest of Tokyo and about 250 miles below the ocean's floor, and therefore had relatively little effect on land despite its strength, the agency said.

It said there was no danger of tsunami, or quake-induced ocean waves.

The Japan Broadcasting Corp. said there were no reports of injuries, damage or fires.

The Richter scale is a gauge of the energy released by an earthquake, as measured by the ground motion recorded on a seismograph. Every increase of one number means that the ground motion is 10 times greater.

A quake of magnitude 6 can cause severe damage in populated areas; one of magnitude 7 is considered a major earthquake, capable of widespread, heavy damage in populated areas.