By Mari Saito
TOKYO (Reuters) - A strong earthquake with a magnitude of 6.9 struck off northeastern Japan on Tuesday, the U.S. Geological Survey said, and small waves were reported along the northern coast but there were no immediate reports of serious damage or injuries.
Japan lifted a tsunami advisory it had issued for the northeastern coast hours after small waves of between 10 cm and 20 cm (4-8 inches) reached the coast off Iwate prefecture, some 600 km (370 miles) north of Tokyo.
Broadcaster NHK said thousands of residents in the area had been ordered to evacuate.
Live video from ports on the Iwate coast showed small waves lapping on the shore. Towns along the northeastern coast of Japan had been leveled in a devastating tsunami in March 2011.
"This quake is an aftershock of the 2011 quake that hit the Tohoku region," Japan Meteorological Agency seismologist Yasuhiro Yoshida told reporters on Tuesday.
The agency warned residents against entering the ocean and said waves could continue to fluctuate for some time.
Tuesday's quake was measured at a depth of about 10 km (6 miles). The Pacific Tsunami Warning Center in Hawaii said there was no danger of a Pacific-wide tsunami but the Japan Meteorological Agency issued its tsunami advisory for Iwate prefecture after the quake.
Tohoku Electric Power Co, which operates the Onagawa and Higashidori nuclear plants in nearby Miyagi and Aomori prefectures, said it saw no irregularities at the facilities after the quake.
All 48 of Japan's nuclear reactors remain offline after a March 2011 earthquake and tsunami, which triggered the Fukushima nuclear disaster in northeastern Japan.
A spokesman for Tokyo Electric Power Co, the operator of the Fukushima Daiichi and Fukushima Daini nuclear plants, said no irregularities were recorded. The quake was felt only weakly in the area, he said.
Unlisted Japan Nuclear Fuel Ltd also said there were no irregularities recorded at its nuclear fuel reprocessing facility or other plants in Aomori.
Earthquakes are common in Japan, one of the world's most seismically active areas. Japan accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.
(Additional reporting by Elaine Lies, Chris Gallagher, Osamu Tsukimori and Olivier Fabre in TOKYO and Jane Wardell in SYDNEY; Editing by Paul Tait)