Japan on Monday revised down the magnitude of a strong earthquake from 7.1 on the Richter scale to 7, Japan Meteorological Agency said.

The quake triggered the agency to issue a tsunami warning, which had later been lifted.

Video image taken on April 11, 2011 in Tokyo shows live news of earthquake in northeastern parts of Japan. A strong quake with a preliminary magnitude of 7.1 jolted eastern, northeastern parts of Japan at 5: 16 p.m. local time (0816 GMT). A tsunami alert was issued, according to Japan Meteorological Agency. [Ji Chunpeng/Xinhua]

The quake struck eastern, northeastern parts of Japan at 5:16 p. m. local time (0816 GMT). And tsunami alert was issued, according to Japan Meteorological Agency.

The agency considered the quake as an aftershock of the March 11 magnitude-9.0 massive earthquake.

The epicenter, with depth of 10 km, was near Hamadori of Fukushima Prefecture. Swaying was strongly felt in buildings in Tokyo, some 200 km south of Hamadori.

The quakes caused the external power supplies at three reactors at the Fukushima Daiichi nuclear power plant, already crippled by the March 11 quake and tsunami disaster, to be cut temporarily, halting operations to spray water onto the spent nuclear fuel pool at the reactors for a while, the plant's operator Tokyo Electric Power Co. said.

There were no reports of damage to nuclear power facilities in Ibaraki, Niigata, Miyagi and Aomori prefectures, according to prefectural governments and plant operators.

Police had received some reports of injuries, public broadcaster NHK said. About 180,000 households suffered blackout after the quake.

The temblor was followed by another earthquake in the same area a minute later with a preliminary magnitude of 6.0.