The strongest aftershock since the catastrophic March 11 earthquake hit Miyagi Prefecture in northeastern Japan late Thursday night.

Video image taken from NHK shows a view of Fukushima Prefecture of Japan when the earthquake occured on April 7, 2011. An earthquake measuring 7.4 on the Richter scale hit the sea near Miyagi Prefecture in northeastern Japan at 23:32 p.m.(1432 GMT) local Time Thursday, according to the Japan Meteorological Agency. [Xinhua Photo]

An earthquake measuring 7.4 on the Richter scale hit the sea near Miyagi Prefecture and its vicinity at 23:32 p.m. (1432 GMT) local time Thursday, according to the Japan Meteorological Agency.

The focus of the quake was located some 40 km under the sea off the northeastern prefecture, said the agency.

Residents have already taken shelters at local facilities and rested in the dark at the time. Tsunami warnings have been lifted so far, the agency said.

Blackouts were seen all over Aomori, Iwate and Akita prefectures as well as several parts of Miyagi and Yamagata prefectures, according to Kyodo news reports.

A report on Japan Today said there were many emergency calls about injured people, fires and gas leakages, according to local police and fire departments. While the quake caused all expressways to be closed in Miyagi Prefecture and most train services to be suspended in the Tohoku region.

As of midnight Thursday, it had left some 3.64 million households without electricity in six prefectures, including those already without power, the report said.

Nuclear power stations in Ibaraki Prefecture were operating normally after the quake, while at the Onagawa nuclear plant in Miyagi Prefecture, which has been suspended, two external power supply units among three have failed, according to the nuclear regulator.

The Tokyo Electric Power Co. said the quake has not brought any major problems to the nuclear power plants in Fukushima Prefecture so far.