The quake struck three kilometers (two miles) from the coastal town of Iwaki at 5:36 am (2036 GMT) at a depth of 21 kilometers, the USGS said, and 80 kilometers south of Fukushima where authorities are battling an atomic emergency,

No tsunami warnings were in effect and there were no immediate reports of damage.

The Japan Meteorological Agency initially said the quake measured 5.1 on the Japanese seismic intensity scale, which runs to seven, but later downgraded the tremor to a 3.7.

The northeastern coast of Japan's main Honshu island was ravaged by a 9.0 magnitude quake and monster tsunami on March 11, leaving 24,525 people dead or missing, according to the latest count by the National Police Agency.

Copyright AFP (Agence France-Presse), 2011

Copyright APP (Associated Press of Pakistan), 2011