The quake was initially measured at 7.3 magnitude but later downgraded to 6.9
A tsunami warning for waves up to three metres has been issued after a 6.9 magnitude earthquake hit northeastern Japan.
People are being warned to evacuate low-lying areas in Fukushima prefecture and Tokyo Electric Power Co was checking its nuclear plants for damage.
TV pictures showed ships moving out to sea as tsunami sirens wailed.
Swells of two to three feet have been reported so far.
The epicentre is not far from Fukushima's Daiichi plant, which in 2011 had a nuclear meltdown following Japan's strongest-ever earthquake - a 9.0.
The plant's owner said things appeared normal after Monday's quake, broadcaster NHK reported, but there are blackouts in some areas.
The quake struck off the Japanese coast, 67km northeast of Iwaki city, at a depth of 11.4km, the US Geological Survey (USGS) said.
A fire department official said there was smoke or a fire at a petrochemical research centre, but that it had been extinguished.
Tokyo, 240km to the south, also felt tremors from the earthquake - which struck at 6am local time (9pm GMT).
Japan's Meteorological Agency initially put the earthquake at 7.3 magnitude, but the reading by the USGS was later downgraded to 6.9.
Japan accounts for 20% of earthquakes greater than 6.0.
The 2011 disaster in Fukushima was the world's worst nuclear disaster since Chernobyl and left more than 18,000 people dead or missing.