An offshore quake with a magnitude of 6.2 shook northeast Japan and swayed buildings in Tokyo on Thursday, but no tsunami warning was issued, seismologists said.
The quake struck at 5:00 pm (0800 GMT) off Ibaraki prefecture in the Pacific, some 10 kilometres (6.2 miles) under the seabed, the US Geological Survey said.
Tokyo Electric Power Company said the quake caused no damage to the Fukushima nuclear power plant which was crippled following the March 11 earthquake and tsunami.
Hajime Motojuku, a TEPCO spokesman said: "So far, no abnormality has been monitored in our water reprocessing and radiation monitoring systems. The earthquake has not affected our operations."
The USGS put the epicentre of the quake 156 kilometres east-northeast of Tokyo, where an AFP reporter said buildings were felt to shake for around a minute.
The website of the Japan Meteorological Agency, which also recorded the magnitude of the quake as 6.2, said there was no risk of a tsunami.
"There may be a slight change in the sea level along the Japanese seacoast, but there is no concern over any damage" from potential high waves due to the quake, the agency said.
The quake came just over six months after a massive 9.0-magnitude undersea quake struck off Japan's northeast coast, generating a huge tsunami that devastated 600 kilometres of coastline and left 20,000 dead or missing.
It also swamped the Fukushima Daiichi nuclear power plant, knocking out cooling equipment and causing explosions, fires and meltdowns that sent radiation into the environment.
Tens of thousands of people remain evacuated from their homes in a 20 kilometre radius around the radiation-leaking plant and from other hotspots further afield, with no definite timescale for when they will be allowed back.
Workers are still grappling with the after effects of the disaster and trying to bring the plant to a cold shutdown.
Sizable aftershocks have continued to rattle Japan's tsunami-ravaged coast, each time raising fears of further damage to the crippled Fukushima plant, where tsunami barriers have yet to be rebuilt.
Japan, located at the junction of four tectonic plates, experiences 20 percent of the strongest quakes recorded on Earth each year.