A powerful 7.1 magnitude aftershock which struck off the north-east coast of Japan killed three people and injured around 100, emergency services say.
The Fire and Disaster Management Agency said a 63-year-old woman in Yamagata prefecture died after her respirator failed when the power was knocked out by the 7.1 magnitude tremor.
Reports said two men aged 79 and 85 were killed in Myagi prefecture _ the area worst hit by the 9.0 magnitude quake of March 11 and the tsunami it spawned.
The quake also triggered tsunami warnings and panics in the wake of last month's devastating magnitude 9 earthquake and tsunami.
The aftershock struck at 11.32pm local time (2.32am NZT) and was centred 66km east of Sendai - which was hammered by the March 11 tsunami - at a depth of 49km.
The quake was initially measured a 7.4 magnitude, but has since been downgraded to a 7.1.
A tsunami warning was issued for the Miyagi Prefecture, and advisories were issued for Iwate, Fukushima, Ibaraki and the Pacific coast of the Aomori prefectures, since the March 11 quake.
The warning was subsequently withdrawn.
No advisories were issued for New Zealand.
The latest aftershock is expected to have caused more damage to already devastated north-eastern Japan.
Buildings as far away as Tokyo, which was about 330 kilometers from the epicentre, shook for about a minute.
Prime Minister Naoto Kan met with staff members in his office shortly afterward, according to deputy Cabinet spokesman Noriyuki Shikata.
NHK World reported there were a handful of injuries following the latest quake, but the National Policy Agency said there had been no reported fatalities.
Jiji Press news agency said there were five fires and 13 gas leaks in Sendai city, according to the Miyagi prefectural office, while NHK World reported three fires in the Iwate and Miyagi prefectures, and 500 households were ordered to evacuate in Iwate Prefecture.
Much of northeast Japan lost power after the quake.
No major damage has been reported.
The Fukushima Daiichi nuclear plant was evacuated after the quake, however no further damage was reported at the troubled plant.
All seven workers at the site were safe, a spokesman for plant operator Tepco said
Japan's nuclear safety agency said power plants along the northeastern coast were under control after backup generators kicked in at three - Rokkasho, Higashidori and Onagawa - that lost power.
The aftershock knocked out two of three power lines at the Onagawa nuclear power plant north of Sendai, which has been shut down since the tsunami.
One remaining line was supplying power to the plant and radiation monitoring devices detected no abnormalities.
The plant's spent fuel pools briefly lost cooling capacity but an emergency diesel generator quickly kicked in.
"It's the way it's supposed to work if power is lost for any reason," said David Lochbaum, director of the nuclear safety project for the US-based Union of Concerned Scientists.
Since the tsunami warning was cancelled 90 minutes after it was issued, there was no reason to believe the facilities' diesel generators would fail like the ones at the stricken Fukushima plant.
The massive wave knocked out cooling systems and triggered a series of mishaps that have left workers struggling to stop radioactivity from spewing nearly a month later.
"That was really the blow that the plant didn't recover from,'' Lochbaum said.
USGS geophysicist Paul Caruso told the AP news agency the 7.1 aftershock overnight was centred about the same location as the March 11 quake.
As of April 6, there 394 aftershocks larger than magnitude 5, three of which have been over 7. The largest to strike since the March 11 quake was a 7.7 magnitude, also on March 11.
On the other side of the Pacific, a 6.5 magnitude earthquake struck 57km south of Las Choapas, Veracrus in Mexico overnight, at a depth of around 167km, at 8.11am local time (1.11am NZT).
- NZ HERALD STAFF/AGENCIESBy Paul Harper @Snappy_nz Email Paul, AFP