Update at 1:11 p.m. ET: NHK World TV reports minor injuries and minor damage from a magnitude-7.1 quake, ranging from cuts and bruises to at least one broken leg. In addition, no major damage has been reported, although power outages have caused considerable travel problems because of outages of street and traffic lights. The quake was initially reported as a magnitude-7.4 quake.

See quake photo gallery here.

Update at 12:29 p.m. ET: Don Blakemore, geophysicist with the National Earthquake Information Center in Golden, Colo., says today's quake "is not at all unexpected." The Pacific Ocean's crustal plate is sliding underneath the crustal plate Japan rests upon, he says, triggering the quakes. "It doesn't give all at once, so we will see aftershocks for a long time," Blakemore says.

Update at 12: 24 p.m. ET: The Japanese meteorogical agency spokesman calls the latest quake the first major aftershock from the huge quake that rocked the country March 11.

Update at 12:01 p.m. ET: NHK World TV reports that two of three power lines servicing the Ongawa nuclear power plant in Miyagi Prefecture were cut because of the quake. NHK reports that the power lines "became unavailable." It says only one line is in service to continue to cool down the nuclear fuel, according to the National Nuclear and Industrial Safety Agency.

Update at 11: 57 a.m. ET: NHK World TV reports that all tsunami alerts have been lifted.

Update at 11:34 a.m. ET: The Japan meteorological agency has issued a tsunami warning for a wave of up to 6 feet. Japan's public broadcaster NHK warns coastal residents to run to higher ground and away from the shore. The warning covers a coastal area torn apart by last month's tsunami, which may have killed about 25,000 people and sparked an ongoing crisis at a nuclear power plant.

Paul Caruso, a geophysicist at the U.S. Geological Survey in Golden, Colo., says the latest quake hit at about the same location and depth as the March 11 quake.

Update at 11:22 a.m. ET: Tokyo Electric Power says workers evacuated the Fukushima Dai-ichi nuclear plant, CNN reports. The AP reports that officials at the nuclear plant say there is no sign of new problems after the strong aftershock.

Update at 11 a.m. ET: A magnitude-7.4 earthquake off Japan triggered a tsunami alert for the country's ravaged northeastern coast, the Associated Press reports.

Officials say the quake hit 25 miles under the water and off the coast of Miyagi Prefecture. Buildings as far away as Tokyo shook for about a minute, the AP says.

Last month's devastating quake that triggered a major tsunami was a magnitude 9.