A powerful 7.1 magnitude aftershock has rocked a large portion of eastern Japan, less than four weeks after an even stronger quake left thousands of people dead and caused a nuclear power disaster, but, this quake seems to have done limited damage.
The earthquake struck at 11:32 on Thursday evening and was centered under the Pacific Ocean off Japan's eastern coast in the same general area as last month's earthquake.
A warning of tsunami waves up to one meter in height was quickly issued, but withdrawn just over an hour later. The quake does not appear to have generated any high waves.
Last month's magnitude 9.0 quake and the tsunami it generated killed more than 12,000 people and left more than 14,000 missing. More than 100,000 people are still living in evacuation shelters.
Japanese Prime Minister Naoto Kan and other senior government ministers rushed to an emergency cabinet meeting after Thursday's quake.
A major concern was further damage to the crippled Fukushima Daiichi nuclear power plant and other nuclear power stations in the area.
Speaking two hours after the earthquake, Deputy Chief Cabinet Secretary Tetsuro Fukuyama said there had been no rise in radiation at monitoring posts near the three nuclear plants closest to the epicenter.
Fukuyama says two of three external power connections to Onagawa nuclear plant are down, but the plant is fine with the remaining connection. Onagawa's reactors were not operating when the quake occured.
The earthquake last month crippled the Fukushima plant, leading to leaks of radioactive steam and water. The plant's operator, Tokyo Electric Power Company continues to struggle to prevent a meltdown of the nuclear fuel in the reactors. After Thursday's quake, TEPCO officials said there were no signs of new damage to the Fukushima plant.
NHK television reported a few light injuries stemming from the quake. The government says damage to the power grid had blacked out large parts of northern Japan.
The quake was felt far down the coast, and Tokyo, 300 kilometers from the epicenter, experienced strong shaking that lasted for at least a minute.