Japan lifted all tsunami alerts after a magnitude 7.4 earthquake off the coast of Fukushima, home to the nuclear power plant crippled in the March 2011 triple disaster.
The temblor, an aftershock of the magnitude 9 quake five years ago, briefly knocked offline a cooling system for spent nuclear fuel at a separate Fukushima plant, and prompted authorities to issue the highest tsunami warnings for five years.
It struck at 5:59 a.m. local time Tuesday at a depth of 25 kilometers (15.5 miles), according to the Japan Meteorological Agency, causing a tsunami of more than 1 meter on the coast of Miyagi prefecture and smaller waves elsewhere.
The weather agency warned that aftershocks of a similar size to todays quake could occur over the next week, triggering further tsunamis, public broadcaster NHK reported. A similar-sized quake struck the region two days before the March 2011 disaster that killed about 18,000 people.
Local authorities and NHK initially urged residents to remember the experience of five years ago and flee to higher ground.
Five people have been reported injured, mostly in northeastern Japan, according to the Fire and Disaster Management Agency. Some bullet train services were suspended and Sendai Airport was evacuated, but Toyota Motor Corp. plants in the region were operating as normal.
A cooling system for a spent fuel pool at the Fukushima Dai-Ni plant shut down automatically after the quake, triggering memories of the early moments following the 2011 disaster, but was restarted less than two hours later. The stoppage of the system, which pumps water to a pool storing 2,544 nuclear fuel rods, would not immediately have led to a release of radiation, Chief Cabinet Secretary Yoshihide Suga said before the restart.
Power would need to be cut for about a week before temperatures in the spent-fuel cooling system would reach the upper safety limit, according to Yutaka Ikoma, a spokesman at the Nuclear Regulation Authority. Temperatures would rise about 0.2 degrees Celsius per hour without the cooling system, reaching 65 degrees Celsius in about seven days, according to the spokesman.
Workers at the Dai-Ichi nuclear plant, crippled following in the 2011 disaster, were evacuated as a precaution after the quake, according to a spokesman.
Suga told reporters in Tokyo that the cause of the pumps malfunction was under investigation, and added that Japan has the strictest nuclear regulations in the world.
These regulations envisage the largest earthquake that could occur on the site of a nuclear plant, and confirm that safety will be assured through individual inspections, Suga said. Tsunami and other natural disasters are also among the possibilities that are envisaged under the new regulations. The government will maintain safety as the top priority in dealing with nuclear plants.
More than five years after the quake, just two of the countrys 42 reactors are in operation. Returning the plants to service is a goal of Prime Minster Shinzo Abes government that aims to have nuclear account for as much as 22 percent of the nations energy mix by 2030. Abe, speaking from Argentina where he is making an official visit, said he ordered ministries to gather information and formulate a response.
Tuesdays malfunction at the nuclear plant wont help the government in its goal to restart the reactors, said Daniel P. Aldrich, a professor at Northeastern Universitys Security and Resilience Studies Program. The public is against restarts primarily because the Fukushima meltdowns revealed the falsity of the 100 percent safety myth promoted by the nuclear power utilities and the central government regulators, Aldrich said.
Adam Pascale, a seismologist with the Seismology Research Centre in Melbourne, played down any connection between Tuesdays quake in Japan and the magnitude 7.8 shaker in New Zealand last week.
Whenever you have two big earthquakes or several big earthquakes in close time proximity, people immediately think that there is a cause and effect thing, but these things happen all the time, Pascale said. There are stress changes throughout the Earth when you do have an earthquake, but the amount of shaking that New Zealand generated and the amount of ground movement in Japan would have been insignificant in terms of generating another fault.
The yen rose as much as 0.5 percent on Tuesday to 110.27 per dollar, heading for its first back-to-back gains in two weeks. Treasuries extended their advance, with 10-year yields declining two basis points to 2.30 percent.