Japan’s biggest aftershock since the day of the March 11 earthquake left two dead and millions without power in the areas hit hardest by last month’s tsunami.
The magnitude-7.1 temblor struck at 11:32 p.m. local time yesterday near the epicenter of last month’s quake, the U.S. Geological Survey reported on its website. About 1.9 million households, mostly in Miyagi and Iwate prefectures, remain without electricity as of 1 p.m. today, said Tohoku Electric Power Co., the main supplier to northern Japan.
“Magnitude-7 level aftershocks may continue to occur even as the frequency of aftershocks will likely fall,” said Yoshihiro Hiramatsu, an associate professor specializing in seismology at Kanazawa University. “Aftershocks will continue for a year or so.”
The aftershock hindered efforts by Tokyo Electric Power Co. to prevent hydrogen explosions at its Fukushima Dai-Ichi nuclear power plant 220 kilometers (137 miles) north of Tokyo. After an evacuation of 15 workers, engineers returned to the plant to pump nitrogen into the No. 1 reactor.
No unusual conditions were observed at the Fukushima Dai-Ichi nuclear station, according to statements by Tokyo Electric and Japan’s Nuclear and Industrial Safety Agency. There have been no indications of changes in radiation levels or damage at the plant, Tokyo Electric spokesman Takashi Kurita said today.
Shares of Tepco, as the company is called, rose 24 percent to 420 yen at the 3 p.m. Tokyo close, its fourth gain in 20 days. The stock has slumped more than 80 percent since the quake on March 11.
“Indications of new leakage or a change in radiation levels will be the only way they’ll tell if there’s further damage,” Murray Jennix, a nuclear engineer who specialized in radioactive containment leaks and teaches at San Diego State University, said in a telephone interview. “You’ve got cracks that could have been made bigger.”
Tokyo Electric injected 800 cubic meters of nitrogen into the No. 1 reactor by 7 a.m. Japan time today, Kurita told reporters at a briefing. The process of feeding nitrogen, the most prevalent inert gas in the atmosphere, into the reactor may take six days, spokesman Yoshinori Mori said yesterday.
“They are manually injecting nitrogen through a very narrow pipe,” Tadashi Narabayashi, a professor of nuclear engineering at Hokkaido University in northern Japan, said by phone yesterday. “High radiation levels in the building are also making it difficult as workers have to keep rotating.”
Tepco is still using emergency pumps to cool the reactors and pools holding spent fuel, four weeks after the initial disaster. Three blasts damaged reactor buildings and hurled radiation into the air last month.
Tohoku Electric said no radioactive water leaked from its Onagawa nuclear station even after being spilt from spent fuel pools and a pumping room. About 11 liters (3 gallons) of radioactive water spilt on the floor because of shaking from the quake, according to the company.
Cooling systems at Onagawa, which was safely shut down after the March 11 quake, were operating normally, he said. Two of three power lines remain disabled to the station, he said.
The company also restored mains power at its Higashidori nuclear plant and restarted three out of six units at thermal plants by 2 p.m. today, the company said.
Japan Nuclear Fuel Ltd.’s power switching station at its Rokkasho nuclear fuel plant in northern Japan started receiving power this morning from an outside source, spokesman Yoshinori Ochiai said by telephone. Cooling, containing, and other major functions at the plant are working properly, he said.
East Japan Railway Co., the nation’s largest rail operator, suspended bullet train services on three of its five lines. The rail company halted the Tohoku, Yamagata and Akita lines, according to a release on its website.
There have been about 900 aftershocks since last month’s temblor, which left more than 27,600 dead or missing and caused an estimated 25 trillion yen ($294 billion) in damage.
Yesterday’s quake struck at a depth of about 49 kilometers, the U.S. Geological Survey reported on its website. A tsunami alert for a possible two-meter wave was canceled by Japan about two hours after the warning was issued. There was a magnitude-7.9 aftershock on March 11 about half an hour after the temblor last month, according to the USGS.
Yesterday’s quake was “tremendously smaller than the main shock,” said Don Blakeman, a geophysicist in the U.S. National Earthquake Information Center in Golden, Colorado. “The main shock caused about 80 times more ground movement.”
The main airport near Sendai in northern will reopen on April 13 after being swamped by the tsunami, Transport Minister Akihiro Ohata told reporters in Tokyo today.