In Japan, Aftershocks Are Also Felt From Within

Aguri Suzuki, a 44-year-old real estate agent, says she sometimes thinks the ground is shaking even when it is not. When she sees a tree branch swaying in the wind, she worries there has been an earthquake.

TOKYO — Aguri Suzuki, a 44-year-old real estate agent, says she sometimes thinks the ground is shaking even when it is not. When she sees a tree branch swaying in the wind, she worries there has been an earthquake.

A spokesman for the Tokyo Electric Power Company after a news conference on Monday in Tokyo on the situation at the Fukushima Daiichi plant.

Doctors here say they are seeing more people who are experiencing such phantom quakes, as well as other symptoms of “earthquake sickness” like dizziness and anxiety.

And it is no wonder. As if the threat of radiation from a crippled nuclear power plant were not enough, Tokyo and the region to its northeast have been under a constant barrage of aftershocks since the magnitude 9.0 earthquake that set off a devastating tsunami on March 11. Two earthquakes were felt in Tokyo on Wednesday morning, three on Tuesday, a large one on Monday and a very large one of magnitude 7.1 last Thursday.

Over all, there have been 400 aftershocks of magnitude 5.0 or greater in northeastern Japan since March 11. That is as many sizable quakes in one month as Japan typically experiences in two and a half years, according to the Japan Meteorological Agency.

The quakes are complicating efforts to control the Fukushima Daiichi nuclear power plant. For instance, the quake on Monday knocked out cooling at the Fukushima plant for nearly an hour.

Every time a sizable quake occurs, the first question on many people’s minds is whether the nuclear plant has been further damaged and whether a new cloud of radiation is on the way. A spokesman for the Tokyo Electric Power Company, the plant’s owner, is then hustled onto television to reassure viewers.

 A POWERFUL 6.6-magnitude aftershock has hit north-east Japan,

Government officials are becoming concerned that in the rush to cool the reactors and prevent hydrogen explosions, the plant’s vulnerability to another tsunami has been overlooked.

“A week ago we thought the major risk was a hydrogen explosion,” a senior official in the office of the prime minister said Tuesday. “I think the major risk at the moment is an aftershock and tsunami.”

Hidehiko Nishiyama, the deputy director general of Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency, said at a news conference on Wednesday evening that three measures are being considered that would allow electricity and cooling at the plant to remain intact even after a tsunami measuring 15 meters, or 49 feet. Right now the site can withstand a tsunami of only about 18 feet, he said. One measure is to interconnect the external power lines that have been built to the power plant, so that if one power line is broken, the others can still carry electricity to the various reactors.

A second measure is to put a generator on a small hill inside the plant site, and the third is to place a fire pumper engine on the hill that could send water into the reactors and spent fuel pools even if electricity was interrupted.

Japan, which sits atop four colliding tectonic plates, has a long history of earthquakes and some sophisticated technology to deal with them.

A detection system transmits warnings of some pending quakes a few seconds in advance to television broadcasters and to many cellphones. In recent weeks it has not been unusual to see nearly all the people in a restaurant or a train suddenly look at their cellphones at the same time.

 2 Killed, 100 Injured in Japan Aftershock

Yurekuru, a free app for the iPhone that delivers such warnings (its name might be translated as “the shaking is coming”), now has 1.5 million users, compared with only 100,000 before the March 11 quake, according to RC Solution, the app’s developer.

Geologists say the frequency of the aftershocks has declined since March 11 and will continue to decline, but will still remain higher than normal for a long time. “There is an increased frequency and it will last for at least five or ten years,” said Ross S. Stein, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif., who has studied the situation in Japan.

The March 11 quake was so strong that a Japan Coast Guard monitoring instrument on the floor of the Pacific Ocean near the epicenter moved 24 meters, or about 79 feet, eastward. The city of Sendai, whose airport was inundated by the tsunami, moved about 13 feet, according to Shinji Toda, a professor at Kyoto University.

Such large movements have shifted stresses in the earth, increasing the likelihood of quakes on some fault lines while reducing the likelihood on others, including the one involved in the 1923 Tokyo earthquake.

But over all, Dr. Stein said, the risks have increased. “There’s this very broad turn-on of seismicity that extends 300 miles from the rupture zone,” he said.

Satoko Oki, an assistant professor at the University of Tokyo’s earthquake research institute, said that an aftershock of the March 11 quake could reach magnitude 8.0.

There is some precedent. The 2004 quake of magnitude 9.1 near Sumatra, Indonesia, which spawned a tsunami that killed more than 200,000 people, was followed three months later by one measuring 8.6 and later by four more huge ones.

But the 8.8 magnitude earthquake in Chile in early 2010 has not yet produced an aftershock larger than 7.1, Dr. Stein said.

To be sure, the spate of earthquakes has not caused the same panic and mass exodus as the fears of radiation in the first week after the nuclear crisis began. Still, with levels of radiation in the Tokyo air having sharply fallen since then, some people interviewed on the street said they worried about the aftershocks more than radiation.

Dr. Hideaki Sakata, director of the Mejiro University Clinic, who is treating Ms. Suzuki, said that feeling the ground shaking when it is not is similar to the continued feeling of swaying when one first gets off a boat onto solid ground.

Dr. Kazuhiro Soeda, an ear, nose and throat specialist in Utsunomiya, outside Tokyo, who also treats patients having trouble dealing with the aftershocks, said: “People are getting too sensitive. This is something we’ve never experienced before.”

New York Times