Seabed split; quake tilted Earth's axis 10 cm

Sunday, March 13, 2011

By ALEX MARTIN
Staff writer

The magnitude 8.8 earthquake that jolted northeast Japan was caused by a tectonic upheaval that created offshore faults stretching for hundreds of kilometers from Iwate Prefecture to Ibaraki, seismologists said Saturday.

Stay close: A mother holds her child as aftershocks rattle a ward office in Sendai on Saturday. KYODO PHOTO

Satoko Oki of the University of Tokyo's Earthquake Research Institute said the massive quake, estimated to be nearly 1,000 times more powerful than the 1995 Great Hanshin Earthquake that killed more than 6,000 people, was caused by a rupture near the boundary between the Pacific and North American tectonic plates.

The quake was created when the Pacific plate slipped under Japan at the Japan Trench, causing tsunami as high as 10 meters to slam the east coast, she said.

Experts estimate the impact of the world's fifth-largest quake caused a displacement of about 20 meters and a fault a few hundred kilometers long.

According to the U.S. Geological Survey, the epicenter of the earthquake was 373 km northeast of Tokyo and 130 km east of Sendai.

Japan's seismic risk map indicates there was a 99 percent risk of a magnitude 7.5 or larger earthquake hitting the region in the next 30 years.

Oki said that while quake eruptions at plate boundaries are relatively common, one of this magnitude was unexpected.

"Magnitude-8.8 is really an enormous quake, the largest ever measured in Japan's vicinity," she said.

Oki warned that increases in seismic activity, especially of the inland type, have been historically noted before and following plate boundary earthquakes, although she said it was difficult to determine whether it had any relation to the magnitude-6.7 earthquake that hit Nagano and Niigata prefectures early Saturday.

"Plate boundary earthquakes happen every 100 years or so, but one of this magnitude happens only once in 1,000 years," Oki said.

Yuji Yagi, associate professor at Tsukuba University, said an earthquake of this scale could trigger other earthquakes at faults that are already on the brink of a tectonic upheaval.

"The stress created by a massive quake increases the possibility of other large tremors; extreme caution is needed," he said.

Sadayuki Kitagawa, an officer at the seismic research division of the government's Earthquake Research Promotion headquarters, said that while they had envisioned a smaller earthquake occurring on the coast of Fukushima or Ibaraki prefectures, they did not expect one so close to land and of such intensity.

"We were predicting an earthquake with a magnitude of 8 or smaller, not one this big," he said, adding that the scale of Friday's quake caused an unusually large series of aftershocks that was expected to last for a while.

Kitagawa said Friday's magnitude-8.8 earthquake was close in size to the 2004 earthquake off Sumatra that generated the huge Indian Ocean tsunami.

Bloomberg News on Saturday reported an Italian geological institute as saying Japan's strongest earthquake probably shifted the Earth's axis by about 10 cm.

The institute said the impact the earthquake had on the Earth's axis was far larger than the impact of the Sumatra quake. The report also quoted experts as saying last year's earthquake in Chile probably shifted the Earth's axis by 7.6 cm.

Oki said that with an average 300 earthquakes happening throughout Japan each day, it is extremely difficult to predict when the next big one might hit.

She also warned that the residents of Tokyo, who experienced the quake at a level of 5.0 on the Japanese intensity scale Friday, shouldn't consider themselves safe and should instead prepare for a large quake striking the metropolis.

Oki said the Tokyo area is prone to two types of earthquakes, one caused by ruptures at plate boundaries, which is the same type that caused the Great Kanto Earthquake of 1923 and which is expected to hit every 200 years or so, or an epicentral earthquake of the Hanshin variety that has a cycle of a few thousand to a few hundred thousand years, making it almost impossible to predict.

The Japan Times