TOKYO – The earthquakes that rocked the Chuetsu region of Japan and surrounding areas Saturday took place in a “seismic gap” – an area many experts had warned could be subject to powerful tremors in the near future.

A seismic gap is defined as the part of an active fault that has experienced little or no seismic activity for a long period, with the build-up of seismic stresses in these areas making an earthquake more likely, according to experts.

Saturday’s temblors, named by the government the Niigata Prefecture Chuetsu Earthquake, are thought to have been caused by a “thrust fault” or “reverse fault,” in which ground on one side moves up and over adjacent ground. This is in contrast to “normal faults,” in which the ground on one side of the fault slides along relative to the other.

As was the case with the 1995 Great Hanshin Earthquake and the Fukui Earthquake of 1948, temblors due to thrust faults are classified as “near-field” earthquakes or “shallow-focus” ones in which the foci are relatively close to the surface compared with earthquakes caused by normal faults, such as those in California.

According to the government’s Geographical Survey Institute, the thrust fault in the Niigata Prefecture Chuetsu Earthquake had been unknown before the quakes and is separate from a group of faults that have been confirmed to run from north to south about 20 kilometers off the Niigata coast.

The institute confirmed Sunday that the first of the series of earthquakes on Saturday, a tremor of a magnitude of 6.8 on the Richter scale that struck at 5:56 p.m., was caused by the previously unknown fault in the Chuetsu region – the central part of Niigata Prefecture.

Analyses of Global Positioning System – data obtained by the institute indicated the initial earthquake took place when the fault caused land on one side to rise up about 1.5 yards.

There was almost no lateral movement along the fault, meaning Saturday’s first earthquake was a typical thrust fault-type quake, institute researchers said.

In the Great Hanshin Earthquake, also classified as a thrust fault-type tremor, there was a maximum 6.3 feet of lateral movement along either side of the fault in addition to a vertical movement of up to 3.3 feet, giving rise to a 5.5 mile-long fissure in the ground on Awajishima island, the researchers said.

The Niigata Prefecture-centered quakes came 11 days after the prediction by the government’s Earthquake Research Committee that the area could be hit by a major earthquake due to the faults off the Niigata coast “with a probability of 2 percent over the coming 30 years.”

The earthquakes Saturday were linked to the previously unknown fault, however.

The seismological experts also pointed out that the Niigata Prefecture Chuetsu Earthquake took place in an area called “seismic gap D”-one of several areas where no major earthquakes have occurred for a long period in spite of evidence that earthquake-causing stresses were building up.

According to studies by Tokyo University’s Earthquake Research Institute and others, there are at least four such earthquake-prone areas from Hokkaido to Toyama Prefecture.

In the area near the epicenter of Saturday’s earthquakes, there have been few big tremors, with the last one in which human fatalities were registered almost 180 years ago in December 1828.

Judging from historic records, that temblor, called the Echigo Earthquake, is believed to have measured M6.9, and left more than 1,400 people dead or unaccounted for and destroyed about 11,000 houses and other buildings.

A quake with a magnitude of more than 6 took place in October 1933 within 20 kilometers of the foci of Saturday’s tremors, but there were no reports of casualties.

Commenting on the Niigata Prefecture Chuetsu Earthquake, Katsumasa Abe, professor of Tokyo University’s Earthquake Research Institute, said there was no doubt it was caused by the previously unknown fault, which lies northwest of a fault that had been forecast to cause an M8-class earthquake.

Given that the foci of the tremors were shallow and located inland, it was normal to see large aftershocks, with more such shocks a real possibility, he said.

Masakazu Otake, president of the GSI’s Coordinating Committee for Earthquake Prediction, said he had been anticipating that a major earthquake with a maximum magnitude of 7.5 would rock the Chuetsu region in gap D – the area from Niigata Prefecture to the northern part of Nagano Prefecture where crustal stresses were known to be building up.

“The earthquakes Saturday should be considered to have fallen short of releasing all earthquake-causing stresses,” Otake said.

“Since only a small portion of the energy is believed to have been released this time, the public should be alert to the high possibility of a rash of aftershocks,” he noted.

(c) 2004, The Yomiuri Shimbun.

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Distributed by Knight Ridder/Tribune Information Services.

AP-NY-10-24-04 1642EDT

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