TOKYO, Japan — The massive 9.0-magnitude earthquake that rocked Japan’s northeast coast last year confirmed scientists’ biggest fears.
The researchers had spent more than a decade mapping every major active fault line in Japan following the 1995 Kobe earthquake that killed thousands, in hopes of gaining information that could help predict another “big one.”
The Kanto region, which is home to four major U.S. military bases — Yokosuka Naval Base, Naval Air Facility Atsugi, Camp Zama and Yokota Air Base — sits atop some of those fault lines.
“We have to prepare for a magnitude 7.0 earthquake in the entire Kanto area,” said professor Naoshi Hirata, director of the Earthquake Prediction Research Center at Tokyo University.
Such an earthquake, caused by movement of tectonic plates under Japan, might kill thousands and cause major damage to buildings and infrastructure, Hirata said.
Yokosuka Naval Base sits close to the Miura Hanto faults, which have a 6 percent to 11 percent chance of causing a major earthquake in the next 30 years, Hirata predicted.
However, the most dangerous scenario at Yokosuka is a large earthquake east of Tokyo, he said, which could cause a huge tsunami.
Tokyo Bay, which is wide, is not likely to see a big tsunami, Hirata said. But Yokosuka, which is close to a narrow channel, could be in danger. After the 1923 Kanto earthquake, which sparked fires that killed 100,000, Yokosuka was hit by a 3.4-meter-high tsunami, he said.
“We expect a 5-meter-high tsunami in Yokosuka [in the event of a large off-shore earthquake],” he said. “But there is a possibility that it could be bigger than that. This is what we learned from the Tohoku earthquake [in March]. The tsunamis were much higher than what seismologists estimated.”
Jeff Lindaman, Yokosuka’s emergency management officer, said last year that the base had little in the way of sea walls to protect it from a tsunami.
In the event of a large offshore earthquake, officials at Yokosuka may have only minutes to act, said Lindaman, who monitors the Internet for earthquake and tsunami warnings posted on Japanese websites. If an earthquake strikes, officials will broadcast on AFN television and radio and send emergency services to warn people on the base to move to higher ground.
Ships might be secured with storm lines or, if there is time, sent out of the harbor to avoid damage, he said.
Yokota Air Base in western Tokyo is close to the Tachikawa Fault, which last moved between 20,000 and 13,000 years ago, causing a quake believed to be between 7.3 and 7.4 on the Richter scale. Seismologists estimate it moves every 10,000 to 15,000 years, Hirata said.
“There is up to a 2 percent probability of a major earthquake on the Tachikawa fault in the next 30 years,” he said. “The expected magnitude is the same as the Kobe earthquake and there were 6,000 fatalities in that case.”
Roland Nishimura, chief of architecture for the 374th Engineering Squadron at Yokota, said on-base buildings constructed after 1981 conform to a Japanese national standard and should remain standing, even in a very large earthquake. Older buildings, including some of the large residential tower blocks on the base, may suffer structural damage in such a quake, he said.
Nishimura added that the on-base towers, even the oldest ones, appear capable of withstanding a quake.
“They have massive beams and columns way beyond what we would design in the U.S.,” he said. “When you have large structural elements you can put in more structural reinforcing steel.”
David Yin, Camp Zama director of public works, said on-post buildings withstood the March earthquake and more than 1,000 aftershocks but there’s no guarantee they’d survive a massive earthquake originating closer to the base.
A reassuring factor is that many of the oldest buildings on Army posts in Japan have been replaced with structures built to the latest Japanese earthquake standards, he said.
Perhaps the most likely U.S. base to get hit would be Misawa Air Base in northern Japan.
Hirata said earthquakes as large as 8.0 regularly strike the region. The chance of another 9.0 quake there is low because there have been only four such quakes recorded in the last century, , according to the U.S. Geological Survey.
In other parts of Japan, the risks are even lower. In Sasebo, for example, there is a chance of only about 1 percent of a 7.1 earthquake in the next 30 years. Such a quake would be caused by the movement of inland faults so there would not be a tsunami, Hirata said.
In Okinawa, there are no large faults under the land near U.S. facilities but the island is close to the Okinawa trench.
“There is a 5 percent chance of a large earthquake there in the next 30 years and there could be a tsunami,” he said. “They have had many tsunamis in that area.”