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Last month an offshore magnitude 6.7 earthquake rocked Japan’s northernmost island of Hokkaido, jolting Misawa Air Base residents more than 200 miles away out of bed shortly before dawn.

And in recent weeks, two moderate temblors have shaken base housing units and office buildings across the Kanto Plain.

Like sushi and spring cherry blossoms, earthquakes are a way of life in Japan, one of the world’s most temblor-prone regions.

In fact, during October, more than 55 earthquakes of magnitude 4.0 or larger were recorded.

For some, Mother Nature’s violent tremors are exhilarating.

“It just feels like you’re on the ride Tilt-A-Whirl,” said Jamie Redrick, 22, the spouse of a Yokota Air Base airman.

Tim McKitrick compares it to rafting a rocky river.

“When you’re lying on the couch, it feels like you’re on a wave,” the Yokota Air Base resident said. “It hasn’t been spooky or anything like that.”

Not so for Camp Zama spouse Cindy Potter, who says she was frightened when a recent temblor rattled her Sagamihara home.

“I was jumping up and down, telling the kids to get under the table or in the door frame,” she said. “My husband was just sitting back like it was no big deal.”

When the earth buckles and lampshades sway, how safe are base residents?

Building experts say military housing and offices, designed to strict Japanese seismic standards, are very safe.

“They build very robust structures,” said structural engineer Bruce Riley of Japan Engineer District, U.S. Army Corps of Engineers, Camp Zama.

The Japanese government builds most base housing under the facilities improvement program, Riley said.

“By agreement, we allow them to build to the Japanese seismic code,” he said.

Anything constructed after 1981 meets Japan’s current building standards, adopted that year when seismic construction codes for concrete-frame buildings were overhauled, and limited changes were enacted for steel-frame structures.

Under current standards, concrete buildings require reinforcing bars every 4 inches, said Keiichi Moriya, chief of Disaster Prevention Section, Housing Bureau of the Land, Infrastructure and Transportation Ministry. Prior to 1981, rebar was required every 6 to 8 inches.

“Just because the standards have changed, doesn’t mean the [older] facilities we have are unsafe,” said Army Lt. Col. Dennis Polaski, director of Directorate of Public Works, U.S. Army Garrison-Japan, Camp Zama.

There was no specific seismic intensity level that the old building codes imposed, according to an official at the Okinawa prefectural government’s Construction Department.

“However, the old building codes required all buildings to withstand a medium level earthquake,” he said.

He then said that the medium level could be interpreted as Level 5 or so.

Under the new codes, buildings are safe without damages when hit by a large-scale earthquake measured at between 5 upper and 6 lower (on the Japanese 10-point seismic intensity scale), Moriya said. “And basically, all the buildings are designed to withstand a quake up to 7.”

The Japanese seismic intensity scale measures the direct impact tremors have on the ground — what people actually feel, he said. The magnitude, or Richter scale, measures energy released at the epicenter.

While no formula converts the Japanese scale to Richter — the measurement used by the United States — Riley said U.S. military buildings on the Kanto Plain built after 1981 were erected based on a 7.0 magnitude baseline.

That means newer buildings, including tower apartments, should not sustain major damage from a quake of magnitude 7.0 or lower, he said.

Beyond that, a building may be damaged beyond repair, but it’s designed not to collapse, regardless of quake magnitude, Riley said.

“I can’t say it would never happen, but it’s designed not to,” he said.

Residents of the nine-story high rises built after 1981 say that although their buildings are supposed to be safe, they can feel their buildings move during earthquakes.

“It feels like a short rumble, then like swinging back and forth,” said Ken Weir, a civilian at Yokota Air Base who lives on the ninth floor of a west-side tower.

A recent earthquake tipped over a few dishes in Weir’s china cabinet, but nothing broke.

“The building seems to sway with the movement of the ground very well,” Weir said.

Despite the rocking, it’s not riskier to live on a higher floor, Moriya said.

As long as a building meets code, the seismic-resistance level is the same on each level, he said. The only advantage of being on the ground floor is that it would be much easier to evacuate.

The towers are typically built on concrete piles (similar to telephone poles) to carry the building load down to firm rock, Riley said. Most also have shear, reinforced concrete walls, which are stronger than partitions or dry walls.

The Great Hanshin Earthquake of 1995, however, proved that modern Japan was not immune to earthquake catastrophes.

The 7.3 Kobe quake killed 6,000 people and destroyed 100,000 houses.

But most damages in the port city erected on soft ground were to buildings erected under older standards, Moriya said.

An EQE International engineering study of structural damage from the Kobe earthquake found that pre-1981 concrete-frame buildings performed very poorly, with many collapses.

“Post-1981 buildings performed much better,” the study said. “Some were extensively damaged, but most had light damage.”

In Japan, about 45 percent of the country’s 3 million public buildings were constructed before 1981, Moriya said. Only about 4 percent of privately owned buildings meet current standards.

With most bases dating back to at least World War II, many older buildings from single-family homes to warehouses still stand.

Few bases, still have towers constructed before 1981.

Of the 20 towers at Yokota Air Base six were built in the mid-1970s, according to Judith Ware, deputy housing chief. The rest were erected between 1982 and 1999.

Camp Zama’s two high-rises were built in the 1970s, said Maj. John Amberg, U.S. Army Japan spokesman. Fifty-one buildings on post were built pre-1981; the oldest houses, dating back to 1951, are two-story dwellings.

“Building safety is a continual process,” Amberg said. “Regular efforts are made to improve projected and existing construction with the best possible earthquake standards.”

The oldest housing facilities at Misawa Air Base are 18, one-story and two-story units built in 1948; the oldest tower was built in 1988, 35th Civil Engineer Squadron officials said.

Some of the oldest housing is at Sasebo Naval Base, which has seven, single-family dwellings built by the Japanese Navy in 1912, base officials said.

And at Iwakuni Marine Corps Air Station, the oldest housing units are Quarters 501 and 504, constructed prior to 1947, base officials said. The oldest tower was built in 1985.

New standards are not only designed to make a building safer, but to lessen the impact of an earthquake, he said.

In fact, the Army recently conducted a seismic study of some older U.S. Army Japan buildings and found they would hold up in a “moderate” earthquake, though temblor intensity was not defined.

An analysis of Hardy Barracks, a 1964 Army lodge and residence hall in Roppongi, concluded the building had no seismic design, Riley said.

But, “in any given earthquake, people wouldn’t be subjected to collapse. It was a cursory examination of the way it was built, figuring a moderate earthquake based on historical” data, he said.

The 1952 U.S. Army Japan headquarters at Camp Zama also were evaluated.

The study determined it needed “a little bit of strengthening” so the building could remain operative in the event of a moderate earthquake, Riley said, adding, “It should survive a pretty good earthquake.”

“We get earthquakes all of the time, and we don’t really see a lot of damage,” he said.

But if the “big one” hits, would older structures remain standing?

“Although the old building standards also required an earthquake-resistant structure, those buildings would not be strong enough to withstand a major earthquake, compared to those built under the new standards,” Moriya said.

Regardless, residents say they feel safe from earthquakes in base housing and at work.

“Even though we are subjected to these earthquakes on a regular basis, the buildings don’t show any evidence of weakness,” said Command Master Chief Mike McCarthy of Misawa Naval Air Facility.

“Everyone’s aware of it, and everyone takes care of each other,” he said.

Bill Turnbull, Navy Misawa safety manager, said most buildings at Misawa are new, so “we’re in very good shape.”

The key to surviving a violent earthquake is knowing how to react.

“The best thing to do is to stay in the building until the earthquake is over,” he said, adding experts recommend sitting on the floor against a wall and away from windows.

Back at Yokota, McKitrick doesn’t worry about the “big one.” But he also doesn’t think people will be prepared if it does strike.

“We haven’t seen the big one. We haven’t even come close to the big one,” he said. “How can you picture how you’re going to prepare for that?”

— Chiyomi Sumida and Jim O’Donnell contributed to this story.

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