Earthquake panel predicts 6 meters of swaying in high-rises if big one hits

December 18, 2015

By ITSUHIRO SUZUKI/ Staff Writer

Skyscrapers in Tokyo, Nagoya and Osaka could sway up to 6 meters during a major earthquake originating in the Nankai Trough off the Pacific coast of Honshu, a government panel said Dec. 17.

The Cabinet Office added that the swaying could last as long as 10 minutes. However, it said high-rise buildings would be unlikely to collapse, even if suffering damage to beams and other support structures.

Still, the Cabinet Office urged management and residents of skyscrapers to take necessary precautions, including steps to keep furniture from falling.

It was the first time the panel at the Cabinet Office has released an estimated impact of long-period ground motion on the top floors of high-rise buildings experiencing a major earthquake.

According to the government, there is a 70 percent probability that a megaquake originating from the Nankai Trough will hit the Pacific coast within the next 30 years.

The panel examined the impact on 100- to 300-meter high buildings that are not equipped with measures to control vibrations.

Long-period ground motion is defined as having a swaying motion that lasts more than two seconds.

The rocking effect tends to be caused by a temblor with a magnitude 7.0 or higher with a hypocenter that is not deep beneath the surface.

Such an earthquake would affect stories 60 meters from the ground, the height of a typical 20-story building.

Although the swaying would be slow, skyscrapers far from the earthquake's epicenter could be affected.

According to the panel, the swaying of up to 6 meters was predicted for buildings measuring 200 to 300 meters high situated on reclaimed land in Osaka's Suminoe Ward.

As for structures of similar height in Tokyo's 23 wards, swaying was predicted at a maximum of 2 to 3 meters.

In Nagoya, buildings standing 100 to 200 meters high in Nakamura Ward would likely sway up to 3 meters.

The calculations were made after the panel of experts analyzed five megaquakes that occurred along the Nankai Trough over the past 300 years, taking into account that one surpassing them is expected in the future.

The panel assessed the impact of long-period ground motion with a rocking motion that occurs every two to 10 seconds on skyscrapers and their interiors.

The duration of ground vibrations was estimated at more than six minutes and 40 seconds in some parts of the coastal areas of Osaka and Kobe, according to the panel.

A swaying of more than five minutes was predicted for some areas in Chiba, Aichi, Osaka and other prefectures.

The panel concluded that high-rises have "certain extra structural strengths that will spare them from collapse," despite damage to their structural supports when the buildings experience violent swaying created from the resonance of a quake's motion combined with the structure's own subsequent movements.

But the Cabinet Office called for exercising precautions, saying that the actual impact could go beyond the panel's estimates.

"Swaying could last more than 10 minutes in structures that are not equipped with vibration-control measures," it said. "People should be aware that the actual swaying, and how long it lasts, could vary from the panel's estimate, depending on the strength of the ground beneath the buildings and the strength of their structures."

According to the land ministry, there are an estimated 3,000 buildings nationwide that are at least 60 meters high, most of them on shaky ground in Tokyo, Nagoya and Osaka.

When the magnitude-9.0 Great East Japan Earthquake hit northeastern Japan in 2011, high-rises as far as Osaka experienced swaying.

http://ajw.asahi.com/article/behind_news/social_affairs/AJ201512180063