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Skyscrapers vulnerable to mega-quake

Although the risk of skyscrapers collapsing due to such motions is deemed small, the government estimate shows that the top floors of such buildings in Japan’s three megalopolises could move 2 to 6 meters in each swing. Such motions could topple shelves and move furniture and office machines violently, and people would find it almost impossible to walk. People may be injured in falls, hit by swaying household items or trapped in elevators, while ceilings could cave in, the report said.

In case elevators stop and people are trapped inside, elevator maintenance firms may not be able to fully cope with the situation. The panel called on building management companies and tenants to take countermeasures based on the assumption that emergency rescue and firefighting forces would not be available.

In addition to fastening their furniture to walls and floors, residents should prepare emergency food and water to last more than a week. Measures to help elderly residents who live on higher floors should be devised in advance.

It also estimates the degree of swaying that would occur in skyscrapers taller than 60 meters from long-period ground motion with a cycle between two and 10 seconds. The top story of a building 200 to 300 meters high, located on reclaimed land in the city of Osaka, would likely rock back and forth by as much as some 6 meters with a cycle of five seconds. Currently, there are about 3,000 skyscrapers over 60 meters high in Japan, with most in Tokyo, Nagoya and Osaka.

The duration of long-period ground motion on land in the event of the largest-scale earthquake is estimated to be the most protracted, at 6.67 minutes or more in some coastal areas in the city of Osaka and the nearby city of Kobe.