A large-scale and long-lasting earthquake would likely cause skyscrapers in coastal districts of Japan’s three major metropolitan areas to sway violently in the event it originated in the Nankai Trough off Japan’s Pacific coast, a government report said.
According to the report, released Thursday by a Cabinet Office panel looking at the issue, coastal districts with soft ground in metropolitan areas in and around Tokyo, Nagoya and Osaka are particularly vulnerable to such tremors from such a Nankai Trough earthquake.
There is a low risk of buildings collapsing, but people may be injured in falls or trapped in elevators, while ceilings could cave in, the report said. In areas near the earthquake epicenter, tsunami could happen, and the collapse of wooden houses and fires are highly likely to occur simultaneously, it said.
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.
The report is modeled on five previous massive earthquakes, including the magnitude-8.6 Hoei quake in 1707 and the magnitude-8.0 Showa Nankai quake in 1946, as well as a possible largest-scale quake with a magnitude of around 9.0 originating off the Kii Peninsula in western Japan.
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 Suminoe Ward in the city of Osaka, would likely rock back and forth by as much as some 6 meters as a result of a long-period ground motion with a cycle of five seconds.
The degree of swaying is estimated at about 2 meters for a building in Nakamura Ward in the city of Nagoya, and some 2 to 3 meters for a building in any of Tokyo’s 23 wards.
The height of such a building is equivalent to the Tokyo Metropolitan Government building and the Sunshine 60 building in Tokyo, both approximately 240 meters high, as well as the Abeno Harukas skyscraper in Osaka and Landmark Tower in Yokohama, both some 300 meters high.
The panel did not show estimates for each of these buildings and instead requested management companies of at-risk buildings to check the safety of their structures and facilities, and make improvements if necessary.
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. It is followed by at least 5 minutes in the prefectures of Osaka, Hyogo, Nara and Shiga, Aichi, Mie and Chiba, according to the report.
The Cabinet Office is slated to set up a new panel in February to come up with estimates regarding a possible huge earthquake originating from the Sagami Trough, which lies in the Pacific off Kanagawa Prefecture.
The new panel is expected to release its estimates within three years. The magnitude-7.9 Great Kanto Earthquake in 1923 originated in the trough.
Following the latest report from the Cabinet Office panel, the land ministry is looking at a requirement for all future skyscrapers more than 60 meters high to have structures resilient enough to long-period ground motion.
In addition, the ministry will request that existing buildings be checked, though it is on a voluntary basis. It plans to provide financial support for renovations.
Currently, there are about 3,000 skyscrapers over 60 meters high in Japan, with most in Tokyo, Nagoya and Osaka.
When a magnitude-9.0 earthquake struck northeastern Japan on March 11, 2011, a 256-meter, 55-story Osaka Prefectural Government building rocked for about 10 minutes and people were trapped in elevators even though it was 700 km from the epicenter of the quake.