The Cabinet Office has unveiled estimates of horizontal vibrations of tall buildings in Tokyo, Nagoya and Osaka in case they are hit by slow shaking motions from a mega-quake predicted to occur along the Nankai Trough in the Pacific Ocean.

The estimates come from the first study of its kind. So far sufficient attention has not been paid to the effects of this phenomenon, which is known as long-period ground motions. Slow-shaking quake motions of this kind can damage skyscrapers even if they are located far from the epicenter.

Organizations and people concerned — businesses, local governments, hospitals housed in such buildings as well as residents of high-rise condominiums — need to take precautionary measures to minimize possible damage to their property and ensure people's safety.

In estimating how buildings taller than 60 meters in Tokyo, Nagoya and Osaka would vibrate horizontally in a Nankai Trough mega-quake, the government said the top floors of such buildings in coastal areas — including central Tokyo, Nakamura Ward in Nagoya and Kita, Abeno and Minato wards in Osaka — would experience swings of up to 2 to 3 meters.

Those in reclaimed land in Osaka's Suminoe Ward close to Osaka Bay were calculated to sway up to 6 meters.

The softer the ground is, the longer ground motions will continue. It is estimated that some areas in Chiba, Aichi, Mie, Shiga, Osaka, Hyogo and Nara prefectures will experience movement lasting five minutes or longer.

In the March 2011 Great East Japan Earthquake, which devastated coastal areas in the Tohoku region with a massive tsunami, the 256-meter-high, 55-story Sakishima Building of the Osaka Prefectural Government, located some 770 km from the seismic center, swung up to 2.7 meters for roughly 10 minutes, damaging interior material and fire doors at some 360 spots and halting all elevators. There were reports that people in the building felt as if they were aboard a rolling ship. In Tokyo, elevator operations stopped in many high-rise buildings, trapping people inside.

Experts say there is a 70 percent chance that a powerful Nankai Trough quake will hit within the next 30 years. The government's worst-case scenario anticipates a temblor with an intensity of 7 on the Japanese scale of 7, more than 300,000 deaths and ¥220 trillion in economic damage — mainly in Pacific coastal areas close to the seismic center. This scenario is based on estimates of strong, relatively fast-shaking ground motions, along with tsunami. The latest estimate shows that high-rises far from the seismic center can also sustain serious damage from slow-shaking ground motions.

Although the risk of skyscrapers collapsing due to such motions is deemed small, the 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.

In case elevators stop and people are trapped inside, elevator maintenance firms may not be able to fully cope with the situation. People in charge of the management of buildings and firefighters should be trained to rescue people trapped in such circumstances.

Lifelines such as city water and gas supply for high-rise condos could be disrupted. 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.

More importantly, slow-shaking quake motions could damage key elements in high-rises, such as girders, walls and ceilings, making the buildings unlivable or unusable. Owners and users of such existing high-rises should immediately have experts check the buildings. If the buildings are likely to resonate with long-period ground motions, they should install equipment to control the vibrations and mitigate the damage.

Construction of super-tall buildings requires approval from the land and infrastructure minister. An approval system requiring designs that minimize vibrations from slow-shaking quake motions should be developed.

Government officials, lawmakers and businesspeople should keep in mind that Tokyo, Nagoya and Osaka are not just the nation's economic centers but also areas that should serve as command headquarters for operations to help people in other areas that have suffered damage from a major quake. It needs to be ensured that these areas will be able to fulfill such functions. From this viewpoint, construction of skyscrapers in the three megalopolises should be carefully weighed against the risk from slow-shaking quake motions and progress in the defense against damage from such motions. This is important because Tokyo will likely be heavily damaged in the event of a major Nankai Trough quake.