A report compiled by the Cabinet Office has pointed out the possibility of skyscrapers in some cities in Japan swaying back and forth by up to 6 meters in the event of a massive Nankai Trough earthquake off Japan's Pacific coast.
Considering geographical features and the sheer number of high-rise buildings, this could cause great damage in areas in and around Osaka, Nagoya and Tokyo.
The damage from such swaying came into focus following the Great East Japan Earthquake in March 2011, but there was a lack of past data, and no clear predictions of damage had been available. The latest prediction was restricted to a Nankai Trough quake, but we must also be on the alert for quakes originating in other areas, such as along the Sagami Trough.
Based on this report, we call for awareness across Japan about the danger posed by long-period ground motion, and urge officials not to overlook preventive measures.
Major earthquakes produce not short-period shaking that rattles buildings, but long-period motions with cycles lasting several seconds. When the cycle is between 2 and 10 seconds, it can cause major damage to high-rise buildings, oil tanks and other such structures due to mechanical resonance.
A characteristic of long-period ground motion is that it can affect places far removed from the earthquake's epicenter. During the Great East Japan Earthquake, the Osaka Prefectural Government's Sakishima office, which has 55 floors above the ground and three basement levels, swayed back and forth by up to 2.7 meters for around 10 minutes, causing damage to walls and ceilings. This is in spite of its long distance from the quake's epicenter -- about 770 kilometers.
Of particular cause for concern are high-rise buildings on the Kanto and Osaka plains, which lie on sedimentary layers. In the latest predictions, the highest floors of skyscrapers on reclaimed land in the city of Osaka could sway back and forward by as much as 6 meters, while those in Tokyo's central 23 wards and Nagoya could sway by up to 3 meters. As for areas besides Tokyo and Osaka, people nationwide could be hit by shaking strong enough to prevent them from standing, forcing them to crawl. This is not just a problem restricted to a particular area; many people would be affected.
In considering long-period ground motion, attention must first be placed on the danger of furniture, electrical appliances, office desks and other equipment toppling over or being thrown across the room. These furnishings must be secured so they don't hit people.
There is also a high risk of people being trapped in elevators or becoming stranded on the upper floors of high-rise buildings, and so we must consider countermeasures. Large oil tanks could also end up in flames. Measures are therefore urgently required.
The Cabinet Office report states that there is no danger of buildings collapsing due to long-period ground motion, but the actual earthquake resistance of the nation's many high-rise buildings is not fully known. We must not neglect making risk assessments of aging skyscrapers. The installation of base isolation and seismic force reduction devices to mitigate the risks is also important. The responsibility for renovating buildings lies in principle with the owners, but there is probably a need to set unified design standards. We hope the Ministry of Land, Infrastructure, Transport and Tourism will quickly set such standards.