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High-rise condominiums and other buildings are at risk of peculiar patterns of shaking when a major earthquake occurs. Members of the public are encouraged to learn how to protect themselves in daily life.
Powerful earthquakes can generate seismic waves that propagate over great distances and cause long-period, slow-motion shaking. They may bring extreme tremors to lofty buildings for several minutes. Vigilance is required even in the case of an earthquake with a faraway epicenter, because such waves can travel great distances.
This type of shaking, known as “long-period ground motion,” has come into the spotlight in recent years. In the 2011 Great East Japan Earthquake, the top floor of a high-rise building in Osaka City, 700 kilometers from the quake’s epicenter, swayed more than 2 meters from side to side, causing damage to its elevator system and other parts of the structure.
Starting this month, the Japan Meteorological Agency has expanded the scope of its earthquake early warning system to cover wider areas and issue alerts where damage from powerful long-period ground motion is predicted when an earthquake strikes.
The number of high-rise buildings has been increasing, especially in large cities, and each complex can have thousands of people inside at a time. This means that the need for early warnings has been growing.
The system issues early warnings via TVs, smartphones and other devices to areas where a quake is expected to register at least 4 on the Japanese seismic intensity scale of 7. From now on, early warnings can also be issued to areas if they are predicted to be hit by strong long-period ground motion — even if a seismic intensity is expected to stand at just 3 or so.
Apart from the seismic intensity scale, long-period ground motion is divided into four levels of intensity, with “class” used as their unit. However, it is hard to say that this four-level scale is widely known.
When high-rise buildings are shaken by long-period ground motion registered as Class 3, people on upper floors find it difficult to keep standing. In the case of the maximum Class 4, it is hard for them to move without crawling on the floor, and furniture and office equipment begin to move and may topple over if they have not been fixed in place. It is important to be fully aware of such risks.
However, details to be delivered by early warnings will be the same regardless of when they are issued for more familiar short-period waves or long-period ground motion. People are urged to take the same steps to ensure the safety of their own surroundings. If people receive such warnings when they are on upper floors at lofty buildings, it is important to exercise vigilance against long-period ground motion.
In a major earthquake, people may not only be injured due to shaking, but may also be trapped inside elevator cars. If people are alerted while traveling aboard elevators in high-rise buildings, they can take such steps as stopping at the nearest floor to evacuate.
It takes some time before buildings far away from the epicenter of a quake start shaking compared to those in closer areas. It is highly likely that early warnings can be used to mitigate damage in such places.
Some buildings have seismically isolated structures that are less vulnerable to shaking, but these features are not a panacea. It is hoped that high-rise condominium residents on upper floors, as well as building managers, will understand what long-period ground motion is so as to help themselves be better prepared for a disaster.
