The Japan Meteorological Agency (JMA) has added predictions of long-period ground motions, which are slow, large tremors that accompany major earthquakes, to the criteria for issuing earthquake early warnings, and began operating new earthquake disaster prevention information on February 1. Of the four classes of seismic ground motion, areas where 3 or higher, the second from the top, are predicted are targeted. Since the number of high-rise buildings has increased, the aim is to call attention to people on high-rise floors and have them take action to protect themselves.
According to the Japan Meteorological Agency, earthquake early warnings have so far been issued to areas where a seismic intensity of 4 or higher is expected for earthquakes with a maximum seismic intensity of 5 lower or higher. Class standards for long-period ground motion were established in 2013. Even in an earthquake with a seismic intensity of 3 or less, if the long-period seismic motion is 3 or more, it is difficult for people to stand, and there is a danger that unfixed furniture will move or topple over.
In conjunction with the operation of this new earthquake disaster prevention information, it became possible to check the observation results of seismic ground motion for each region on the Japan Meteorological Agency’s website about 10 minutes after an earthquake occurred.
Long-period ground motion is characterized by a long shaking period of about 2 to 20 seconds and is slow, and occurs when a large-scale earthquake occurs. Compared to short-period earthquakes, it travels farther and lasts longer. The closer the period of the shaking of the building to the period of the seismic motion, the easier it is to resonate. Prediction is made by applying the estimated scale of the earthquake, the distance from the epicenter, the ground conditions, etc. to the calculation formula.
This seismic motion became a focus of attention after the 2003 Tokachi-oki earthquake, which caused a fire in an oil tank in Tomakomai, Hokkaido, about 250 kilometers away from the epicenter. When the Great East Japan Earthquake struck in 2011, skyscrapers in Tokyo, about 400 kilometers away from the epicenter, shook violently. A 55-story high-rise building in Osaka, about 700 kilometers away, experienced strong shaking for about 10 minutes, damaging fire doors, ceilings and floors.
According to the Japan Meteorological Agency, looking back up to 2000, before the classification was established, earthquakes thought to be accompanied by long-period ground motion of class 3 or higher since that year were the Niigata Prefecture Chuetsu Earthquake in 2004, the Great East Japan Earthquake in 2011, and the Kumamoto Earthquake in 2016. It is said that it counts a total of 33 times.
Since the Great East Japan Earthquake, the construction of high-rise buildings has increased rapidly, mainly in urban areas. A Nankai Trough megathrust earthquake, which is predicted to occur with a 70% to 80% chance of occurring within the next 30 years, is expected to cause great damage even with long-period seismic motion. In light of this situation, the Ministry of Land, Infrastructure, Transport and Tourism and the Japan Meteorological Agency have begun to operate new earthquake disaster prevention information. The seismic and volcano department of the agency said, “Even if an earthquake early warning based on the class criteria for long-period ground motion is announced, the basic actions to be taken are the same as those in previous early warnings, so please stay calm and protect your own safety.” .