「長周期地震動」東日本大震災ではタワマンも被害　揺れ「半減」“画期的技術”とは？(2026年3月11日)

Fifteen years ago, urban areas far from the epicenter experienced "long-period ground motion," which severely shook high-rise buildings. As the threat of a Nankai Trough earthquake is increasingly recognized, preparations are growing. We take a closer look at the latest technology to reduce shaking by half. ■What is "long-period ground motion"? When the Great East Japan Earthquake occurred, Sendai City, Miyagi Prefecture, recorded a maximum seismic intensity of 6+. A 31-story tower apartment building also suffered significant damage. Mori Haruo, resident on the 25th floor of Lions Tower Sendai Nagamachi "It was a complete mess." "All the cupboards fell over and the glass shattered." The apartment building was earthquake-resistant, but the windows in the room shattered, and the walls of the elevator hall were cracked... Sendai City, located approximately 170 kilometers from the epicenter, was hit by "short-period ground motion," which is characterized by rapid, small shaking. The complex distortion of the entire apartment building caused energy to concentrate on the middle floors, causing damage. Meanwhile, approximately 400 kilometers from the epicenter, central Tokyo was also hit by massive shaking. Skyscrapers can be seen swaying violently from side to side. What was occurring at the time was "long-period ground motion." Long-period ground motion is the slow, long-period, large shaking that occurs during earthquakes. Unlike the shaking observed in Sendai, higher floors shook more strongly and for longer, and it could be felt even far from the epicenter. Professor Yoshiaki Hisada, Faculty of Architecture, Kogakuin University "In the Great East Japan Earthquake in Tokyo, the seismic intensity generally was lower than 5. On higher floors, this amplified to upper 5 or lower 6." The Japan Meteorological Agency classifies long-period ground motion into four levels. At the highest level, "Level 4," it is said to be "unable to stand." Seven long-period ground motions have been observed so far, including the January earthquake in eastern Shimane Prefecture and the Noto Peninsula earthquake two years ago. Long-period ground motion has been observed 157 times overall since 2013. ■What will happen? During Long-Period Ground Motion So, what kind of dangers exist indoors during long-period ground motion? E-Defense, the world's largest disaster prevention testing facility, was granted special permission to visit. The facility houses a 40-meter-tall testing building, capable of recreating all types of earthquake motion using a shaking table. On this day, a full-scale hospital will be recreated to examine the differences in shaking caused by long-period ground motion on lower and higher floors. This time, we will be conducting an experiment using a seismic intensity of 5+. We will start with the lower floors. The mannequin appears to be shaking slightly. Since this is the first floor, no major disturbances were observed inside the room. However, on the higher floors, the room is significantly disturbed. There was a loud bang, and even the lockers collapsed. Clearly different from the shaking on the first floor, beds with unlocked casters were thrown outside with such force. The space where the kitchen was located has collapsed, making it impossible to enter. Medical equipment had also fallen over, and books had been knocked out of the bookshelf in the back. Eiji Sato, Senior Researcher, National Research Institute for Earth Science and Disaster Resilience "The shaking on the first floor was a seismic intensity of 5+. (Higher floors) The shaking was amplified to a seismic intensity of 7 on the seismic intensity scale. (Higher floors) suffered greater damage. It has become clear that there is a high risk." ■ Experience the "Shaking" of Long-Period Ground Motion Announcer Kentaro Izawa actually experiences the shaking of a high-rise building caused by "long-period ground motion." This simulator precisely reproduces all kinds of shaking. This time, it's set up as if you're on the 30th floor of a tower apartment building. Announcer Izawa "Wow! The amplitude of the shaking is getting quite large. It's shaking to the right, left, forward and backward, and my body is shaking quite a bit. Standing on this is definitely impossible. Is this long-period ground motion?" The amplitude of the shaking is clearly different from that on lower floors, and the amplitude of the shaking is greater all at once, making it more dangerous indoors on higher floors. Izawa "I was sweating. The shaking was so intense it made my heart rate shoot up and I was panicking." Norito Teshiba, Disaster Prevention Systems Division, Hakusan Kogyo "(Q: What's the risk of heavy objects falling over?) They're prone to tipping over, and if you place heavy objects on top, they're more likely to fly over rather than fall." ■Can the shaking of high-rise buildings be "halved"? Long-period seismic motion was observed in Tokyo during the ...