Skyscrapers are seen in downtown Tokyo in this file photo taken in May 2021. (Mainichi/Natsuki Nishi)

       The Mainichi Shimbun answers some common questions readers may have about the long-period ground motion observed during the magnitude 5.9 earthquake that hit Tokyo and surrounding areas on the night of Oct. 7.

       Question: How did the Oct. 7 earthquake affect high-rise buildings?

       Answer: The Japan Meteorological Agency (JMA) said long-period ground motion, which causes the upper floors of high-rise buildings to sway greatly, was observed in northwest Chiba Prefecture and Tokyo's 23 wards during the strong earthquake, whose epicenter was in northwest Chiba Prefecture. Long-period ground motion is divided into four levels of intensity, and on Oct. 7, level 2 intensity -- the second weakest -- was measured.

       The JMA explains that with level 2 intensity, an individual would feel like they need to hold onto something to walk. At level 4, people would not be able to keep standing.

       Q: How is this different from regular earthquake shaking?

       A: It's characterized by the longer duration of time it takes for the earthquake wave to travel back and forth compared to what is seen during a regular quake. In long-period ground motion, the period of the wave ranges from 2 to 10 seconds, and the vibration is more like slow swaying back and forth, rather than intense shaking.

       Each building has a period at which they shake easily, dependent on the size of the structure. When the natural resonance period of the building corresponds with the wave period of an earthquake, it causes sympathetic vibration and the building shakes violently and for a long time. In calculations, the number of the floor times 0.1 is an indicator of the building's periodic property. For example, the 48-story Tokyo Metropolitan Government building in Shinjuku Ward has a period of four to five seconds. As a result, even when an earthquake is low on the Japanese seismic intensity scale, it could cause large vibrations on the upper floors of the building.

       Q: Has long-period ground motion caused major destruction?

       A: Yes. Long-period ground motion tends to be bigger when a strong but shallow earthquake hits. In the 2011 Great East Japan Earthquake, skyscrapers in the Shinjuku area shook back and forth for over 10 minutes, while walls and ceilings of high-rise buildings in the city of Osaka, more than 700 kilometers away from the hypocenter, were damaged. In the 1985 Mexico City earthquake, more than 500 buildings were completely destroyed, causing mass casualties.

       While the Oct. 7 earthquake didn't cause intense long-period ground motion, it's predicted that up to 20,000-plus people would be stuck in elevators in a megaquake feared to occur in the Nankai Trough area off the Pacific side of western Japan. Particularly in the plains where major cities are concentrated, vibrations are amplified. We recommend reviewing at-home quake prevention measures such as having tall pieces of furniture fixed to a wall or other stable objects.

       (Japanese original by Tomohiro Ikeda, Science & Environment News Department)

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