News Navigator: What is “long-range ground motion” in earthquakes?

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

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

Question: How did the October 7 earthquake affect tall buildings?

Answer: The Japan Meteorological Agency (JMA) said that long-range ground motion, which causes the upper floors of tall buildings to sway greatly, was observed in Chiba Prefecture, northwest of Chiba Prefecture, and the 23 Tokyo wards during the strong earthquake, whose epicenter was in the north. western. Chiba Prefecture. Earth’s long-range motion is divided into four levels of intensity, and on October 7, intensity level 2 – the second weakest – was measured.

The JMA states that with level 2 intensity, an individual may feel the need to hold on to something to walk. At level 4, people will not be able to continue standing.

Q: How is this different from a normal earthquake?

A: It has a longer duration of time for an earthquake wave to travel back and forth than is seen during an ordinary earthquake. In the long-term motion of the Earth, the wave period is 2-10 seconds, and the vibration is more like a slow swing back and forth, rather than an intense one.

Each building has a period in which it vibrates easily, depending on the size of the structure. When the period of the building’s natural resonance corresponds to the period of the earthquake wave, it causes sympathetic vibration and the building vibrates strongly and for a long time. In the calculations, the number of floors multiplied by 0.1 is an indicator of the periodic characteristic of the building. For example, the duration of the 48-storey Tokyo Metropolitan Government Building in Shinjuku is 4-5 seconds. As a result, even when an earthquake is low on the Japanese seismic intensity scale, it can cause significant vibrations on the upper floors of the building.

Q: Did the long-term ground movement cause great damage?

A: Yes. Long-range ground motion tends to be greater when a strong but shallow earthquake strikes. In the 2011 Great East Japan Earthquake, skyscrapers in Shinjuku shook back and forth for more than 10 minutes, while walls and roofs of tall buildings were damaged in Osaka, more than 700 kilometers from the hypocenter. In the 1985 Mexico City earthquake, more than 500 buildings were completely destroyed, causing mass casualties.

While the October 7 earthquake did not cause intense, prolonged ground movement, it is expected that upwards of 20,000 people will be stuck in lifts in a massive earthquake feared in the Nankai Basin region off the Pacific side of western Japan. The vibrations are amplified especially in the plains where major cities are concentrated. We recommend reviewing earthquake prevention measures in the home such as fixing tall pieces of furniture to the wall or other fixed objects.

(Japanese original by Tomohiro Ikeda, Science and Environment News Division)

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