A magnitude 5.9 earthquake that struck in the early hours of August 23 triggered Level 2 on Japan’s “long-period ground motion” scale, which indicates the degree of shaking in high-rise buildings, across Tokyo’s 23 wards, northwestern Chiba Prefecture and southern Saitama Prefecture.
The shaking was strong enough that people needed to hold onto something to walk, prompting experts to renew calls for vigilance and disaster preparedness, particularly in the high-rise-dense capital region.
Tremors Have Traveled Over 700 Kilometers
Long-period ground motion refers to the slow, large oscillations generated by major earthquakes, with each back-and-forth cycle lasting roughly two to eight seconds. The higher a building’s floor, the greater the sway, raising the risk of furniture toppling over or elevators breaking down.
Because seismic intensity alone cannot capture how strongly high-rise buildings will shake, the Japan Meteorological Agency (JMA) uses a separate “long-period ground motion class” scale.
The agency defines Class 4, the most dangerous level, as conditions in which “people cannot move without crawling,” Class 3 as conditions in which “it becomes difficult to remain standing,” and Class 1 as conditions in which “many people feel the shaking.”
Since 2023, the JMA has also issued early warnings when Class 3 or higher motion is forecast.
Long-period ground motion is also known to travel unusually long distances.
During the Great East Japan Earthquake in 2011, the 55-story Osaka Prefectural Government Sakishima Building, located over 700 kilometers from the epicenter off the coast of Miyagi Prefecture, shook for roughly 10 minutes due to long-period ground motion, damaging ceilings and fire doors and halting elevators.
Expert Warns of Amplified Shaking in Kanto
In the August 23 earthquake, elevators at the Adachi Ward office in Tokyo were among the facilities that stopped functioning.
Nobuo Fukuwa, professor emeritus of earthquake engineering at Nagoya University, pointed out, “The Kanto Plain has a thick sedimentary layer underground, which makes shaking prone to amplification.”
He noted that particular caution is needed in the capital region, where high-rise buildings are concentrated.
When shaking is felt, people are advised to move away from glass and take shelter in areas with few objects, such as hallways. Those in elevators should press the buttons for every floor and exit as soon as possible.
In office buildings and high-rise condominiums, strong shaking can cause furniture to shift and even break through window glass, creating a risk of objects falling outside.
“In addition to routine measures such as securing furniture, it’s necessary to decide in advance the order in which each floor should evacuate, to avoid congestion,” Fukuwa said.
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(Read the article in Japanese.)
Author: Momoka Nagare, The Sankei Shimbun
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