An earthquake with a distant epicenter and a relatively modest seismic intensity could still cause such large and lengthy swaying in high-rise buildings that would topple people on the upper floors.

The swaying in tall buildings is caused by long-period seismic waves that can travel farther than short-period waves based on which expected seismic intensity is calculated and “earthquake early warnings” are issued.

The Japan Meteorological Agency will begin issuing “earthquake early warnings,” starting Feb. 1, also to areas expected to undergo what is scientifically known as “long-period earthquake ground motion,” which affects high-rise buildings.

The term refers to slow and large movements where it takes the ground several seconds or more to move in a cycle of reciprocal motion. It also tends to amplify in plains covered with thick layers of sediment.

“Long-period seismic waves may travel far from the epicenter, so an earthquake early warning could be issued more than 10 seconds before the ground motion begins,” said Itsuki Nakabayashi, a professor emeritus of urban disaster management with Tokyo Metropolitan University. “That would allow people enough time to take action to protect themselves, making it an effective piece of disaster-management information.”

Long-period ground motion also causes resonance by coinciding with the characteristic vibrations of tall buildings, causing large swaying in the upper levels of high-rise buildings.

“People will be advised, just like before, to take shelter in a safe place and prepare for shock waves,” a JMA official said. “They are advised to react in the same way they have always done.”

The JMA has so far been issuing earthquake early warnings--in the event of a seismic event expected to measure a maximum of “lower 5” or more on the Japanese seismic intensity scale of 7--to areas expected to undergo motion of a seismic intensity of 4 or more.

The seismic intensity, however, is a measurement of relatively short-period motions and therefore cannot amply represent the dimensions of shakes caused by long-period ground motions.

The hope is to reduce earthquake damage on the upper floors of high-rise buildings, primarily on the 14th or 15th floors and above.

The Great East Japan Earthquake of March 2011 caused large and long-lasting swaying in the upper levels of buildings in Tokyo’s 23 wards.

It also left interior finishing materials, fire doors and elevator ropes damaged in a 55-story building in Osaka, about 700 kilometers from the epicenter, where the seismic intensity was only a 3.

The JMA in 2013 released a long-period ground motion scale of four grades.

Under the long-period ground motion of grade 3, for example, people find it difficult to remain standing, and furniture with caster wheels significantly move about. When the motion is of a grade 4, people can only move by crawling, and most unsecured furniture is displaced.

When this scale is applied, the Great East Japan Earthquake measured up to grade 4 in Tokyo and grade 2 in Osaka.

In some areas, including in the central Tokachi district of Hokkaido, the northern Tsugaru district of Aomori Prefecture and the Kaetsu district of Niigata Prefecture, the 2011 quake measured only 4 or below on the seismic intensity scale but a grade 3 or 4 on the long-period ground motion scale.

The latest decision says an earthquake early warning will be issued to areas with expected long-period ground motion of a grade 3 or more.

When issuing a similar alert, however, officials will not say whether the issuance is based on the seismic intensity or on the long-period ground motion grade.

“Long-period ground motion causes large swaying and lasts for a while,” Nakabayashi said. “People are advised to draw on this opportunity to thoroughly prepare themselves for such an event, including by securing furniture.”

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Long-period ground motion scale

[Grade 1]
Most people feel the swaying. Window blinds significantly sway.

[Grade 2]
People feel large swaying, making it difficult to walk without holding onto something stable. Furniture with caster wheels slightly moves, and objects fall from shelves.

[Grade 3]
People find it difficult to remain standing. Furniture with caster wheels significantly move. Unsecured furniture may move and topple over. Cracks may form in partition walls.

[Grade 4]
People find it impossible to remain standing or move without crawling. Most unsecured furniture moves and may topple over. Cracks are more likely to form in partition walls.