JMA earthquake early warning to cover vibrations affecting tall towers

An earthquake with a remote epicenter and relatively modest seismic intensity could cause such a large and long sway in tall buildings that it would knock people down to higher floors.

Swinging in tall buildings is due to long-range seismic waves that can travel farther than short-range waves based on calculating expected earthquake intensity and issuing “early warnings of earthquakes”.

The Japan Meteorological Agency will start issuing “early earthquake warnings” from February 1, also to areas expected to undergo what is scientifically known as “the ground movement of a prolonged earthquake”, which affects tall buildings.

The term refers to slow and large motions where the Earth takes several seconds or more to move in an alternating cycle of motion. It also tends to swell in plains covered with thick layers of sediment.

“Long-range seismic waves may travel far from the epicenter, so an early earthquake warning can be issued more than 10 seconds before ground motion begins,” said Itsuki Nakabayashi, professor emeritus of urban disaster management at Tokyo Metropolitan University. “This will allow people ample time to take action to protect themselves, making it an effective part of disaster management information.”

Long-range ground motion also causes resonance by synchronizing with the characteristic vibrations of tall buildings, causing significant oscillation in the upper levels of tall buildings.

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

The JMA has so far issued 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 seismic movement of intensity 4 or more.

However, seismic intensity is a measure of movements of a relatively short period, and thus cannot adequately represent the dimensions of vibrations caused by long-range Earth movements.

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

The Great East Japan Earthquake of March 2011 caused a large and prolonged swaying in the upper levels of buildings in the 23 Tokyo wings.

Interior finishing materials, fire doors and elevator ropes were also left for damage in a 55-story building in Osaka, about 700 kilometers from the epicenter, where the earthquake intensity was only 3.

In 2013, JMA released a four-degree long-range ground motion meter.

Under a grade 3 long floor motion, for example, people find it difficult to stay standing, and furniture with movable casters moves wildly. When the movement is grade 4, people can only move by crawling, and most of the unsafe furniture is shifted.

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

In some areas, including the central Tokachi district of Hokkaido, the northern Tsugaru district of Aomori Prefecture, and the Keetsu district of Niigata Prefecture, the 2011 earthquake measured only 4 or less on the seismic intensity scale but a 3 or 4 on the long-term Earth motion scale for a period.

The latest resolution says earthquake early warning will be issued for areas where long-range ground movement of Class 3 or more is expected.

However, when issuing a similar alert, officials will not say whether the release depends on the intensity of the earthquakes or on the degree of long-term ground movement.

“Long-term ground motion causes a big swing and lasts for a while,” Nakabayashi said. “People are advised to take advantage of this opportunity to fully prepare themselves for such an event, including by securing furniture.”

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long range earth motion meter

[Grade 1]Most people feel swayed. Window blinds swing high.

[Grade 2]People feel a large swing, which makes it difficult to walk without holding on to something stable. Furniture with caster wheels moves a little, and things fall off the shelves.

[Grade 3]People find it difficult to stay standing. Furniture with caster wheels is very mobile. Unlocked furniture may move and fall apart. Cracks may form in the partition walls.

[Grade 4]People find it impossible to stay standing or move without crawling. Most unlocked furniture moves and may tip over. Cracks are more likely to form in the dividing walls.

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