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Large Earthquake "Bounces" Are Stronger Than Gravity

Ker Than
for National Geographic News
October 30, 2008
 
Side-to-side shaking during earthquakes can also be accompanied by up-and-down jolts, which may increase the threat to buildings and other structures, scientists say.

A new study documented unusually strong vertical "bouncing" motions during a magnitude 6.9 earthquake in Japan's Iwate Prefecture in June 2008.

Instruments recorded forces of up-and-down movements reaching up to four times that of Earth's gravity. (See photos of a quake that struck Japan in 2007.)

"Having a vertical acceleration is not unexpected. What's unusual about this is how large it is," said Bill Leith, an earthquake program manager at the U.S. Geological Survey who was not involved in the study.

It's unusual for quakes to have more than the force of Earth's gravity, and records of two times that force are very rare, Leith said.

The study will appear tomorrow in the journal Science.

Twice the Power

The vertical motions were also noteworthy because they packed nearly twice as much energy as the earthquake's sideways shaking.

Study author Shin Aoi, a seismologist at the National Institute for Earth Science and Disaster Prevention in Tsukuba, Japan, noted that sideways shaking is usually twice as strong as vertical movements.

To explain the anomalies, Aoi and his team speculate that a layer of loosely packed soil bounces up and down on a quivering rock layer below it, much like a person jumping on a trampoline.

The vertical earthquake waves detected in the study did not actually cause buildings or loose rocks to bounce up and down, because they were very high frequency waves and thus relatively weak.

Vulnerable Buildings?

If the waves had been low frequency, damage to overlying structures could have been severe, commented Dan O'Connell, a senior geophysicist at the California consulting firm William Lettis & Associates.

Most earthquake-reinforced buildings today are designed to withstand mostly horizontal shaking.

"A large vertical [movement] really changes the equation," said O'Connell, who wrote an accompanying article about the study in Science.

"It could locally compress a building and make it feel a much higher effect of gravity," he said.

This in turn can increase the potential for damage, he added.

(Related: 'Supercities' Vulnerable to Killer Quakes, Expert Warns" [May 2, 2003].)

USGS' O'Connell noted, however, that most building foundations are compacted soil or rock, and that more studies are needed to determine whether such buildings would respond to vertical earthquake waves in the same way as those atop loosely packed soil.

It's not clear that "this is really a motion that is going to be imposed on buildings …," O'Connell said.
 

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