Washington, Oct 31: A new study that documented unusually strong vertical "bouncing" motions during a magnitude 6.9 earthquake in Japan in June 2008, which was four times stronger than Earth''s gravity.

According to a report in National Geographic News, the study suggests that 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.

"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.

"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," he added.

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.

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 and Associates.

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

"A large vertical movement really changes the equation," said O''Connell. "It could locally compress a building and make it feel a much higher effect of gravity," he added.

"This in turn can increase the potential for damage," he further added. (ANI)