Earthquake's trampoline effect

Published 31 October 2008

During earthquakes the ground not only shakes from side to side, but also bounces up and down; this has important implications for designing quake-proof structures

Earthquakes shake the ground from side to side, but they also cause the ground to bounce up and down. That the ground ground moves up and down during a quake should not be a surprise, but during the magnitude 6.9 Iwate-Miyagi earthquake that struck Japan on 14 June, the ground accelerated upward at nearly 4 g — an unprecedented level.

New Scientist’s Emma Young writes that this acceleration was twice that of the horizontal movements, which normally account for most of the ground shaking in a quake. Evidence of the violent jolt was found in data from the quake by a team led by Shin Aoi at the National Research Institute for Earth Science and Disaster Prevention in Tsukuba, Japan. The team also found that the ground was pushed upward much more powerfully than it fell back down, a phenomenon which cannot be explained by existing ground motion models. The researchers suggest, therefore, that during the upward motion the compressed soil behaved elastically, but on the way down the particles separated and fell back relatively slowly. This is similar to a person bouncing on a trampoline: the upward force of the trampoline is larger than the downward force of gravity.

The team also analyzed data from about 6,800 other Japanese earthquakes and found another two with unusually strong upward movements. “We think this occurs only under conditions of very large shaking,” says Aoi. “But we can’t judge whether it is common or not under these conditions because we don’t have enough records.”

The research team does not know what ground conditions are required to produce the phenomenon, but in principle it could happen in other countries as well as Japan, says Aoi. A full explanation would help with earthquake hazard assessment studies, he adds.

The team’s finding have important implications for the design of buildings. Most quake-proof structures are designed to withstand only horizontal shaking, but strong vertical shaking could also damage buildings, says Daniel O’Connell, a geophysicist at consulting firm William Lettis & Associates in Golden, Colorado, who wrote a commentary on the research. Foundations for buildings compact soil, however, which might prevent the “trampoline effect.” A larger-scale deployment of motion sensors at the foundation level of buildings is needed to find out, O’Connell says.

-read more in Daniel R. H. O’Connell, “Assessing Ground Shaking,” Science 322, no. 5902 (31 October 2008): 686-87 (DOI: 10.1126/science.1166149) (sub. req.)