Gravitational force: why large landslides flow like fluid decoded

Vibrations generated by large slides can cause tonnes of rock to flow like a fluid, according to a new study that may explain why some landslides travel much greater distances than expected. The “runout” distance of most landslides the distance debris travels once it reaches flat land tends to be about twice the vertical distance that the slide falls.

If a slide breaks loose a half-mile vertically up a slope, it can be expected to run out about a mile. However, “long-runout” landslides, also known as sturzstroms, are known to travel horizontal distances 10 to 20 times further than they fall, according to Brandon Johnson, an assistant professor at Brown University in US. “There are a few examples where these slides have devastated towns, even when they were located at seemingly safe distances from a mountainside,” said Johnson.

“It has been known for more than a century that very large, dry landslides travel in a fluid-like manner, attaining speeds of more than 100 miles per hour, travelling tens to hundreds of kilometres from their sources and even climbing uphill as they overwhelm surprisingly large areas,” said Jay Melosh, professor at Purdue University in US. “However, the mechanism by which these very dry piles of rock obtained their fluidity was a mystery,” Melosh said

Scientists developed several initial hypotheses. Perhaps the slides were floating on a cushion of air, or perhaps they ran atop a layer of water or ice, which would lower the friction they encountered. However, the fact that these types of landslides also occur on dry, airless bodies like the Moon cast doubt on those hypotheses.

In 1995, Charles Campbell from the University of Southern California created a computer model that was able to replicate the behaviour of long-runout slides using only the dynamic interactions between rocks. However, due to the limitations of computers at the time, he was unable to determine what mechanism was responsible for the behaviour.

For the new study, Johnson was able to resurrect that model and run it on a modern workstation to capture the dynamics in finer detail. The new model showed that vibrations do reduce the effective friction acting on the slide.

The amount of friction acting on a slide depends in part on gravity pulling it downward. The same gravitational force that accelerates the slide as it moves downslope tends to slow it down when it reaches flat land.

However, the model showed that vibrational waves counteract the gravitational force for brief moments. The rocks tend to slide more when the vibration reduces the friction effect of the gravitational force.

Because the vibrational waves affect different rocks in the slide at different times, the entire slide tends to move more like a fluid. The study was published in the Journal of Geophysical Research: Earth Surface.