Some landslides run greater distances than expected

Some landslides travel much greater distances than scientists would normally expect. Now a team of researchers has come out with an explanation for this phenomenon using a sophisticated computer model.

A team of geoscientists from Brown University, Purdue University and the University of Southern California in the US has found that vibrations generated by large slides can cause tonnes of rock to flow like a fluid, enabling the rocks to rumble across vast distances.

According to the study's lead author Brandon Johnson, an assistant professor at Brown, 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.

So if a slide breaks loose a half-mile vertically up a slope, it can be expected to run out about a mile.

But "long-runout" landslides, also known as sturzstroms, are known to travel horizontal distances 10-20 times further than they fall.

"There are a few examples where these slides have devastated towns, even when they were located at seemingly safe distances from a mountainside," Johnson said.

Scientists developed several hypotheses to explain long-runout slides. But none could convincingly explain their behaviour.

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.

"The model showed that there was something about rocks, when you get a lot of them together, that causes them to slide out further than you expect," Johnson said. "But it didn't tell us what was actually happening to give us this lower friction."

For this new study, described in the Journal of Geophysical Research: Earth Surface, Johnson was able to resurrect that model, tweak it a bit, and run it on a modern workstation to capture the dynamics in finer detail.

The new model showed that, indeed, 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. But 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.