Vibrations make large rocky landslides flow like water

by Mary Caperton Morton
Wednesday, July 20, 2016

There is a rule of thumb in geology for how far a landslide will run out: Most landslides travel roughly twice the vertical fall distance from where they fall off their parent slope. But certain types of dry landslides, called sturzstroms, can travel 20 to 30 times farther, without water or mud to lubricate the flow. Scientists have long hypothesized about exactly how this occurs, and new computer models seem to back up their hypotheses: that vibrations generated by dry rocky landslides can cause the slides to flow like a fluid and spread out across surprisingly large areas.

“Sturzstroms seem to be able to happen just about anywhere you have large amounts of intact rock and a high degree of topography,” says Brandon Johnson, a geophysicist at Brown University and lead author of a new study detailing the model results, published in the Journal of Geophysical Research: Earth Surface. “Instead of the slopes undergoing a number of smaller landslides over time, they experience catastrophic slides.” Debris fields from sturzstroms have been observed in a number of geologic settings on Earth, as well as on the moon, Mars, Venus and several other moons in the solar system. But, until now, computer models have not been able to adequately model the complex forces involved in the large, chaotic slides.

In the mid-1990s, when co-author Charles Campbell, a geologist at the University of Southern California, first began running models of dry landslide fluidization theories, “they took over a year of computing time,” Johnson says. With modern computing power, however, Johnson is now able to run his models in a matter of days. They’ve shown that vibrations produced from such large volumes of rock can indeed reduce the friction acting on the slide by counteracting the effects of gravity as the rocks bounce and slide, causing the entire slide to move like a fluid.

“This is what we call an ‘emergent phenomenon,'” Johnson says. “It’s something that you don’t see if you’re only looking at a few particles, but when you get enough particles together, they start to act strangely.” The finding aligns with the acoustic fluidization hypothesis, first proposed in 1979 by co-author Jay Melosh at Purdue University, who suggested that vibrations might reduce the effects of friction on moving rocks.

The model seems to be a good fit for explaining sturzstroms observed on other planetary bodies, says Kelsi Singer, a geophysicist at Southwest Research Institute in Boulder, Colo., who was not involved in the new research. For example, Saturn’s moon Iapetus is home to the longest runout landslides in the solar system, extending for 80 kilometers from the parent slope.

“This is a good working hypothesis for explaining these dry runout slides, which should be extremely frictional, but for some reason overcome that friction and keep sliding for incredible distances,” Singer says. The findings may also have some applications to other moving rock phenomena, such as earthquakes and fault movements where the faults shift farther than would be expected given the friction in the system, Singer says.

More work will be needed before scientists can begin to predict where such slides might occur in the future, Johnson says. “The model tells us all you need is a big enough volume of rock to slide to generate these vibrations and cause a long runout. So in theory, they could happen anywhere you have enough rock,” Johnson says. The next step will be to model such slides in three dimensions, he adds. “This model is rather simplified but we’re working on adding in more details, working our way up to the next level of complexity.”

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