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The Shaky Side of Landslides
21 March 2013 2:10 pm
Giant landslides have a seismic fingerprint that allows researchers to estimate their size, duration, and even how far they travel across the landscape, new research reveals. The finding may be particularly useful in identifying and characterizing landslides that occur in steep, remote areas where few people live—not because of their immediate effects, but because such slumps can block rivers and impound lakes that could later breach the natural dams and threaten populated areas downstream.
Seismometers and other such instruments record ground motions occurring at all frequencies, but seismologists typically pay attention to only those in ranges where the telltale signals of earthquakes reside. The first seismic waves generated by temblors and explosions—those from mining operations as well as underground nuclear tests—are sharp and distinct, says Göran Ekström, a seismologist at the Lamont-Doherty Earth Observatory in Palisades, New York. But the low-frequency waves generated by giant landslides are occasionally hidden in the mix of seismic vibes rumbling through Earth's crust, too, he notes.
Of the 29 largest known landslides worldwide from 1980 through 2012, ground motions from the 27 largest—including the massive slide that triggered the eruption of Mount St. Helens in May 1980, the greatest one yet observed and recorded—were detected by seismic instruments that were part of a global network of instruments. Seismic vibrations produced by the other two slides showed up well on regional networks, Ekström says.
When Ekström and colleague Colin Stark analyzed the seismic data associated with those major landslides, they realized that certain characteristics of the slumps were contained in the ground motions—similar to the way that researchers can use seismic data to estimate the size of a quake and the directions at which the two sides of a fault zone slipped past each other. For instance, when rock falls off a mountainside, the peak is suddenly lighter—so, according to Newton's laws of motion, the mountain springs upward and away from the falling rock, generating initial ground motions that reveal the size of the landslide as well as its direction of travel, Ekström says.
Subsequent ground motions can reveal other aspects of the slump, including how long it lasts, how far it travels, and the type of terrain over which the material is sliding, the researchers report online today in Science. For instance, Ekström says, in several cases major landslides have fallen upon glaciers and then scooted nearly friction-free across several kilometers of ice—which tends to muffle seismic vibrations until the speeding material slams into the opposite side of the valley.
"It's been known for a long time that landslides can generate a seismic signal," says Randall Jibson, a research geologist with the U.S. Geological Survey in Golden, Colorado. "I'm happy that these guys have tackled the problem and analyzed them systematically."
And because seismic data offers clues about how landslides unfold, it may help researchers develop better models of how landslides behave, Ekström and Stark contend. "People rarely see large landslides happen; they typically only see the aftereffects," Ekström notes.
Jibson agrees: "With landslides, what we typically see is a huge deposit. … It's often really challenging to reconstruct what happened."
Indeed, Ekström and Stark's analysis revealed that a set of landslides that fell onto the Siachen Glacier near the India-Pakistan border in September 2010 actually included seven slides that occurred over a period of 4 days. "If we'd only seen this deposit in the field, we'd likely have thought it was formed by one or two landslides, Ekström says.
While some landslides occur in populated areas and are observed directly, many happen in remote, unpopulated regions. Although such landslides fortunately don't affect people immediately, they can have devastating long-term effects, particularly if they block rivers to form large lakes that can suddenly breach the natural dams, flooding populated areas downstream.
"In such a case, there may be only a few days or weeks to assess the hazard and figure out what to do," Jibson says. If researchers have a way to identify such landslides quickly, they can possibly minimize damages and loss of life, he suggests.