A new method for detecting big landslides is allowing scientists to understand the dynamics of these elusive events almost instantly, without traipsing to remote mountains or scrambling up rugged peaks months, or even years, later. In a recent study in the journal Science, Göran Ekström and Colin Stark, geophysicists at Columbia University’s Lamont-Doherty Earth Observatory, have catalogued the 29 largest landslides since 1980 using satellite images and recordings from a global network of seismic instruments. A third of the avalanches are documented now for the first time.
“These are very large geophysical events involving many cubic tons of rock and ice,” said Ekström, the study’s lead author. “In this modern age, it’s surprising that we don’t know more about where and when they occur.”
The biggest slide detailed in the study was triggered by the 1980 eruption of Mount St. Helens and produced the equivalent of a magnitude 5.6 earthquake. The largest landslides to be uncovered by the study are a wave of seven events in 2010 on Siachen Glacier, in the eastern Karakoram range of the Himalayas, which produced the equivalent of magnitude 4.6 to 5.1 earthquakes. The Global Seismographic Network records ground shaking at all frequencies but seismologists usually only pay attention to the sharp, high-frequency signals emitted by earthquakes and explosions. Landslides, by contrast, produce low-frequency vibrations easily overlooked in the noise.
A decade ago, Ekström and his colleagues began looking at these “slow” signals closely in an effort to locate volcano-generated earthquakes. Their search led to an unexpected discovery: low-frequency signals in Greenland, later dubbed “glacial earthquakes,” produced as calving glaciers dump walls of ice into the sea. In 2009, Ekström noticed a similar cluster of signals in Taiwan not long after Typhoon Morakot struck. The signals were later traced to landslides set off by the storm’s torrential rains which wiped out the village of Xiaolin, killing 400 people. “Initially, no other agency had detected or located the four events that we had found, so it seemed very likely that we had detected something special,” Ekström told Smithsonian.
With data from the Xiaolin landslide, Ekström and Stark developed an algorithm to search out the seismic signatures of other big landslides in past and ongoing earthquake recordings. In a companion piece to the study in Science, David Petley, a researcher at Durham University in the U.K., praised the technique. “It opens the way to a true global catalog of rock avalanches that will advance understanding of the dynamics of high mountain areas,” he wrote.
It may have practical benefits, too, providing an early warning system to vulnerable communities. “In Taiwan, it took 48 hours for the authorities to realize a landslide had happened, valuable time that could have helped people to be rescued,” Ekström told SciDev.Net.
Giant Landslides Identified by Seismic Fingerprints, SciDev.Net, March 29, 2013
Landslide “Quakes” Give Clues to the Location and Size of Debris Flows, Smithsonian, March 27, 2013
The Shaky Side of Landslides, Science, March 21, 2013
Landslides Detected from Afar, Science News, March 21, 2013
Listening with Seismology Could Predict Landslides, Live Science, March 21, 2013
Earth Institute news story: Investigating Giant, Unseen Landslides, June 12, 2012