In China's mountainous south, geologists think they have discovered a new kind of explosive super landslide that gives no warning and jumps rivers in a single bound.
At least six of what they are calling mass ejection landslides were triggered by the great Wenchuan earthquake of 2008, and are thought to have launched whole mountainsides into the air at more than 160 miles per hour (250 kph), leaping over rivers -- not even stopping to dam them -- and crashing into the mountains on the opposite sides of their valleys.
That is, at least, the interpretation of Hui-Ming Tang and his coauthors, in a study they published in a special edition of the journal Engineering Geology.
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"This type of landslide is characterized by concealed development, sudden break-out, dramatic ejection of landslide material through the air, and devastating consequences," explained Tang, who is a researcher at the Three Gorges Research Center for Geo-hazards at the China University of Geosciences in Wuhan, China.
What makes these events particularly dangerous, Tang explains, is that they happen on slopes that appear to be relatively stable -- under normal circumstances. But the Wenchuan earthquake was far from normal. Such a rare, high magnitude earthquake is the trigger for these otherwise stable slopes to collapse.
By analyzing six of the landslides -- out of the tens of thousands of landslides triggered by the quake -- the researchers came up with four ingredients for the mass ejection landslides. They are: plenty of height for them to fall, a slope of at least 30 degrees, lots of room to maneuver and a very high starting speed (helped by the earthquake).
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"The term 'take-off speed' is exactly the speed when the landslide mass has moved a considerable distance and reaches the 'ski jump,' where the rocks and rubble are torn from the mountainside and become airborne," the researchers explain. That speed, they calculated, is 110 to 160 miles per hour (180 to 250 kph).
The ski jump, they explain, is the bottom of the spoon-shaped depression in the mountainside that is being emptied of rocks in the landslide. It's sort of like the bottom of playground slide, but with rock and debris flying off the through the air.
The very best example the researchers found of a mass ejection landslides was the Jingu landslide in Beichuan County, Shichuan. At Jingu "the huge mass was launched...," they wrote. "After flying several hundred meters, the ejected mass seriously impacted the area across the river, and formed a huge area of debris splash." In other words, it shot through the air and missed the river.
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Not everyone is ready to accept this interpretation of the Jingu landslide, however.
"I am afraid that I am somewhat doubtful of this," observed David Petley in The Landslide Blog. Petley is a landslide geologist and Pro-Vice-Chancellor at the University of East Anglia in the UK.
"The behavior seems too extreme, the velocities so high, and the mechanism so exotic that I think there must be a simpler explanation.... To me a much simpler explanation is that the landslide failed conventionally, descended the lower (steep) slope conventionally, achieving a high velocity. The mass then traveled across the valley with almost no deposition before striking the opposite valley wall and stopping. Thus, it left little material in the river."
It might also help if something similar were found elsewhere in the world, for comparison. However nothing like these landslide has been described anywhere else, according to Petley. "Now, if the paper by Tang et al. is correct, these landslides are truly miraculous."
Note: Independent science writer Larry O'Hanlon is also the blogs manager and social media coordinator for the American Geophysical Union, which hosts The Landslide Blog.