Sichuan Province China May 12 2008

The devastating magnitude 7.9 earthquake in the Longmenshan ranges of Sichuan Province, China, triggered thousands of landslides, including several gigantic landslides that buried several villages and blocked the Qingshui and Hongshi Rivers, forming two lakes, in Donghekou and Shibangou. Two of the first scientists to reach and study the Donghekou landslide after the earthquake were the British geologist Jian Guo Liu and the American geologist T. M. Kusky, who estimated the volume of material that collapsed and buried local villages. They mapped the thickness of the toe of the deposit, the length of the slide (>2 miles, or 3 km) and width (average >2,500 feet or 1 km), and estimated that at least 20-30 million cubic yards (15-25 million m3) of material raced down the slopes in this single landslide. The staggering reality is that during the earthquake, there were hundreds, if not thousands, of similar giant rock avalanches. The landslides and mass wasting were responsible for a large portion of the known 87,587 deaths from this earthquake.

A mountain on the outskirts of the buried village of Donghekou collapsed, and a large mass of earth slipped downhill at such a high speed it jumped a small mountain, plunged across a river, and then ran a thousand feet (300 m) up a mountain slope across from the village. The top of the mountain has many huge displaced blocks that slid away from the headwall scarp, suggesting that the event started as a massive rock slide/fall, and as the mass moved downhill, it quickly became a debris avalanche as the material in the fall was pulverized into a mixture of large boulders and finer material. Debris from the avalanche formed a natural dam in the river, blocking it and forming an avalanche lake. The scale of this massive landslide is impressive, covering several square miles (km2) of the surface, and being up to several hundred feet (hundred or more m) thick. The Donghekou landslide is about 30 percent larger than the giant landslide/debris avalanche that buried the community of Barangay Guinsaugon in Leyte, Philippines, on February 17, 2006, killing 1,450 people.

Massive rockfall avalanches and rockflow rubble streams can travel at high speeds over large distances, particularly where a minimum of 650,000 cubic yards (500,000 m3) of rock falls at least 500 feet (150 m) onto a slope equal to or exceeding 25°. These landslides are able to travel so far because they must conserve mass and momentum. Mapping of the Donghekou landslide by geologists Liu and Kusky suggests that the momentum was maintained by a loss of friction along the base of the landslide, by having the falling mass ride on a cushion of compressed air. The Donghekou and Shibangou landslide avalanches were associated with air blasts that shot out from beneath the falling debris. Rapidly falling rock and soil compressed the air beneath the avalanches and allowed the material to move downhill quickly on a cushion of compressed air for great distances, and as the avalanches came to a halt the air was ejected from beneath the debris in strong jets that knocked down trees in radial patterns reaching a thousand feet (several hundred m) up opposing mountain slopes from the site of the avalanche tongues, in patterns reminiscent of bomb blasts.

The dam blocking the course of Qingshui River was channelled by the army using explosives to allow water to discharge gently, avoiding catastrophic dam collapse and flooding. The devastating consequence of landslides in the region is obvious, with several villages and schools buried by this slide. The landslide in Shibangou also buried a village and blocked the Qingshui River a second time, forming another lake upstream. The landslide at Shibangou is interesting in that the amphitheatre-shaped headscarp detachment surface at the top of the slide is visible, and it shows a typical slumplike crown with transverse cracks and fault zones, large slide blocks moving into the main slide, back rotation of slump blocks on the upper part of the slide, then a transition where the slide turns into an avalanche and eventually a debris avalanche grading into a debris field. This slide was also associated with powerful airblasts, as the trees are blown over in complex patterns going uphill and down valley away from the toe of the avalanche.

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