from China Journal
The Science Behind the Continuing Quake Risks Chinese officials have said that the dams in Sichuan (both man-made and earthquake-created) are safe, though they acknowledge the risks of heavy rains and aftershocks in the coming days, and earlier this week a big landslide in the region killed 158 relief workers. One person who's been following the post-earthquake damage closely is David Petley, professor of geography at Durham University in the U.K. and author of Dave's Landslide Blog. We caught up with Prof. Petley to pose a few quake-related questions. In a recent blog post, you noted five major threats associated with landslides in the aftermath of the Sichuan earthquake. Which of these do you think poses the biggest risk and why?
In my view the biggest short-term threat is that of a dam break flood because these events have the potential to cause huge amounts of destruction for tens of kilometers. Assuming that this can be managed, then the next big threat is landslides induced by rainfall. We know that the area of landsliding is likely to increase dramatically in heavy rain, and much of the debris released by landslides will be mobile. This will represent a fearsome threat. A good example is the Central Cross Island Highway in Taiwan, which was destroyed by the earthquake in 1999. This has been rebuilt twice only to be destroyed again each time. The Taiwan government has now abandoned its attempts to rebuild. pakistan_art_257_20080521060743.jpg Photo by David N. Petley There's some debate over what to do about the natural dams and lakes created by the quake. What do you think is the best approach to take to these? This is very simple - the best approach is to build a channel across the dam. [Pictured is] a photograph of a team from the Pakistan army doing exactly this to a landslide in Kashmir after the 1999 earthquake. The channel must be carefully built and the bed must be protected against erosion (this should not be difficult). This will allow the river flow through the blockage without creating a sudden release. Blasting is not the answer as it would create a sudden, unpredictable and potentially very damaging flood. Why is a shallow and large earthquake likely to trigger more landslides? Because the earthquake waves are more concentrated. In a deep earthquake the waves are more spread out when they reach the surface. In a shallow event they are very focussed, which means that the shaking is more intense. It is this shaking that causes the landslide. We've seen several references to mountains collapsing in Sichuan province. How is a "mountain collapse" defined and how do mountains collapse in an earthquake? A mountain collapse is essentially a giant landslide that occurs when a large part of a mountain collapses - it is as simple as that. They are terribly catastrophic as the volumes are large (sometimes cubic kilometers in volume), they can move at hundreds of kilometers per hours and they often travel very long distances. They occur because the shaking is very intense, which both increases the forces pushing them down the slope and reduces the forces that hold them in place. Big earthquakes in mountainous areas nearly always trigger this type of landslide. -Sky Canaves
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