COULD a way to identify areas at risk of earthquakes be hiding in plain sight? That鈥檚 the claim of a team of geologists, who reckon that the shape of some mountains can betray the presence of active faults beneath the surface, such as the one that caused the earthquake in China鈥檚 Sichuan region in May.
About seven years ago, Eric Kirby of Pennsylvania State University and colleagues trekked out to the mountains above Chengdu in south-west China to study how steep the riverbeds were there. 鈥淚f you were sitting in Chengdu, and could lift all the clouds and smog, you would see an incredibly steep mountain front rising almost 6000 metres above you just 50 kilometres to the west,鈥� says Kirby. This is where the eastern Tibetan plateau pushes against the tectonic plate beneath the Sichuan basin. Rivers flow out of the range into the Sichuan basin, perpendicular to a series of fault lines (see Map).
The researchers noticed that the riverbeds were in some parts far steeper than elsewhere. Kirby wondered whether this abrupt change in gradient indicated a region rising faster than the surrounding landscape. Such rapid uplift would signal that a fault was at risk of slipping below.
Advertisement
Though they couldn鈥檛 predict when, the team proposed in 2003 that the southernmost part of the Beichuan fault within the Longmen Shan mountain range was more active than surrounding faults and was most likely to slip (Journal of Geophysical Research, ). Sure enough, this part of the fault shifted by 5 metres on 12 May, killing 70,000 in the Sichuan earthquake.
For similar reasons, the team argue that another fault in the Sichuan region, the Huya fault, could produce a quake at some point in the future. Kirby is also analysing topography maps of the Andes and Himalayas to test whether his idea applies in other regions (Nature Geoscience, ).
The researchers point out that the connection between mountain shapes and earthquake risks are not always easy to interpret. 鈥淚t is not as simple as saying 鈥楢ha! the mountains are steeper here so they must be rising faster, and this is where we鈥檙e going to see an earthquake鈥�,鈥� says Kirby. Riverbed steepness, for instance, can be affected by the hardness of the underlying bedrock.
鈥淵ou could say it is a quick and dirty reconnaissance tool,鈥� he says. 鈥淢ountain ranges can be vast, and identifying the zones that are most likely to rupture is a challenge, especially in remote and rugged ranges.鈥� High-resolution maps of the surface can now be obtained from planes and satellites, which help distinguish abrupt changes in the slope of riverbeds.
For Alex Densmore of the University of Durham, UK, that鈥檚 as far as the method can go. 鈥淏ut that in itself is a huge step compared to nothing,鈥� he says. 鈥淚f you can direct the selection of sites and go to areas that are most likely of being at risk, then you have done something worthwhile.鈥�
