Plans are afoot to improve landslide monitoring for the endangered town of Zhangmu in Tibet. Credit: Purbu Zhaxi/Xinhua Press/Corbis

The Tibetan town of Zhangmu is on edge — in an emotional and physical sense. Perched precariously on a mountainside, the growing trading and tourist centre lives under the constant threat of landslides, the result of a formidable combination of geological, climatic and developmental factors. The settlement, whose population reaches 40,000 in summer months, is built on the unstable debris of past landslides. As more buildings appear, the risk of a catastrophic collapse increases.

Social sciences suffer from severe publication bias Ebola virus mutating rapidly as it spreads Scientific advice: Crisis counsellors

Many settlements across the globe face a similar predicament. With extreme weather events becoming more common, land resources dwindling and urban development spiralling, landslides “are increasing in frequency, scope and destructive capacity”, says Sálvano Briceño, chair of the scientific committee at Integrated Research on Disaster Risk, an international research programme headquartered in Beijing.

But the risks are being addressed. At the third World Landslide Forum in Beijing last month, researchers met to discuss ways to improve the monitoring, prevention and management of these lethal phenomena. Presentations included technologies for mapping hazards and providing early warnings, as well as computer models that simulate the effects of rain­water and earthquakes. “With the projected increase in extreme rainfall, communities in landslide-prone regions will be more vulnerable,” said Rex Baum, a geologist with the US Geological Survey in Golden, Colorado.

Slope failures are the biggest landslide threat. These occur when a chunk of slope becomes detached from a hillside. As the mater­ial descends, shearing forces increase the pressure of water in the gaps between soil and rock particles (the pore-water pressure), causing clumps of slope materials to collapse. This process, called liquefaction, can be a result of rainfall-induced increases in water volume or seismic waves, and greatly accelerates the landslide because the water acts as a lubricant.

Catastrophic slides are frequent. In 2010, heavy rains in Zhouqu in northwestern China unleashed a torrent of mud and rocks that engulfed 550 houses and killed nearly 1,800 people. And in May this year, a rain-drenched scarp in northeastern Afghanistan gave way, sweeping away the village of Ab Barak and killing more than 2,000 inhabitants.

Developing countries are worst hit (see ‘Danger zones’). A study by geologist Dave Petley at Durham University, UK, shows that, of the 32,322 landslide fatalities between 2004 and 2010, most occurred in Asia, especially in the Himalayas and China (D. Petley Geology 40, 927–930; 2012).

Credit: Dave Petley

But advances in remote-sensing technologies are making hazards easier to detect. Satellite and airborne laser and radar instruments, such as LiDAR and InSAR, can be used to monitor ground movements, enabling accurate mapping of potential landslide sites.

“We are getting pretty good at spotting areas susceptible to landslides,” says Baum. “But we still can’t quite predict, if a slope fails, how big it will be or how far it’s going to go.” The landslide that struck in Washington state on 22 March, killing 41 people, took many by surprise. “We didn’t really expect that a slope coming off a block that was only 200 metres high could have flowed over one kilometre,” Baum says.

A big unknown is how rainfall, which triggers two-thirds of landslides, can change ground­water dynamics and the strength of soil and rock particles, says Kyoji Sassa, a geologist at Kyoto University in Japan. At the forum, his team presented a lab-based landslide simulator that tests how pore-water pressure and the strength of slope materials change with increasing rain. By feeding the data into a computer model designed to reproduce both the initiation and movement of a slide — a first for a landslide model — they have been able to replicate past events.

In a US$5-million project funded by the Japanese government, Sassa and his colleagues are testing the approach on a notoriously unstable slope in southern Vietnam, where annual precipitation is more than 4,000 milli­metres. They will combine rainfall records and weather forecasts to see if the simulator and model can predict how the slope will react to further rain. The ultimate goal, says Sassa, is “to develop a model that could be applied in all monsoonal regions”.

In the meantime, Zhangmu, which is prone to earthquakes and heavy rainfall, needs a contingency plan. A survey led by Wei Fangqiang, deputy director of the Chinese Academy of Sciences’ Institute of Mountain Hazards and Environment in Chengdu, found that the 49–78-metre-deep layer of previous landslide debris below the city is already moving, albeit slowly. It identified 21 potentially dangerous sites, some of which could produce several million cubic metres of debris (the Washington slide generated about 7.6 million cubic metres).

Last month, the Chinese government approved a $483-million project to improve monitoring in the Zhangmu region. Engineers will install sensors to determine pore-water pressure and implement measures to stabilize slopes, drain rainwater and block debris flow.

Critics warn that many governments tend to invest much more in disaster mitigation and relief than in reducing exposure to hazards. “Many mountainous regions are being developed rapidly without proper planning or risk assessment,” says Briceño. “What kills people are not natural phenomena, but poorly built or wrongly located houses.”

The second phase of the United Nations’ Hyogo Framework for Action, a ten-year plan aimed at reducing the impact of natural disasters, including landslides, should help to address such problems, he adds. Its latest incarnation, which is expected to tackle the challenges of extreme climate events and land-use changes, is due to be adopted next March. “Risk reduction is the key,” says Briceño. “It should go hand in hand with climate-change adaptation and sustainable development.”