15.4 Deadly Mass Movements

Assess the threats to people from mass movements and explain how their vulnerability to those threats can be reduced.

Mass movement events are deadly hazards for people. According to the USGS, mass movement events are among the world’s most deadly of natural disasters. It is estimated that $2 billion to $4 billion in damage and an average of 50 deaths are caused by mass movements in the United States annually. A study published in the journal Geology in 2012 found that between 2004 and 2010, there were 2,620 deadly mass movements worldwide, which killed about 32,300 people (Figure 15.32).

Figure 15.32

Deadly mass movements. This map shows the locations of 2,620 mass movement events that took place between 2004 and 2010 and resulted in human deaths. Landslides that were triggered by earthquakes are not included. If they were included, this number would be considerably higher.

Causes of Deadly Mass Movements

As we have seen, the factors that cause most mass movement events are the undercutting of steep slopes, the removal of vegetation, earthquakes, and heavy and prolonged rainfall. Many mass movement disasters have been preceded by deforestation and slope steepening through development, which weakens the slopes. An earthquake, or saturation of the ground by heavy rainfall, then triggers slope failure. Major mass wasting events causing 1,000 deaths or more have occurred, on average, about every 4.5 years in the last 50 years (Figure 15.33).

Figure 15.33

Recent deadly mass movements. (A)On February 27, 2006, half the village of Guinsaugon, on Leyte Island, in the Philippines, was buried by this debris slide. The slide traveled quickly down the steep slopes over a distance of 3.8 km (2.3 mi) before coming to rest. Some 1,100 people died in this single slide. (B) This table lists recent mass movement disasters that took 10,000 lives or more. Note that in some cases, multiple mass movement events are grouped together under a single trigger event, such as the Sichuan earthquake in 2008 and Hurricane Mitch in 1998.
(AP Photo/Wally Santana)

A less obvious hazard is created by landslides when rivers are blocked by debris. River canyons are frequently sites of slides because stream erosion cuts steep walls that become unstable through time. These slides often settle across rivers, damming them and creating lakes behind the dam debris. The 2008 Sichuan earthquake created 66 new lakes as landslide debris dammed the streams. More than 40 of these debris dams had to be carefully removed because scientists feared that the dams would suddenly break and the impounded water would submerge communities downstream.

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Assessing the Risk

Many developing countries are particularly vulnerable to mass movement disasters. Factors that increase the possibility of a disaster include heavy rainfall brought by the summer monsoon (see Section 4.4) or by tropical cyclones (see Section 5.3), farming on steep slopes (see Section 9.1), shantytown (slum) settlements on steep hillsides, large populations, and poverty. In many large urbanized regions, such as Rio de Janeiro, Brazil, or Manila, in the Philippines, human population growth and economic marginalization have driven uncontrolled settlement expansion onto steep and unstable slopes, where mass movements are more likely to occur.

Mapping the spatial variation in mass movement risk is a relatively straightforward task. Using satellite data and a GIS (see Section GT.4), scientists have mapped areas of greatest mass movement susceptibility (Figure 15.34). But as useful as it is for identifying areas of risk, such maps do not provide real-time warnings as changes in weather elevate the risk of mass movements.

Figure 15.34

Global landslide risk. This map of landslide risk, which includes all types of ground movement, was made using satellite data and a GIS. To make the map, scientists analyzed slope steepness (steeper slopes have a higher risk), land-use types (land with less vegetation has a higher risk), and soil types (coarse-grained soils have a higher risk). The black dots show areas where significant landslides occurred between 2003 and 2006.

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To address this problem, scientists have developed a risk-assessment program that is based on rainfall rates and amounts. It is available in real time to anyone with an Internet connection. The TRMM (Tropical Rainfall Measuring Mission), run by NASA and the Japan Aerospace Exploration Agency (JAXA), evaluates landslide risk by using satellites to measure rainfall amounts and rainfall intensity for all tropical areas. These data are used in conjunction with the mass movement risk map above to provide real-time warnings (Figure 15.35).

Figure 15.35

TRMM map of potential landslide areas. This map shows landslide risks for South Asia for August 22, 2013. This map is based on 7-day average rainfall intensities and rainfall totals. Areas with increased landslide potential are circled in yellow. Areas that are likely or very likely to experience landslides are colored orange and red.
(Harold F. Pierce/NASA Goddard Space Flight Center)

At a global scale, identifying broad regions most at risk of landslides is a relatively simple task. However, at a large geographic scale, identifying which specific slopes will fail, and which towns and villages are at risk, is difficult. Although monitoring techniques like those shown in Figure 15.26 can detect ground movements, there is not yet a warning system for mass movements analogous to the U.S. tornado warning system (see Section 5.2), which has proved so effective at giving people enough lead time to get out of harm’s way. Scientists are trying to develop a similar system that gives residents warning of landslide hazards. These efforts could reduce our vulnerability to these natural disasters.

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