Landslide hazards in the central and southern Andes

Landslides are among the most destructive natural phenomena of the central and southern Andes, being comparable in property damage and loss of life due to vibrational destruction and collapse of buildings during earthquakes. The region considered in this report, which includes the Andean Highlands where elevations are generally above 4,000 m, the Andean front ranges, and the low-lying coastal desert region of western South America, shows great diversity of geology, topography, and climate. It is a region of active continental uplift and intense seismic activity. Most of the hundreds of landslides that occur each year are triggered by heavy rains and by earthquakes. Major earthquakes having magnitudes greater than 7, of which at least one will occur during any given 10 year period, commonly trigger hundreds to thousands of landslides, many of which have been among the largest and most destructive historic landslides in the Andes. Landslide distributions and styles differ according to geology and topography. The most destructive tend to be those occurring in areas of great relief in the Andean Highlands and in deep valleys along the eastern and western flanks of the Andes. High-speed debris avalanches and debris flows (formed by the rupture of glacial lakes) are characteristic of glaciated mountain ranges of the Highlands. More frequent but smaller and less destructive rock falls, rock and debris slides, and soil slips also occur in this region and in deep valleys along the Andean front. Exceptionally heavy rains in the Andean Highlands also cause destructive floods, debris flows, and mudflows in these valleys, particularly along the arid western Andean front and in the coastal region. Earthquake-triggered slumps and lateral spreading in water-saturated, unconsolidated sediments have caused extensive damage to settlements and cities in the Andean Highlands and along the Pacific coast. Statistical data about landslide frequency and destruction in the Andes are scanty, and so accurately estimating property damage and loss of life due to landslides is not possible. Nevertheless, available data suggest that in years without major landslides, destruction costs range from a few million to several tens of millions of U.S. dollars and deaths due to landslides are a few to a few tens of individuals. On the other hand, catastrophic landslides, which occur at 5- to 10-year intervals, may cause property damage in the hundreds of millions of dollars and kill hundreds or even thousands of persons.