Geology Cafe

Landslide Hazards—An Awareness Guide

Table of Contents

Landslides Hazards—Basic Concepts and Understandings

Landslide is a general term covering a wide variety of mass-movement landforms and processes involving the downslope transport, under gravitational influence, of soil and rock usually within a relatively confined zone. Landslides have a great range of shapes (morphologies), rates of motion, patterns of movement, and can range is size from a small area to affecting a region of many square miles. Landslides, in forms both fast and slow, are naturally occuring. The modern world with its global information network frequently get to hear about landslide disasters that impact whole communities or even region when "big events" occur. However, perhaps the greatest costs comes from the cumulative total of relative small-scale landslide-related activity that affect roads, property, and infrastructure in a region prone to landsliding.

Aerial view of the La Conchita Landslide in California. This landslide formed on the hillslopes above the community built on a narrow coastal plain along the Pacific Ocean between Santa Barbara and Ventura. The landslide formed in soft (poorly consolidated) sediments on the western slope of Rincon Mountain. On January 10, 2005, an earthflow (landslide) at La Conchita destroyed or seriously damaged 36 homes and killed 10 people. Another landslide occurred in the same vicinity in 1995 (without catastrophic results) and the hillslopes in the area display abundant evidence of landslide activity in the prehistoric past. The disaster at La Conchita illustrates the problems of development activities in landslide-prone areas.

For more information see Landslide Hazards at La Conchita by R.W. Jibson []. (Photograph by R.L. Schuster, U.S. Geological Survey, 1995.)

Landslides constitute a major geologic hazard because they are widespread throughout the United States and locations around the world.
  • In the United States alone, landslides and debris flows cause $1-2 billion in damages and more than 25 fatalities on average each year.
  • Landslides pose serious threats to manmade features (homes, highways, and infrastructure such as pipelines, canals, power lines, and reservoirs). Landslides and debris flows impact activities and structures associated with transportation, fisheries, tourism, and timber harvesting, and mining and energy production.
  • The expansion of urban developments into hillside areas results in ever increasing numbers of residential and commercial properties that are threatened by landslides.
  • Landslides also commonly occur in connection with other major natural disasters such as earthquakes, volcanic eruptions, and floods (particularly in areas recently made barren by wildfires before heavy precipitation occurs in mountainous regions).
  The May 18, 1908 eruption of Mount St. Helens produced one of the greatest landslides in recorded history.