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• driven mass-wasting processes transfer the products of weathering downslope where an erosional agent, in this case a valley glacier, carries the debris away
• Even in areas with steep slopes, catastrophic landslides are relatively rare occurrences
• On October 5, 2005, torrential rains from Hurricane Stan triggered mudflows in Guatemala that buried the village of Panabaj
• On October 8, 2005, a magnitude 7.6 earthquake struck the Kashmir region between India and Pakistan, triggering hundreds of landslides
• On February 17, 2006, extraordinary rains triggered a lethal mudflow that buried a small town on the Philippine island of Leyte
• Mass wasting


  The downslope movement of rock, regolith, and soil under the direct influence of gravity
• Mass wasting is the step that follows weathering in the evolution of most landforms
• The combined effects of mass wasting and running water produce stream valleys, which are the most common and conspicuous of Earth's landforms
• If dynamic internal processes did not continually produce regions having higher elevations, the system that moves debris to lower elevations would gradually slow and eventually cease
• As mountain building subsides, mass wasting and erosional processes lower the land, and steep and rugged mountain slopes give way to gentler, more subdued terrain
• Trigger


  An event that initiates downslope movement, but is not the sole cause of the mass-wasting event
• Triggers of mass wasting

  • Saturation of material with water
  • Oversteepening of slopes
  • Removal of anchoring vegetation
  • Ground vibrations from earthquakes
• Angle of repose


  The steepest angle at which unconsolidated, granular particles remain stable
• Plants protect against erosion and contribute to the stability of slopes because their root systems bind soil and regolith together
• When anchoring vegetation is removed by forest fires or by people, surface materials frequently move downslope
• Post-fire debris flows are most common in the two years after a fire, and some of the largest debris-flow events have been triggered by the very first intense rain event following the wildfire
• Earthquakes can dislodge enormous volumes of rock and unconsolidated material, triggering mass-wasting events
• Even in dry weather, the soil tends to move down steep slopes
• Fire can "bake" the ground, creating a water-repellant layer at a shallow depth
• The water-repellant layer prevents or slows the infiltration of water, resulting in increased surface runoff during rains
• The consequence can be dangerous torrents of viscous mud and rock debris
• Conditions that favor mass wasting

  • May exist in an area for a long time without movement occurring
  • An additional factor is sometimes necessary to trigger the movement
• Triggers


  Factors that initiate mass wasting movement
• Earthquakes and their aftershocks can dislodge enormous volumes of rock and unconsolidated material
• Memorable United States example

  • January 1994 quake in Los Angeles region
  • Produced over 11,000 landslides in an area of about 10,000 square kilometers
• China example


  • May 12, 2008 magnitude 7.9 earthquake near Chengdu
  • Triggered hundreds of landslides that destroyed roads, bridges, and buildings, and dammed rivers creating lakes
• Liquefaction


  Intense ground shaking during earthquakes can cause water-saturated surface materials to lose their strength and behave as fluidlike masses that flow
• Liquefaction was a major cause of property damage in Anchorage, Alaska, during the 1964 Good Friday earthquake
• Southern California lies astride a major plate boundary defined by the San Andreas Fault and numerous other related faults
• Landslides occur when gravity prevails over the forces pushing the landscape upward
• Some landslides in Southern California are triggered by earthquakes, while others are related to periods of prolonged and intense rainfall
• The 2005 debris flow in La Conchita, California involved the remobilization of a portion of a large landslide that destroyed homes in 1995
• Historical accounts indicate that landslides in the La Conchita area have been a regular occurrence
• The most significant contributing factor to the 2005 La Conchita debris flow was prolonged and intense rain
• Debris flows are to be expected at La Conchita when the amount and intensity of rainfall is sufficient
• Rapid mass-wasting events sometimes occur without being triggered by any discernible external conditions
• Slope materials gradually weaken over time under the influence of long-term weathering, infiltration of water, and other physical processes
• Eventually, if the strength falls below what is necessary to maintain slope stability, a landslide will occur
• The timing of such events is random, and thus accurate prediction is impossible
• Landslide potential in the US

  • Greater in mountain areas
  • Lower in Florida and the central plains