What is a Landslide? A landslide is soil, rock and other debris
giving way to gravity. CREDIT: Mark Reid, USGS. When soil, rock,
and other earth debris can no longer hold it together and gives
way to gravity, landslides happen. The downward force of a
landslide can move slowly, (a mere millimeters per year) or
quickly with disasterous effects. Landslides can even occur
underwater, causing tidal waves and damage to coastal areas.
These landslides are called submarine landslides. Landslides can
be triggered by earthquakes, volcanic activity, changes in
groundwater, a disturbance or change of slope. Intense rainfall
over a short period of time tends to trigger shallow, fast-
moving mud and debris flows. Slow, steady rainfall over a long
period of time may trigger deeper, slow-moving landslides.
Different materials behave differently, too. Every year as much
as $2 billion in landslide damage occurs in the United States.
In a record-breaking storm in the San Francisco area in January
1982, some 18,000 debris flows