Landslides are rock, earth, or debris flows on slopes due to gravity. They can occur on any terrain given the right conditions of soil, moisture, and the angle of slope. Integral to the natural process of the earth's surface geology, landslides serve to redistribute soil and sediments in a process that can be in abrupt collapses or in slow gradual slides. Such is the nature of the earth's surface dynamics. Also known as mud flows, debris flows, earth failures, slope failures, etc., they can be triggered by rains, floods, earthquakes, and other natural causes as well as human-made causes, such as grading, terrain cutting and filling, excessive development, etc. Because the factors affecting landslides can be geophysical or human-made, they can occur in developed areas, undeveloped areas, or any area where the terrain was altered for roads, houses, utilities, buildings, and even for lawns in one's backyard. They occur in all fifty states with varying frequency and more than half the states have rates sufficient to be classified as a significant natural hazard.

The U.S. Geological Survey, working with other federal agencies, has efforts underway to study, plan, and mitigate landslide risks. So have some communities across the country. Many deal with landslides as part of flood control, erosion control, hillside management, earthquake hazard mitigation, road stabilization, and other programs.

Perhaps the most common reminders of landslide risks are those "Watch For Falling Rocks" highway signs. Although "sliding rocks" is more apt, very few get to see a land slide. Occasionally we see small rocks or debris on the pavement, but a large size slide usually starts with such small incidents. Visually, a landside resembles a snow avalanche, only with a louder rumbling noise, and is capable of generating enough force and momentum to wipe anything in its path. One such devastating landslide wiped entire towns and villages in Columbia in 1985 when 20,000 died.

The pictures you see on this web site are recent examples from around the country. They show what's left after a slide. In some cases, only the rail or pavement is mangled, in others a house or building crushed, but in almost every aftermath, the losses are real, the damages total, and the terrain changes permanent.

Landslides cause one to two billion dollars in damage each year in the US and claim as many as fifty lives. Although damage estimates vary by how one classifies disasters, it is not insignificant. Landslides affect utilities, transportation, and all other forms of infrastructure, whether public or private.

As development pressures around the country increase, so does the likelihood of building in areas susceptible to landslides. Such areas are neither isolated nor far in-between. They span the entire eastern part of US, from New England to the Appalachian region encompassing some of the most scenic areas in the east as well as large urban areas. Landslide risks loom through them all. Pittsburgh and Cincinnati are two examples of urbanized areas with frequent landslides where developments on hills and hillsides are common. In the Great Plains, heavy rains combined with loss of vegetation due to wildfires trigger landslides in clay-rich rocky areas. On the west coast, earthquakes add to the causes of landslides. For example, the 1994 Northridge earthquake triggered many thousand landslides in the Santa Susanna Mountains. In short, no region of the country is safe from landslides, whether caused by geophysical or human-made factors.

Other related materials:

• See an online slide presentation (ppt), prepared by Paula Gori, USGS.
• See a computer simulation of a landslide, prepared by the USGS after the 1997 storms in the San Francisco Bay Area created this and several other massive landslides.