Landslides can occur in all the fifty states. The primary regions of landslide occurrence and potential are the coastal and mountainous areas of California, Oregon, and Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. Alaska and Hawaii also experience all types of landslides.
Landslides are not currently amenable to risk assessment since there is no basis to determine the probability of landslides occurring within a given time period. Hazard assessments are possible and can be used in place of risk assessments. Hazard assessments are estimations of an area’s susceptibility to landslides based on a few key factors. These are each capable of being mapped and allow land areas to be evaluated on their relative susceptibility to landslides.
Most assessment procedures for landslide hazard zonation employ a few key or significant physical factors to estimate relative landslide hazard. These include:
Past Landslides and Their Distribution
Interpreting the likelihood of future landslide occurrences requires an understanding of conditions and processes controlling past landslides in the area of interest. This can be achieved by examining and mapping past landslide activity in the area.
Type of Bedrock
Bedrock influences landslide occurrence in several ways. Weak, incompetent rock is more likely to fail than strong, competent rock. On slopes where weak rock overlain by strong rock is exposed, the difference in strength increases the potential for landsliding in the stronger rock as well since the weak rock tends to erode and undermine the stronger rock.
Slope Steepness or Inclination
The influence of slope steepness on landslide occurrence is the easiest factor to understand. Generally, steeper slopes have a greater chance of landsliding. This does not prevent failures from occurring on gentler slopes. Other factors may make a gentle slope especially sensitive to failure, and thus in this situation could be determined to have a relatively high hazard potential.
Water is recognized as an important factor in slope stability-almost as important as gravity. Information on water table levels and fluctuations is rarely available. To represent the hydrologic factor in landslide hazard assessments, indirect measures can be used which can be mapped to show the influence of the area's hydrology, such as vegetation, slope orientation (aspect), or precipitation zones.
In addition to natural phenomena, human activities may increase the natural tendency for a landslide to occur. Landslides which result from development activities are usually the result of increasing moisture in the soil or changing the form of a slope. Development activities such as cutting and filling along roads and the removing of forest vegetation are capable of greatly altering slope form and ground water conditions.
Landslide/Mass Movement Vulnerability Assessment Questions
To assess your vulnerability to the threat of a Landslide/Mass Movement to your family and property, perform an assessment to determine if you have knowledge of, are prepared and/or have a plan by answering the following questions:
General All-Hazard Questions:
- Have you determined the disaster risks in your locale and the hazards that accompany them?
- Do you know the emergency warning signals and alert notifications used in your community?
- Have you instructed family members how to shut off water, gas and electricity to your house?
- Have you made the necessary property preparations to reduce the damage from the hazard?
- Do you have a backup generator in case of a prolonged power failure?
- Have you purchased insurance (property, health, life, and/or hazard type)?
- Have you made the necessary financial arrangements in case of a sudden evacuation and power outage that shuts down local ATMs and banks?
- have you organized important documents and records and stored them in a portable lock box or safe-deposit box?
- Have you performed a home inventory video taping the contents stored them in a portable lock box or safe-deposit box?
- Does your family have an established Emergency Communication Plan and evacuation plan in place and asked an out-of-state person to serve as the “family contact”?
- Have you assembled a shelter-in-place Emergency Supplies Kit in case you have to shelter at home and you are without power?
- Have you assembled a mobile Emergency Supplies Kit that can serve as a “grab and go” bag?
- Are you or someone in your family trained in first aid and CPR?
- Have you made they necessary preparations and arrangements for pets, seniors, and the disabled?
- Have you familiarized yourself with the emergency plans of your family member’s employment building, school, day care center, or nursing home?
Hazard Specific Questions:
- Is your home built near steep slopes, close to mountain edges, near drainage ways, or natural erosion valleys?
- Has a ground assessment been made of your property?
- Are gas appliances fitted with flexible connections and/or a breakaway gas shut-off device?
- Have changes occurred in your landscape such as patterns of storm-water drainage on slopes (especially the places where runoff water converges) or have you noticed any land movement, small slides, flows, or progressively leaning trees?
- Have doors or windows begun to stick or jam for the first time?
- Have new cracks appeared in plaster, tile, brick, or foundations?
- Have outside walls, walks, or stairs begun pulling away from the building?
- Have slowly developing, widening cracks appeared on the ground or on paved areas such as streets or driveways?
- Has bulging ground begun to appear at the base of a slope?
- Do fences, retaining walls, utility poles, or trees tilt or move?
This map was prepared by evaluating formations or groups of formations shown on the geologic map of the United States as being of high, medium, or low susceptibility to landsliding and classified the formations as having high, medium, or low landslide incidence (number of landslides).
Susceptibility to landsliding was defined as the probable degree of response of the areal rocks and soils to natural or artificial cutting or loading of slopes or to anomalously high precipitation. High, medium, and low susceptibility are delimited by the percentages given below for classifying the incidence of landsliding. Susceptibility is not indicated where lower than incidence. The effect on slope stability caused by earthquakes was not evaluated, although many catastrophic landslides have been generated by ground shaking during earthquakes.
|Landslide Incidence Legend
Color-coded Pacific & Southwest Landslide Susceptibility/Incidence Map
Color-coded Pacific & Northwest Landslide Susceptibility/Incidence Map
Color-coded Midwest Landslide Susceptibility/Incidence Map
Color-coded Atlantic & Southeast Landslide Susceptibility/Incidence Map
Color-coded Atlantic & East Central Landslide Susceptibility/Incidence Map
Color-coded Atlantic & Northeast Landslide Susceptibility/Incidence Map
Color-coded Northcentral & Michigan Landslide Susceptibility/Incidence Map
- USGS – Landslide Types and Processes – Fact Sheet 2004-3072: http://pubs.usgs.gov/fs/2004/3072/fs-2004-3072.html
- USGS – Landslide Overview Map of the Conterminous United States – Website: http://landslides.usgs.gov/learning/nationalmap/
- Image Source: http://landslides.usgs.gov/learning/nationalmap/images/psw.gif [Accessed July 14, 2013]
- Image Source: http://landslides.usgs.gov/learning/nationalmap/images/pnw.gif [Accessed July 14, 2013]
- Image Source: http://landslides.usgs.gov/learning/nationalmap/images/midwest.gif [Accessed July 14, 2013]
- Image Source: http://landslides.usgs.gov/learning/nationalmap/images/ase.gif [Accessed July 14, 2013]
- Image Source: http://landslides.usgs.gov/learning/nationalmap/images/ecentral.gif [Accessed July 14, 2013]
- Image Source: http://landslides.usgs.gov/learning/nationalmap/images/ane.gif [Accessed July 14, 2013]
- Image Source: http://landslides.usgs.gov/learning/nationalmap/images/michigan.gif [Accessed July 14, 2013]
- Image Source: http://earthobservatory.nasa.gov/Features/Landslide/ [Accessed August 22, 2013]