“Landslide” is a general term for the downslope mass movement of soil, rock, or a combination of materials on an unstable slope. It can be rapid, very slow, or in between. It can involve large or small areas and volumes of material.
The principal types of movement are falling, sliding, and flowing, but combinations of these are common.
As human impact on the land increases, we both cause more landslides and are increasingly affected by natural ones. In some parts of Pennsylvania, landslides are a significant hazard.
The Pennsylvania Geological Survey's educational series booklet Landslides in Pennsylvania (PDF) contains information on landslide types and their geologic settings, the effects of landslides and the associated costs, and measures that can be taken to prevent landslides from occurring or to make them less catastrophic.
Areas of Pennsylvania Susceptible to Landslides
Southwestern Pennsylvania has by far the highest concentration of landslides. Outside that region, high susceptibility areas are mostly smaller and have more varied geology and topography.
Across the state, many landslides occur in colluvium (loose soil) and old landslide debris on steep slopes. Mountainsides have experienced debris avalanches associated with extreme rainfall or rain-on-snow events. In very susceptible materials, landslides can even develop on relatively gentle slopes.
Bedrock landslides have been known to happen where weak rock layers or fractures provide slip surfaces. Most major and minor highways have sections cut in rock or soil that can fail.
Glacial and glacial-lake sediments underlie stream-bank and lake-bluff slumps and other failure areas across much of the northern part of the state.
Effects and Costs of Landslides in Pennsylvania
Urban and rural land development is increasing both the number of landslides and the economic effects of natural slides. Major highway construction with large excavations and fills located in mountainous areas creates the potential for many landslides.
Most Pennsylvania landslides are moderate to slow moving and damage things rather than people.
Landslides cause damage to transportation routes, utilities, and buildings, and create travel delays and other side effects. Fortunately, deaths and injuries due to landslides are rare in Pennsylvania. Almost all of the known deaths due to landslides have occurred when rockfalls or other slides along highways have involved vehicles.
Storm-induced debris flows are the only other type of landslide likely to cause death or injury. As residential and recreational development increases on and near steep mountain slopes, the hazard from these rapid events also will increase.
“Backyard” landslides, common in the Pittsburgh area, are usually repaired incompletely or not at all. Cost estimates of several hundred thousand dollars for stabilization and repair of a landslide affecting two or three properties are typical. With repair estimates exceeding the value of the properties, abandonment is a frequent solution. Insurance covers landslide damage only for some business situations.