Landslides: What We Know about this Deadly Phenomenon

The mudslide in Washington has left at least 18 dead. - Marcus Yam/The Seattle Times

Mudslides (also called landslides) are the movement of rock and dirt (consequently homes, trees, and whatever happens to be on top of that rock and dirt) down a slope. On average, each year, in the United States alone, landslides cause $3.5 billion in damages and claim 25-30 lives.

There are three causes for mudslides: the materials in the ground, the structure of those materials, and human activity. Different layers of the earth have different strengths, making some stronger and better able to support the weight placed on it. In addition, if the ground is cracked, the ground is more susceptible to a mudslide. Drought and erosion also cause mudslides. These natural processes weaken the ground, leaving areas vulnerable to mudslides. Another structural cause of landslides is fault lines. Along fault lines, the earth is apt to separate, causing the rocks and dirt to slide along the separation. Agriculture, irrigation, and construction - all human activities - contribute to the weakening of the earth's surface. These activities can leave natural slopes open to mudslides.

Furthermore, there are two types of landslides: translational and lateral. Translational mudslides occur when the surface of the earth is separated from the more stable underlayers. Lateral landslides move the weakened material from side to side. Earthquakes are often culprits for both translational and lateral mudslides. Landslides resulting from volcanic activity are called lahars.

Landslides are rather difficult to predict, as the natural, environmental factors are the most common causes of mudslides.