Where and Why Do Landslides Occur?

Last month's mudslide in Oso, Snohomish County, Wash., has killed more than 30 people and has left many more still unaccounted for.

The rural neighborhood is now submerged in earth, mud and debris. According to the Seattle Times, a 1999 report written by geomorphologist Daniel J. Miller and filed with the U.S. Army Corps of Engineers warned of "the potential for a large catastrophic failure."

According to the U.S. Geological Survey, a landslide is caused by gravity acting on an over-steepened slope. A variety of contributing factors are often responsible for their occurrence, but precipitation often plays a major role in failure.

Some water on Sunday, March 23, 2014, had breached the dam created by the landslide on the Sillaquamish River at Oso, Wash. (Photo/Washington Department of Transportation)

"Excess weight from accumulation of rain or snow... may stress weak slopes to failure," USGS reported. "Almost every landslide has multiple causes."

In addition to erosion from waterways, glaciers and waves, earthquakes and volcanic eruptions can cause landslides. Human activities can also contribute to landslides.

"Another cause of landslides is often man-induced because of logging or excavation," AccuWeather.com Meteorologist Jim Andrews said.

The May 1980 eruption of Mount St. Helens caused a massive landslide, Andrews said. The Mount St. Helens landslide was 57 times larger than than another recent major landslide, the April 2013 rockslide near Salt Lake City, according to the Associated Press.

Some parts of the United States are more prone to landslides based on the geology of the region.

According to the USGS's Landslide Overview map, the Appalachians, areas in the Rockies and mountain ranges near the Pacific coast are more prone to landslide than other parts of the county.

"Landslides are a serious geologic hazard common to almost every state in the United States," according to the USGS website. "It is estimated that in the United States, they cause in excess of $1 billion in damages and from about 25 to 50 deaths each year."

Globally, hundreds of thousands of deaths and injuries are caused by landslides each year, along with hundreds of billions of dollars in damages, the USGS reported.

Throughout geologic history, massive slides have left their imprint across the globe.

The most expensive landslide in the history of the United States occurred in the spring of 1983 in Thistle, Utah, which is now a ghost town.

"The Thistle, Utah, landslide cost in excess of $200 million to fix (1984 dollars - adjusted for inflation, this would be more than $400 million in 2010 dollars)," according to the USGS report.

(Photo/Google Maps)

Andrews said there was an El Niño winter season, making Utah exceptionally wet due to a high snowmelt along the slopes because of warmer weather.

As debris slid down the slopes, it dammed the Spanish Fork River, he said, causing the town of Thistle to flood. It also pushed across Highway 89.

"It buried the highway and drowned the town," Andrews said.

According to the USGS report, the slide reached a state of equilibrium across the valley, but fears of reactivation caused the railway to construct a railroad tunnel through the bedrock around the slide zone. Highway 89 had to be realigned around the landslide.

In this image provided by the Washington State Patrol, debris is shown from a landslide Saturday, March 23, 2014, between Arlington and Darrington, Wash. (Photo/Washington State Patrol)

On April 8, Pittsburgh city officials temporally closed LeMont restaurant located on Mt. Washington due to potential landslide dangers after large boulders and dirt fell from the slope, according to the Pittsburgh Tribune-Review.

"An engineering consultant said rain, runoff and perhaps the severe winter combined to loosen clay and shale over an area estimated ‘as wide as a football field'," the Tribune-Review reported.

On April 7, Pittsburgh received a half of an inch of rain with temperatures hovering in the low to mid-50s, AccuWeather.com Meteorologist Brian Edwards said.

Between April 2 and 4, Pittsburgh was hit with 1.5 inches of rain.

"They've seen seen twice their normal amount of precipitation for the first 10 days of the month," Edwards said.