The Landslide: Gravity Never Stops

By Jordan Chapman
Special to the Center for Disaster Philanthropy

Workers search through sludge following the 2014 Washington Landslide. (Photo courtesy of FEMA).

Unlike any other natural disaster on Earth, landslide events occur almost on a daily basis because of one constant phenomenon in the universe.

“Gravity never stops,” said Jonathan Godt, Ph.D., program coordinator at U.S. Geological Survey (USGS).

The term “landslide” is a broad term and, like many other natural disasters, can be classified into a series of different types: a rotational or translational slide, a block slide, a rock fall, a topple, a debris flow or avalanche, an earthflow, a creep or a lateral spread.

In the United States these events cause about $3.5 billion in damage and kill between 25 to 50 people annually. Though landslides in the U.S. generally occur in states compromising the intermountain west and the mountainous and hilly regions in the east, landslides can and have occurred in all 50 states and U.S. territories.

“Anywhere there is a mountain or hill there is an erosion process taking place. It’s a ubiquitous process that, at a low level, happens almost all of the time…if you look at mountains around the world, landslides are a frequent and common occurrence,” Godt said.

Water is the biggest culprit, according to Chris Humphrey, engineering geologist with the U.S. Army Corps of Engineers. If hilly or mountainous slopes were divided in two, on top would be the driving force, while the bottom contains the resisting mass. “If (the resisting force) is removed for some reason, as a result of river erosion or even grading activity…you’re removing material that’s holding up the slope,” he said. “The mass above, essentially, wants the material to move down.”

Most landslides happen without consequence, which means most of them aren’t deadly to humans, but the ones that are can cause long-term economic disruption, population displacement, and negative effects on the natural environment.

“[Gravity is] always tugging down,” Godt said. “Where you have water eroding a steep landform, [landslides are possible].”

Major landslide events are usually the result of some greater natural phenomena, like an earthquake, volcano or tsunami, and are referred to as a secondary hazard. Godt used the 2008 Wenchuan Earthquake in China as an example, which prompted tens, if not hundreds of thousands of landslides.

The landslide event can and does happen as its own signature event. The United States experienced two major landslide events within six months of one another during the past year.

The landslide in Oso, Washington, occurred in March 2014. Godt was on site afterwards to help assess any continuing hazards that would hinder the recovery effort. He explained Oso was a highly unusual slide because it did claim so many lives. “The slide moved so far, so fast,” he said. “[It] basically caused roughly 100 percent destruction of a small community…the buildings, everything was pushed to the very edge of the landslide deposit,” he continued. “The search and rescue was quite difficult because people and equipment were working in this soupy mess to try and find people.”

The other recent event in the United States took place mid-September 2013, in Northern Colorado and is a prime example of a debris flow, the cause of which was nearly continuous rainfall that resulted in floods and landslide events. “[Debris flows] move fast and they tend to be destructives. We had 1,000 debris flows over an area of about 100 km north to south and maybe 50 km east and west,” Godt said.

Landslide Causes

Much speculation has circulated through the scientific community as to whether or not there has been an increase in landslide activity and if so, what that can be attributed to.

Godt recalled records indicating not an increase in rainfall amounts, but perhaps an increase in rainfall rate, which could be attributed to climate change. “If we look at the records of landslide activity there’s no clear signal that… activity has increased,” he said.

There’s a greater and much more relevant culprit that beats out climate changes: increase in land use.

The United States Census Bureau estimates there is a baby born in the United States every eight seconds. This growing human population needs places to live, and when humans move to utilize more area, the landslide frequency doesn’t change, but the fatality rate in areas where landslides occur may go up.

“As populations increase in the United States people are looking for more areas to live,” Godt said, which is when humans can unknowingly place themselves into a complicated equation with many variables. “The change in land use has a much greater effect than climate change.”

Disturbing or changing drainage patterns and removing vegetation often caused by irrigation, lawn watering, draining or creating reservoirs and deforestation are common human-induced changes that can initiate landslides.

“Shallow landslides often happen and are the result of the weathering of topsoil and upper bedrock,” Humphrey stated, and the weathering of topsoil is more likely when the vegetation is removed.

Deforestation operations are a large part of this problem, illustrated by one of many articles covering the topic. One study in 2006 detailed slope instability in Sierra Norte, Puebla, Mexico between 1989-1999, when vegetation in the area had been reduced by 70 percent.

Vegetation density decreased from 1245 square kilometers to 363. Findings showed that 85 percent of slope instability could be found on surfaces that were bare and contained low vegetation and that land use change and land degradation are precursors to environmental hazards, mass movement events in particular.

Mitigating Landslide Risks

Dr. David Petley, A.K.C., Ph.D., F.R.G.S., F.H.E.A., CGeog, dean of Research and Global Engagement and Wilson Chair in Hazard and Risk in the Institute of Hazard, Risk and Resilience at Durham University, explained that no matter a person’s location in the world—be it a rich or poor country—if one is unlucky enough to be caught in a high devastation landslide—like the one in Oso—the chances of survival are low.

“The rate of loss of life for people who are involved (in landslides) are much higher than pretty much any other natural disaster,” he said.

Petley does much of his research in high mountain areas and poor countries. “They have a pretty good knowledge and understand that slopes weaken through time and heavy rainfall is the trigger,” Petley said of the people he’s studied.

“People will make decisions that on the outside that appear to be very irrational decisions,” he said, using a family who moved near a road—but in a high risk area for landslides—as an example. “It appears dangerous to the outside world, but because they live near the road their kids can go to school and they can easily attain work down the street in a local café or quarry…They live with multiple risks, in which landslides are just one. From their perspective, what they’re doing is actually looking over their whole portfolio of risk and reducing it, even though their risk of landslides is going up.”

No matter the area a person is living in, simple defense and prevention of landslides is always the same: avoidance. But as populations grow, experts and policy makers know that’s not always practical.

Humphrey explained that many residents in the U.S. don’t know they live in an area at high risk. “People that have lived in an area for decades, some of them were never told that there is a problem,” he said, noting it’s more of a local city and county initiative to create the appropriate ordinances and rules that can notify residents about landslide risk. “It’s variable. Some counties do a good job at doing landslide assessment,” he said, but even then, notifications are usually only given to areas of newer construction. “If your house was around in the 1940s and your family has owned it the whole time and you don’t keep track of current publications, you may never know you’re on a landslide.”

There are ways to self-diagnose areas of concern. “What people can look for, number one, is a steep slope. Anywhere that is steep has potential for landslide activity, and there are a number warning signs if you’re on the slope: trees moving, sounds, cracks opening up in the ground,” Godt said.

Citizens and business owners who are concerned about landslide activity can also consult a professional, such as a geotechnical or civil engineer, as they have the training to solve instability problems. Local companies or county and city municipal offices may have individuals who are also professional geologists, planners or building experts. (2)

“USGS scientists produce maps of areas susceptible to landslides and identify what sort of rainfall conditions will lead to such events,” one article read (3), but not even the most sophisticated studies can always predict the worst disasters.

In the 1970s Hong Kong experienced a slew of landslides and the government chose to mitigate risks by engineering and securing slopes that were landslide prone.

“Of all natural hazards—with the right resources and the right skills—landslides are…the most manageable of them, so long as you’re willing to put the financial capital into the landslide mitigation program,” Petley said. “[Hong Kong is] now down to small amount of fatalities per decade.”

The United States, and many other countries, isn’t in the practice of securing dangerous slopes as Hong Kong did, which required the relocation of thousands, the construction of temporary homes and slope maintenance after implementation. “In terms of engineering measures to prevent landslides, that’s a very expensive undertaking. Typically in the United States, that’s not what we do,” Godt said. “Avoidance is a much more cost effective way to approach the problem.”

Cheaper options to prevent landslides include controlling the water on any given slope, either by rerouting it entirely or safely through a slope in way that doesn’t add pressure of the resisting forces holding a slope up. Cheaper options yet include building buttresses and walls, or inserting anchors into the base of hills and slopes that show a high probability for landslide events.

“In most poor countries, there’s neither the financial capital nor the skill set to be able to put forward such programs,” Petley said, noting the drastic difference in response efforts between poor and rich countries, which manifests itself mainly in response time, labor skills, financial capital and good governance.

“In rich countries what you see is a very rapid response with search and rescue teams going in, organizing searches and of the…percentage who aren’t killed, there is a reasonable chance of recovering people,” he said.

Responses in the United States Compared to Elsewhere

In the United States, specifically, there are a number of agencies that respond to a large disaster occurrence. Aside from local and state emergency personnel, high impact disaster areas may see: USGS workers, whose primary purpose is to conduct studies for underlying causes and to help design and monitor early warning systems; Federal Emergency Management Agency workers, who do search and rescue and clean up; and U.S. Army Corp of Engineers, who can be called in by FEMA to assist with search and rescue, though they primarily keep to engineering and construction to stabilize and reestablish roadway infrastructure.

Compare that to a poor country. “There’s a complete lack of organization,” Petley said, and unfortunately, it’s in the poorest countries where Earth’s highest probability for landslide activity resides. “The geographical fact is that [these] areas of the world carry the [perfect landslide recipe]…What you need is an area with steep slopes and weak rocks with heavy rainfall; especially if that rainfall is seasonal,” he said, not to mention the ever present changes caused by the exponential increase in human activity—farming and the removal of vegetation—that can drastically weaken a slope.

Petley called out the following regions that carry a high need for landslide assistance: large portions of the Himalayas, China; large parts of Indonesia; the Philippines; Central America and parts of South America.

Godt noted that the deadliest landslide in history occurred in a poorer region of the United States. Le Mameyes, Puerto Rico was victim to a devastating landslide in 1985, destroying 120 houses and killing at least 129 people.

Can landslides be stopped or prevented entirely? The short answer: no. People will always be in need from such events, and the need is great. They’ll always need your help.

“Fortunately in the United States, landslides that cause great loss of life are quite rare,” Godt said, but they do happen, as the last year has shown.

Help can be given in many ways, by either providing relief to disaster areas or funding technology research that can continue to help mitigate, prevent or clean up landslide events.

For Godt, the hardest part when dealing in landslide clean-up is seeing the impact such events can have on families and homes. He explained nothing can prepare a person for what he or she will be faced with, and nobody should need to be.

It’s the kind of place where the image of a stuffed bunny, once plush and pink—but now matted brown and stuck in the mud—can a stop a person flat in their tracks to wonder: where is the owner? And, are they okay?


  1. USGS Landslide Types and Processes
  2. Landslide Handbook
  3. USGS Features: Landslide in Washington State
  4. USGS Fact Sheet: Landslides in Northern Colorado
  5. USGS Landslide Events
  6. NOAA Landslide: The Mameyes, Puerto Rico