The Deadly Dynamics of Landslides

They result from a simple mix of gravity, friction, and momentum, but take on a stunning variety of forms—with similarly diverse human consequences.

Physics Geology

Current Issue

This Article From Issue

July-August 2014

Volume 102, Number 4
Page 298

DOI: 10.1511/2014.109.298

In 1978 a hapless farmer in a small town in Norway discovered what happens when liquefaction occurs on a hillslope. His farm was set on a gentle slope along the coast of Lake Botnen.

Eleven thousand years ago, an ancient sea that developed at the end of the last ice age covered this area. As the glaciers advanced about 14,000 years ago, they ground rocks in their path into fine powdery silt and clay. Then, as the climate warmed and the glaciers melted, water carried the clay and silt particles into the fjords at the sea. There the particles fell out of suspension in the water to form muds on the bottom of the sea. Chemical reactions between the muds and the salt in the seawater strengthened the muds—a process rather like the setting of freshly poured, watery cement. Later, after the weight of the glaciers had been removed, Scandinavia rebounded upward, elevating these cemented muds as much as 600 feet above sea level.

Elaine Thompson/AP Images

Ad Right

Over the next thousands of years, more reactions between the excavated muds and rainwater transformed the top 20 feet of these clays into a stiff layer suitable for building and farming. At the same time, groundwater circulation deeper in the deposits leached the salt out of any seawater that had been trapped in the pores of the clays. And therein lies the rub, and the problem for the hapless farmer: When the salt concentration of water in the pores of clay-rich sediments drops below a critical threshold (to about 0.1 ounce per gallon of water), the clay takes on an amazing character. At one moment it can be strong, almost like a solid, but the next moment, under certain conditions of stress, the state changes nearly instantaneously into a liquid. This liquefied material, “quick clay,” resembles its more famous relative, quicksand, except that the particles are smaller. In Norway the areas underlain by quick clay constitute some of the best farming land, so they are heavily populated.

In order to construct a new wing on his barn, the farmer had removed 900 cubic yards of soil and piled it aside at the shore of Lake Botnen. That’s when disaster struck. The slight redistribution of weight by the farmer from the barn area to the shore of the lake triggered a slide of over 7 million cubic yards. The relatively small volume removed by stockpiling paled in comparison to the amount of material in what is now known as the “Rissa slide” by a ratio of about 8,000 to 1. During several pulses, material flowed inexorably toward the lake over a period of five minutes. A witness said that one pulse “came towards me like a sea wave.” Immediately after this pulse stopped, a new and larger flake broke off, and a video of the event shows the farm floating down the quick-clay river at a speed of over 20 miles per hour. Forty people were caught in the slide, one died, and seven farms and five homes were destroyed or abandoned.

Many regions formerly occupied by ice-age glaciers are vulnerable to quick-clay slides, particularly in Norway, Russia, Finland, Sweden, the United States (Alaska), and Canada. Even though quick-clay slides move fairly slowly in comparison to some other landslides, they are often deadly because they can occur without warning. A quick-clay slide in Saint-Jude, Quebec, in 2010 happened so suddenly that the members of one family died where they had been sitting: watching an ice hockey game on television.

During the Rissa slide, fine-grained materials flowed down a gentle slope. At the other extreme, some landslides consist mainly of huge rocks that roar down steep slopes. The small village of Elm sits below a mountain in an area of Switzerland where slate—a rock that fractures nicely to expose flat surfaces that make great blackboards—was quarried with explosives for decades. The introduction of mandatory public education in the mid-19th century greatly increased the quarrying activities. In the late 1800s, cracks developed in the mountain, but the quarrying was too lucrative to give up in spite of these ominous signs, and so it continued—even after an exceptionally large crack opened in the rocks above the village in 1876.

For five years not much happened, but on September 11, 1881, after two months of heavy rain, a giant slab of the mountain disintegrated. For about 20 minutes, rock rained down on the village of Elm, culminating in 10 million cubic yards of rock roaring into the valley, across its floor, and 300 yards up the other side. One hundred and fifteen people were killed.

A Diversity of Form

The events at Lake Botnen and Elm illustrate a basic truth. Landslides are like people: Every one is different. They occur in settings ranging from jungles to deserts. The materials involved range from mud to rock to ice, including mixtures of all three. Some slides are wet; others are dry. Some roar down steep mountainsides; others creep along at barely perceptible rates. Some are so rigid that they are truly “slides”; others, so fluid that they are best described as “flows.” But the motion of landslides, small or large, is always governed by the conservation laws of physics.

Even when a hillside is static and stable, gravity is tugging everything downward. Usually, hillsides are strong enough to resist the pull, but all materials have zones of weakness, and when one of these zones fails, disaster looms. Conservation of momentum in the form of Newton’s second law (force = mass × acceleration) determines the details of the resulting slide motion. This deceptively simple equation is actually very complicated because many forces act on a landslide. Specifying them all and adding them together is usually difficult.

Most landslides do not tumble down vertical cliffs but slide down slopes, sometimes very gentle slopes. The force of gravity driving them is reduced by an amount proportional to the slope. Gravity also has a highly effective opponent: friction. At the base of the slide where it moves across the ground, friction between the slide and the ground opposes the motion just as it slows a book when you try to slide it across a table. And within the slide, materials shear and deform, creating friction internally. The balance between gravity driving the slide downhill and friction resisting this motion determines the speed of the landslide and how far it will travel; the distance that a landslide travels is called its runout. The wide variety of landslide settings and material properties results in wildly different forms from one landslide to another—with wildly different human consequences as well.

Reliable statistics on landslide fatalities are difficult to obtain, but by conservative estimates, nearly 90,000 people died in landslides between 2002 and 2012. During these years, two catastrophic slides caused by earthquakes accounted for a large fraction of the fatalities. The 2005 magnitude 7.6 earthquake in Kashmir killed at least 30,000 people; the 2008 magnitude 7.9 Szechuan earthquake killed at least 25,000. Historical, earthquake-induced landslides have killed hundreds of thousands at a time in China and the Far East, and destroyed vast areas of agricultural land. Even when earthquake-induced landslides are removed from the statistics, an average of 4,000 people per year are killed in landslides.

Landslides descending from hillsides into river valleys often block the river channels, forming a dam behind which the water from the river ponds to form a “quake lake” that can also be deadly. In 1786 a magnitude 7.8 earthquake in southwestern China triggered a landslide that blocked the Dadu River. About 100,000 people drowned in the flood. Although the United States has far fewer fatalities from landslides than does China, the annual cost of landslides here amounts to billions of dollars.

Geology + Weather = Human Danger

Slopes develop a predisposition to landslides because of several processes. At the surface of the Earth, weathering produces a soil overlying the rocks. Vegetation holds the soil together; changes in vegetation, either natural or human-caused, can destroy the root systems and weaken the soil. Often there is activity at the base of a slope, such as erosion by a river or waves from an ocean, or land modification by humans. All of these processes predispose a slope to fail and slide when a trigger occurs, but by far the most frequent triggers are rainfall and earthquakes.

Rahmat Gul/AP Images

Nowhere in the United States does the effect of rainfall on landslides seem to impact so many as in California. When I was a graduate student at the California Institute of Technology in the 1960s, we loved to take a spin up the spectacular, cliff-hanging California Highway 39 that ascends through a 30-mile stretch from Azusa to the crest of the San Gabriel Mountains. It was fairly isolated then, but now nearly 3 million people a year drive this highway. In addition to providing a thrill for joyriding tourists, the highway serves residences of 500 people and access to three flood control dams, as well as to the Angeles National Forest for firefighters. Here, as everywhere else in southern California, brush fires occur frequently during the dry seasons. These fires destroy the vegetation that holds the soils together. If heavy rainy seasons follow a big fire season, landslides frequently cover the highway. Maintenance costs its owner, Caltrans, $1.5 million a year.

A quick-clay slide in 2010 happened so suddenly that the members of one family died where they had been sitting: watching an ice hockey game on TV.

The devastating and ongoing consequences of even small landslides are well illustrated by events in La Conchita, a small town covering 28 acres in Ventura County, California. It sits on low land abutting, to the east, a cliff that rises 500–600 feet above the town. At the top of the cliff, a ranch of avocado and citrus trees extends inland for about a half mile. To the west, waves from the ocean lap at the land. It is the perfect setting for a collision of geology, weather, and humans.

In 1995, the ground failed along a fault plane about 100 feet deep. A slow landslide, moving at a few yards per minute, buried nine homes but caused no casualties. During a period of heavy rain in 2005, part of the 1995 slide mobilized again, this time moving tens of feet per second, destroying 13 houses, and killing 10 people. The slide buried four blocks of the town with more than 30 feet of earth, a total volume of nearly 2 million cubic yards of material. USGS scientists estimated that liquefaction occurred “nearly instantaneously.”

Townspeople have sued the avocado ranch numerous times. Lawyers for the residents argued that irrigation practices on the ranch caused the slide because irrigation raised the water levels in the hillslopes, weakening them and setting up conditions for the slides. Lawyers for the ranch argued in return that the irrigation couldn’t have been a factor, because there was a history of landslides in this area dating back to at least 1865, when a wagon trail had passed through this part of the coast, well before extensive agricultural watering began. The lawyers for the ranch also cited a study that showed a strong correlation between intense rainfall and landslides in southern California, and then noted that the La Niña conditions of 1995 had dumped 18 inches of rain on this area, making it the wettest season ever recorded. The 2005 slide also occurred during wet conditions, at the end of a 15-day period of near-record rainfall.

Image courtesy of the U.S. Geological Survey.

After years of legal battles back and forth, the assets of the ranch plus a cash settlement of $5 million were turned over to the plaintiffs. In an interesting political and ethical decision, Ventura County rejected pleas of homeowners to stabilize the hillside, arguing that no one should be living in this unsafe area and that any intervention would put the county at risk of future lawsuits. Subsequent research has shown that the 1995 and 2005 slides are part of a much larger prehistoric slide that includes the ranch on the bluffs. Future slides are inevitable.

Although rains are a frequent trigger of landslides, sometimes there is no obvious immediate trigger. The lack of a clear trigger is most common with larger landslides. An earthquake in 1994 shifted a mass of land near the village of Attabad in northern Pakistan, and earthquakes over the next 15 years shook the region, causing minor damage but no major landslides. Then, without warning, in January 2010 a landslide of nearly 40 million cubic meters in volume struck, damming the Hunza River, killing 14 people, stranding about 25,000, and inundating 12 miles of the Karakoram Highway, a major trade route between Pakistan and China. Later that year, months of heavy rainfall triggered more flooding and landslides, affecting over 20 million people, killing 2,000, injuring 3,000, and causing $20 billion in damages.

The situation was even more dire in China in 2010, when floods created especially destructive mudslides there. One such slide, the August 8 Gansu mudslide, was triggered when a debris dam that blocked a small river failed. A five-story-high muddy flow roared through downstream villages, killing more than 1,500 people. Floods and landslides in China that year affected more than 230 million, with over 15 million being evacuated. Even when the torrential rains and high rivers abated, conditions failed to improve, because enormous piles of quick mud were left behind, impeding and endangering rescuers. The United Nations secretary-general pointed out that the number of people directly affected by this flooding in China exceeded the entire population hit by the Indian Ocean tsunami, the Kashmir earthquake, Cyclone Nargis, and the earthquake in Haiti—combined.

Flying Slides and Alien Worlds

Despite the importance of rain and earthquakes as triggers, a landslide can occur at any time. All rocks are riddled with microscopic flaws and fractures. Stresses in rocks are concentrated around the tiny flaws and at the tips of the microscopic fractures. Stresses are always changing on rocks and slopes, not just when there is rain or an earthquake. For example, the normal tides in the oceans caused by the changing positions of Earth, the Sun, and the Moon also occur in the solid earth, giving rise to continuously changing stresses. Natural or human-caused variations of water levels in reservoirs, such as those behind a dam or in natural lakes, also change the distribution of forces on a slope.

One intriguing hypothesis is that landslides can be transported like flexible sheets over a cushion of trapped and compressed air—like a flying carpet.

When a new stress first appears, old imperfections absorb some of the stress and new, isolated microscopic fractures might form. At first nothing dramatic happens, because the old and new microcracks are isolated from each other. But when the overall stress exceeds a critical value, local stresses at the flaws become large enough to break molecular bonds, and the microcracks propagate. The cracks take off running, so to speak, merging to form an ever-larger network of cracks. Ultimately, and usually catastrophically, they grow into the huge surface along which a landslide moves.

Approximately 17,000 years ago, a volume of rock equal to a cube about a half-mile on a side roared out of a steep canyon in the San Bernardino Mountains in southern California. It originated 1,500 feet above the canyon bottom. Rocks in the slide, already fractured at the start of the event, shattered on impact with the canyon bottom, forming intricate three-dimensional jigsaw puzzles. When this event, known as the Blackhawk slide, exited from the canyon, it ran out across a nearly flat valley floor for five miles. Amazingly, the pieces of the jigsaw puzzles stayed together as the slide zoomed along at nearly 75 miles an hour. A similar landslide triggered by the 1964 Alaska earthquake traveled three miles across the nearly level Sherman Glacier before coming to rest. Where the base of the landslide could be seen on the glacier, it rested on—believe it or not—undisturbed snow. In other places it left alders, mosses, and small plants completely undisturbed.

The observations from Elm and Alaska, and geological evidence from the Blackhawk landslide, led to the intriguing hypothesis that landslides can be transported like flexible sheets over a cushion of trapped and compressed air—literally like a flying carpet. Imagine such huge masses of rock roaring down a canyon, hitting a resistant ledge, being launched hundreds of feet into the air, settling back onto a blanket of compressed air only a few feet thick, and then hurling out at breakneck speeds onto the desert floor until the air leaks out and the slide gently glides to a halt, the whole event taking perhaps a minute or two. In this “air lubrication” hypothesis, the landslide floats as a nearly rigid slab on its cushion of air, so fragile jigsaw pieces of rock such as those observed at Blackhawk are preserved.

Image courtesy of NASA.

Although the air lubrication hypothesis may work for specific slides, two arguments suggest that it cannot explain all long runouts. First is the question of whether the air can stay trapped under the slide long enough for the runout, because it would tend to diffuse through the slide and around its edges. The second problem, which became obvious as a result of unmanned spacecraft looking at the planets and their satellites in the Solar System, is that long-runout landslides occur on our Moon and on four other moons in our Solar System that have no atmospheres—Io, Callisto, Phobos, and Iapetus. Such slides also occur on Mars, which currently has only a very thin atmosphere, although it is not known what the atmosphere was like when the landslides formed in the past. Long-runout slides on airless or nearly airless bodies have forced geologists to look at explanations other than air lubrication.

One group of theories takes into account the fact that landslides are not monolithic slabs of rock, but consist of rock fragments of many different sizes. They fall into the broad category of materials called “granular matter” that have unique properties. The cereal in your breakfast bowl provides an example. Sometimes these materials behave very much like a solid, and other times they flow like a liquid. Grains can flow, slosh, and reflect from boundaries like a liquid, they can erode channels just like flowing water, and in some instances they can produce hills and gullies that mimic features formed by flowing water.

Increasingly, evidence has been mounting that lubrication is enhanced also by liquid water, ice, wet debris, or mud at the base of the slide, or perhaps water within the slide. Even if landslides are not saturated with water, they are unlikely to be completely dry; they will always contain some liquid water (on Earth) or ice (on Earth and the other planets or satellites). Water is effective at lubricating debris and mudflows, such as the recent ones in Washington state and Afghanistan (see page ). Sometimes water on the surface of the Earth even provides a layer over which a landslide hydroplanes like a boat.

Even these possibilities do not exhaust the ideas proposed for long-runout slides. By studying landslides at these many scales, geoscientists have come up with such a bewildering array of proposals for how they move that it may sound as if we simply don’t know what we are talking about. In truth, some of the processes proposed almost certainly occur some of the time in some of the landslides, and not all of the processes occur all of the time or in all places. The large number of hypotheses and mechanisms proposed is testimony to the awesome complexity of our world.

Natural disasters of all kinds result from changes of state in a system through modification of its materials, or changes in the forms of its energy and motion, or both. Landslides are a perfect example of such changes of state. The materials involved are soils or fractured rock that consist of individual grains or particles separated by pores or fractures. On a stable slope, the pressure of individual particles on each other where they are in contact holds the materials together. Pores and fractures are filled with air, or with a bit of water.

Ted S. Warren/AP Images

Periods of heavy rain, such as those preceding all four recent landslides, change the properties of the slope materials. As rain soaks into the ground, the voids fill with water that reduces the pressure on the contact points, and hence the strength of the materials. Sometimes the water also wets unstable clays in the soil. Under such conditions, normally strong materials can turn to mush nearly instantaneously, a change of state called liquefaction. In the liquefied state, material has unusual mobility, running out at high velocity over great distances, even uphill. The local sheriff in Colorado reported that the slide traveled up and over a hill during its long runout.

The recent events underscore that each landslide has its own characteristics. The Oso event appears to have contained fine-grained sand and clay that form a nasty substance called, depending on grain size, “quick sand” or “quick clay.” Quick clay was also the cause of the Elm landslide of 1881. The Afghanistan slide, on the other hand, seems to have involved loess—sediment composed of angular, wind-blown dust particles—which is highly porous and has weak cohesion. Scientists understand a lot of these details about how landslides work, but understanding is not enough. We also need to make sure that updated geologic information is incorporated into public understanding and policy on local, national, and global scales.

For more news on disasters as they occur, visit the author’s blog:

(Adapted from The Dynamics of Disaster by Susan W. Kieffer. Copyright © 2013 by Susan W. Kieffer. Reprinted with permission of the publisher, W. W. Norton & Company, Inc. This selection may not be reproduced, stored in a retrieval system, or transmitted in any form by any means without prior written approval of the publisher.)