On March 22, 2014, a massive landslide obliterated the town of Oso, Washington, flattening 49 homes in a little more than two minutes, killing 43 people. The news was shocking, but this kind of catastrophe is far from an isolated incident, as the PBS "Nova" special "Killer Landslides" underscores by including examples from Afghanistan and Nepal. What causes landslides, and is it possible to prevent them? We posed these questions and others to David Montgomery (pictured), a geologist at the University of Washington, author of "The Rocks Don't Lie: A Geologist Investigates Noah's Flood" and the expert who appears in the Nov. 19 special.

Dave Montgomery

MNN: Landslides seem to be more prevalent worldwide. Why is this?

David Montgomery: Landslides are a common process in steeplands around the world — they have been shaping mountains for millennia. But are they occurring with greater frequency today, or are there now more places where people are living in their path? Or do we simply hear about more of them in our increasingly interconnected world? Those are hard questions to answer without comprehensive regional landslide studies that extend back in time. But an increase in the intensity of storm events would indeed translate into more landslides.   

What are the main factors that converge to make the Earth prone to landslides? 

Steep slopes, weak geological materials, and heavy rains, are natural factors that contribute to high potential for landsliding. Human modifications to the land surface, through things like road building, timber harvest, and drainage changes that concentrate runoff onto potentially unstable slopes also greatly contribute to the potential for landsliding. But there are many different types of landslides, some of which move rapidly and other more slowly, and the factors that influence them vary to some degree by landslide type.   

How has technology taught us more about how they work and how to predict them? What innovations and experiments enable you to understand how they work, and what can we do to extrapolate from them and apply that to real situations?

The high-resolution topography generated by LIDAR (Light Ranging and Detection) surveys has given geologists brand new glasses in terms of being able to read the landscape for signs of past landsliding. And INSAR  (Interferometric Synthetic Aperture Radar) data allows detecting the movement of even slowly creeping landslides. Experimental facilities such as the U.S. Geological Survey's debris flow flume have greatly increased our understanding of the mechanics of landslide initiation and run-out. The challenge now is how to integrate such advances into a new generation of landslide hazard and risk assessment programs in a time of serious budgetary constraints at all levels of government. My students, colleagues and I have been working with LIDAR data on analyzing landslides in a variety of settings, focusing on both how landslides shape topography over geologic time and on how such data influences our ability to identify landslide hazards and better assess risks.  

Do you think we will be better able to predict landslides via an early warning system?

There are examples of warning systems that have been used to identify when a region is approaching rainfall totals that are predicted to be capable of generating landslide activity. They can give warning of when one might expect landslides to occur in a region. And there are monitoring systems that have been used to track the ongoing movement of existing landslides, and thereby try to predict their reactivation. But it is quite difficult to predict which potentially unstable slope(s) will fail in a future storm event. There are models to predict the location of slopes particularly prone to slope failure (high hazard slopes), but accurately forecasting which of them will fail in a particular storm is very difficult due to generally incomplete knowledge of the subsurface hydrological conditions in a complex natural slope.  


What messages do you hope viewers of the documentary take away?

I would hope that viewers will take away a greater appreciation for the potential for rapid and unexpected earth movement in geologically unstable terrain, and for the role of landslides in shaping mountain ranges. We will never be able to prevent all landslides from happening, but the more people are aware of the nature of the hazard — and what we can and cannot do about it — the greater the likelihood of reducing the impacts from catastrophic landslides when they do happen. I would hope that viewers of the documentary would be better able to recognize potential landslide hazards — and when to seek geological advice — after watching the program.  

What can people who live in landslide-prone areas do?

Become aware and educated about the potential for slope instability; learn how to look for signs of incipient failure and to identify the types of conditions associated with high landslide hazards —and to consult with a landslide geologist if there is reasonable suspicion of landslide potential. And, by far, the best way to prevent landslide disasters is to not have anyone living in their run-out path, although with appropriate (and sometimes expensive) hazard mitigation measures the risks can be substantially reduced.

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Photo of Dave Montgomery courtesy of WGBH.