The mile-long mudslide that buried homes along a bend in the Stillaguamish River near Oso, Wash., some 55 miles north of Seattle, leaving at least 14 dead and more than 100 missing, occurred at a site that was known to be landslide-prone.
Much-heavier-than-normal rainfall had been recorded for the month before the March 22 slide in the hills above the site, which for decades regularly has experienced landslides, albeit on a smaller scale.
That such a disaster could occur – the death toll is expected to rise as rescue workers hunt for the missing – even with that history highlights the challenge faced by researchers who are trying to refine ways to assess landslide risks and provide timely warnings when conditions are ripe to trigger landslides and mudslides.
It's a daunting challenge but one with a potentially significant pay-off, say scientists involved in the effort. Landslides add an average of more than $2 billion a year to the nation's annual disaster-recovery bill and lead to between 25 and 50 fatalities a year, according to the US Geological Survey (USGS).
Moreover, in the face of climate change, landslide hazards are likely to increase where rainfall intensifies over regions with already slide-prone slopes, researchers suggest.
At the moment, however, no detailed landslide-hazard map exists for the country as a whole. A prototype map researchers offered up two years ago takes a first step by showing the relatively few locations where the hazard is virtually nonexistent. No national database exists of past landslides and the variety of conditions under which the earth moved. And the nature of the slides themselves vary widely – from the catastrophic release of a hillside, as happened near Oso, to the slow creep of an 82-acre landslide in 2011 near Keene, N.Y., on the slopes of Little Porter Mountain in the Adirondacks.
"There's a whole zoo of different kinds of landslides," says David Montgomery, a geologist at the University of Washington in Seattle.
The slide that buried homes outside of Oso was one of a series of slides at that location dating back to the 1950s, says Dr. Montgomery.
"You can imagine it as a slide that's been failing in pieces that eat farther back into that hill about once a decade," he explains. "This current one took off a bigger piece of the hill and went farther faster."
During the winter, western Washington experienced unusually heavy rains, with totals topping 12 inches above normal during the past month, according to Cliff Mass, a meteorology professor at the University of Washington. A weather station in the hills above the slide zone recorded 24 inches of rain for the month.
Large, deep-seated slides such as this one cut loose in response to a thorough, persistent soaking, rather than a single sudden downpour, Montgomery explains, adding that "it leads to this odd phenomenon that on a nice day, a whole hill can come down."
Essentially, the water percolates down through the deep layers of glacial soil to the water table, raising it and allowing the water to saturate the deep soil. In effect, this increases the buoyancy of the hillside, counteracting part of its weight and making it easier for gravity to pull it down the slope.
Although researchers have been chipping away at the landslide hazard and warning problem since the mid-1980s, when one of the first systems was set up in Utah, the move to develop operational warnings at the federal levels emerged in 2005. That year, the National Oceanic and Atmospheric Administration and the USGS published the results of a joint study exploring the feasibility of providing warnings for a type of landslide known as debris flows. These tend to be triggered by heavy rain or rapid snow-melt and drive a slushy mix of mud and stone downhill.
They are especially troublesome in regions prone to wildfires, which can destroy vegetation that might otherwise hold the soil in place. The National Weather Service forecast offices in Oxnard and San Diego, Calif., have been combining precipitation forecasts with maps of burn zones to provide warnings for debris flows.
During heavy rains that hit southern California just before the Oscar ceremony in Hollywood in early March, forecasters were able to give emergency managers in Glendora and Monrovia, at the base of the San Gabriel Mountains, enough lead time to evacuate residents. Mud flowed, and while some homes were lost, no lives were lost, according to Eric Boldt, warning coordination meteorologist at the forecast office in Oxnard.
Landslide warning systems for shallow slides, where the depth of soil moving is 10 feet or less, have been set up for Seattle, western Oregon, and the Appalachians.
But the deep slides, such as the one that hit outside of Oso, remain problematic.
If natural-hazard specialists know of a particularly vulnerable area – a roadway set into a hillside or a community at the base of a hill or mountain – they can devote resources to monitoring it with a range of well-established tools, says Jonathan Godt, a researcher with the USGS's landslide hazards program in Denver.
But monitoring for slides on a state-wide or even regional scale "is really difficult, especially in places like Washington that have all this terrain that has landslide potential, at least at some point," he says.
Remote-sensing techniques can help. For instance, researchers at NASA's Jet Propulsion Laboratory (JPL) in Pasadena and the University of California at Berkeley have been using a form of radar known as interferometric synthetic aperture radar to track the movement of slow-moving landslides – about 7 millimeters a year – in the Berkeley hills.
The creep is hardly dramatic, but over time it can have serious consequences for water mains, sewer pipes or building foundations, says Eric Fielding, a geologist at JPL who has been working on the project. And the pace can accelerate when heavy rains arrive.
The airborne radar the researchers used, however, isn't sufficient to provide regular, repeated coverage of a continent to identify places with ground on the move.
Help may be on the way, however, from a new generation of satellites that carry the interferometric synthetic aperture radar.
On April 3, the European Space Agency is set to launch Sentinel-1A, the first of two satellites carrying the radar, which can detect ground movements of a few millimeters. In May, Japan is launching its own radar satellite for land observations.
And last week, NASA gave Dr. Fielding and colleagues the green light to begin formulating a synthetic-aperture-radar mission for a joint US-India operation that is still years away from launch. Although the European satellite can capture larger landslides, the US-India mission is being designed to detect much smaller slides and their precursor movements as well.
The Japanese satellite aims to overfly a given location every 14 days, rather than every 46 days, as its predecessor had done. When Europe's two satellites are operating together, the same spot will be recorded every six days.
"Forty-six days is kind of a long time to wait if you want to know if the hill behind your house is unstable," Fielding says.
"We're really looking forward to these new systems."