WASHINGTON — New research has found that 6.5 million Americans live in homes on or near landslide-prone slopes. In a single map, local governments and residents can now see landslide hazard hotspots at the regional, state, and county level to make sure they are aware and prepared for an emergency.
"We have never considered the human geography of landslides; this is really the first attempt to do that," said Joseph Wartman, a civil engineer at the University of Washington and co-author of the study.
Although much of the U.S.'s steepest terrain is in the west, the study finds that over half of the people exposed to landslides live closer to the east coast, simply due to higher population densities in that region. The researchers also found that people in rural areas have less capacity to recover than their urban counterparts.
The study will appear Wednesday, 29 July in Earth's Future, AGU's journal for research on the state of the planet and its inhabitants, sustainable and resilient societies, the science of the Anthropocene and predictions of our common future.
Mapping the human side of landslides
Unlike other natural disasters like earthquakes, hurricanes and wildfires, which tend to impact large areas, a single landslide might only affect a small part of the landscape, leaving an adjacent hillside untouched.
"The susceptibility at one location can be vastly different than the susceptibility at a neighboring location just 10, 20 or 30 meters away," Wartman said.
To capture the hazard to people on this fine scale, the study combined maps of landslide-susceptible areas from the United States Geological Survey, spatial footprints of every recorded building in the U.S. and nighttime building occupancy estimates to quantify how many people are inside buildings that could be struck by landslides. Using nighttime occupancy counts the population when they are least able to react to a hazard and move to safety but doesn't consider movements throughout the day, for example tourists flocking to the steep, scenic slopes of the Smoky Mountains. The study also did not account for landslide frequency, which tends to be higher in the west.
Although the Rocky Mountains, Alaska, and Hawaii encompass plenty of steep slopes, these regions account for just 4% of people living under landslide hazard because the population is so sparse. By contrast, the flatter but more densely populated interior plains, including states like Nebraska, Minnesota and Illinois, account for over 700,000 landslide-exposed people.
"This is really a template that we should be able to duplicate in other nations and other parts of the world," said Daniel Acosta-Reyes, a civil engineer at the University of Washington and the study's lead author.
The urban-rural divide
The researchers also captured people's ability to recover by layering in demographic and economic data from the U.S. Census Bureau. Wealth can help people bounce back after a landslide. However, factors like poverty, disability, living in a very isolated area or low English proficiency can make recovery more difficult, as well as limit access to resources like warning systems and aid networks. When three or more of these factors impact a census tract, the U.S. Census Bureau identifies that population as vulnerable after a natural disaster.
"Technically, a landslide is not insurable, so if you lose your home in a minor landslide event, that's going to be a personal expense and a catastrophic loss for the homeowner," said Ben Mirus, a research geologist at the USGS who was not involved with this study. In general, rural residents — who account for 62% of people living in landslide territory — are less able to recover from such major losses after a disaster.
"We have found that this urban-rural divide plays out across the entire United States," Wartman said. Urban areas tend to be better resourced, and cities in hilly regions often add housing by building upwards in flat areas rather than outwards into the mountains. Cities and towns also tend to alter the landscape around them as they grow, helping stabilize nearby slopes.
The largest rural, landslide-vulnerable population is in the Appalachian Highlands from northern Maine to central Alabama. In places like West Virginia, where 70% of the state is landslide terrain, flatter land outside towns and cities tends to be more expensive, driving low-income people into steep regions. These households are less likely to have the funds to rebuild after a landslide. Here, Wartman and his colleagues suggest local investment in resilience infrastructure like early warning systems as well as access to state and federal funding for recovery efforts.
Where urban populations interact with landslide terrain, the picture often differs. Steep hillsides around cities like San Francisco and Seattle attract high-income residents seeking scenic views despite the greater landslide hazard. For these groups, the study recommends that cities use regulations to deter building in landslide-prone areas and leave more of the costs of landslide recovery with homeowners who can afford it.
