Photograph by Scott T. Smith, Corbis
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Most people don’t think of earthquakes when they think of Salt Lake City, but Utah is among the top ten states in terms of people at risk from a damaging quake in the next 50 years.

Photograph by Scott T. Smith, Corbis

If You Think You're Safe From Earthquakes, You May Be Wrong

It’s no surprise that Californians are in danger, but scientists have also flagged residents of Missouri, Tennessee, Arkansas, Illinois, and South Carolina.

More than 143 million Americans live in an earthquake zone, a new analysis shows, though many of them may be surprised to learn that dangerous shaking is possible in the places where they live.

These people face at least some risk of a damaging earthquake by the year 2065, the U.S. Geological Survey reports—almost twice as many as the 75 million cited in a similar study in 2006.

If you focus only on those at highest risk, says seismologist Mark Petersen, of the USGS Earthquake Hazards Program, a co-author of the new report, “we’ve identified that there are 28 million people living where you’d expect to see pretty strong ground shaking in next 50 years. That’s a pretty significant number.”

The states in the most danger danger are California, with the highest number of people at risk, followed by Washington, Utah, Tennessee, Oregon, South Carolina, Nevada, Arkansas, Missouri, and Illinois.  

The new study, published in the journal Earthquake Spectra, is based on an earthquake hazards map of the lower 48 states released by the U.S. Geological Survey (USGS) in 2014. “The guys who created it,” says University of Washington seismologist John Vidale, “make the best survey maps in the world. This is solid work.” Vidale was not involved in the new research.

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But that 2014 map focused only on where quakes are most likely to happen. “This is a new twist,” says Petersen. In the 2014 study, he says, “we were looking at where the ground will shake, but not how that shaking will affect people and structures.” To make the new update, Petersen and his co-authors overlaid their hazards map with a population map from Oak Ridge National Laboratory in Tennessee and maps of where buildings and other infrastructure are located from the Homeland Security Infrastructure Program.

The dramatic jump in the at-risk population from the 2006 study, Petersen says, comes from two factors. The first is simple population growth. The second is that scientists now have a significantly better grasp of the threat from specific faults.

The Cascadia subduction zone, for example, where a huge slab of the Earth’s crust at the floor of the Pacific Ocean triggers quakes as it forces itself beneath the landmass of the Pacific Northwest, was poorly understood a decade ago, Petersen says. Seismologists have also learned much more about the behavior of the Wasatch Fault underlying Utah and the New Madrid fault, which caused three devastating earthquakes in eastern Missouri, western Kentucky, and Tennessee in the early 1800s. They even have a better handle on the infamous San Andreas Fault, which caused the great San Francisco earthquake of 1906 and which has been under surveillance more or less ever since.

What the hazards map leaves out, says Petersen, is earthquakes caused by “induced seismicity”—that is, quakes caused by humans as we pump wastewater from fracking or carbon dioxide deep underground. “We’ll be studying that in the future,” Petersen says “but it changes  so rapidly that it’s difficult to incorporate into our hazard maps.”

Even without those human-caused quakes, the new report is likely to be invaluable to those who anticipate and respond to disasters. “All of us, in our profession take these sorts of things very seriously, and use them to inform a whole range of our activities,” says Patrick Massey, Federal Preparedness Coordinator of the Federal Emergency Management Agency’s (FEMA) Region 10, which covers Alaska, Idaho, Oregon, and Washington.

That region is even now gearing up for a major exercise in 2016 called Cascadia Rising, which will be based on a hypothetical 9.0 magnitude Pacific Northwest earthquake followed by a major tsunami. “We’ll have emergency response centers from counties, cities, Native American tribes, federal and state agencies all working together on this,” Massey says. The exercise was in the works long before the new report was released, but, he says,“our understanding of risk is always based on the science.”

So while the USGS report won’t be incorporated in this particular exercise, it will be crucial in planning future exercises, here and elsewhere.

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