Add in Alaska and Hawaii, and just about half of Americans live in potential earthquake areas. Of them, 28 million live in places with "high potential" for ground shaking and 57 million live in "moderate" hazard zones.
These latest figures represent a major increase over previous estimates showing 75 million people living in 39 states were exposed to such a risk.
Mark Petersen, study co-author and chief of the USGS National Seismic Hazard Modeling Project, said the new estimates are higher in part because of new data showing an increase in overall U.S. population, as well as a concentration of population in high-risk areas, such as California.
Another reason: the addition of more earthquake zones, the result of more advanced research methods honed over many years. These estimates include the Cascadia Subduction Zone in the Pacific Northwest, which was featured in a terrifying New Yorker story last month.
Not surprisingly, people living along the West Coast are the most at risk.
While the Carolinas and the Northeast show up on these earthquake maps, "many of these places are not going to experience a rash of earthquakes," Petersen said. "There are places in the United States where we think ground shaking could be significant and we ought to look at those areas to see if they are built to the safe levels that would correspond to these ground shaking levels."
The report also shows "there are thousands of schools, fire stations, hospitals and other facilities" in areas that have the potential to experience strong ground shaking, Petersen said. While the findings should prompt officials to consider ground shaking in their community planning, he added, "it's also important that people who live in these areas understand they live in earthquake country and they need to understand that they should maybe prepare themselves."
The study doesn't include earthquakes resulting from human activity, so-called "man-made earthquakes" that are typically related to oil or gas extraction. This kind of seismic activity -- mostly in Colorado, New Mexico, Texas, Oklahoma and Arkansas -- can change quickly and often occurs when fluids are pumped back into the ground.
"There will be populations that will be subjected to strong ground-shaking in these states if the pumping continues the way it is," Petersen said.
Researchers restricted the chances of an earthquake happening to a 50-year-time frame, a typical building lifespan. Petersen said building code groups look to USGS for "the best available science on ground shaking levels," and that these new figures should provide some insight on evaluating risk.