Some regions of the United States -- specifically along the East Coast and in the Midwest -- are thought to face higher risks for powerful earthquakes than once believed, the U.S. Geological Survey reports.
The risks are revealed in the USGS's recent update of its U.S. National Seismic Hazard Maps, the first since 2008.
While the country's traditional earthquake zones of California, the Intermountain West and the Pacific Northwest are still prominent on the hazard map, areas of the central and eastern U.S. are now showing an increased risk, the USGS is reporting.
Earthquake frequency in those regions has increased five-fold, reaching an average of 100 annually in 2011 through 2013, up from just 20 each year for the three decades leading up to 2000, it says.
"We know the hazard has increased for small and moderate size earthquakes," USGS scientist William Ellsworth says. "We don't know as well how much the hazard has increased for large earthquakes. Our suspicion is it has but we are working on understanding this."
One of the East Coast's strongest temblors in 100 years hit in 2011, a 5.8 magnitude earthquake centered in rural Virginia that was nevertheless sensed by hundreds of thousands, including many residents of Washington, D.C.
Data gathered during that earthquake "helped us understand better ground shaking in the central and eastern United States," USGS seismologist and report lead author Mark Petersen says.
The USGS report listed the states with the highest earthquake risks, including California, Alaska, Montana, Oregon, Nevada, Illinois, Arkansas, Hawaii, Kentucky, South Carolina, Utah, Wyoming, Idaho, Missouri, Washington and Tennessee.
All of them have experienced earthquakes of at least magnitude 6 in historical times, the USGS said.
"While all states have some potential for earthquakes, 42 of the 50 states have a reasonable chance of experiencing damaging ground shaking from an earthquake in 50 years," the report said.
Of significant concern, the report indicated, is the Cascadia Subduction Zone, running from Northern California up to British Columbia.
Powerful earthquakes in similar subduction zones around the globe, including the 9.0 earthquake off Japan in 2011 and an 8.2 magnitude shaking off Chile's coast earlier this year, suggest the southern Cascadia might someday be the site an earthquake as strong as magnitude 9.3, the report authors say.
Although the map highlights the areas of the U.S. considered at highest risk, that doesn't mean any region can consider itself completely risk free, Petersen says.
"These maps are refining our views of what the actual shaking is," he says. "Almost any place in the United States can have an earthquake."