When it comes to earthquakes in the United States, most people assume the dangerous shaking is confined to California.
But a new study published in Earthquake Spectra Monday found that the numbers of people at risk of potentially dangerous temblors are much more widespread across the United States than previously thought.
It found that 143 million Americans living in the 48 contiguous states are at risk of ground shaking from earthquakes. When you include earthquake-prone areas of Alaska, Hawaii and U.S. territories are included, the number rises to nearly half of all Americans.
"The new exposure estimate is nearly double the previous 2006 estimate of 75 million Americans in 39 states, and is attributed to both population growth and advances in science," said William Leith, senior science advisor for earthquake and geologic hazards at the U.S. Geological Society and a co-author of the study. "Populations have grown significantly in areas prone to earthquakes, and USGS scientists have improved data and methodologies that allow for more accurate estimates of earthquake hazards and ground shaking."
The figure would even be higher, the scientists said, if they had included earthquakes that have been linked to the oil and gas industry in places like Texas and Oklahoma. But maps reflecting those induced earthquakes are not ready.
When looking at the highest populations exposed to the strongest shaking California leads the way followed by Washington, Utah, Tennessee, Oregon, South Carolina, Nevada, Arkansas, Missouri and Illinois.
While the study found tens of thousands of schools and hospitals would be a risk, it cautioned that the chances of a damaging quake are still quite rare in many places. Overall, only 28 million people could experience violent shaking in their lifetime.
"The key is that 143 million people live in places where large earthquakes have occurred and ground shaking could be high if those earthquake occur again," Mark Petersen, a co-author on the paper who is also the chief of the national seismic hazard model project at the USGS, told CBS News. "But they don't occur very often."
The difference between those areas at risk from moderate versus strong shaking depends on a range of factors, including the location of fault lines and the seismicity rates of the area.
The USGS shaking calculations consider the chance of an earthquake occurring in a 50-year time frame, since that is the typical lifetime of a building. This time frame is thought to be reasonable for life-safety considerations when designing buildings and other structures.
The new estimates are derived from the recently updated U.S. National Seismic Hazard Maps, which identify where future earthquakes will occur, how often they will occur, and how strongly the ground will likely shake as a result. Researchers analyzed high-resolution population data and infrastructure data to determine populations exposed to specific levels of earthquake hazard. The population data are from LandScan, and the infrastructure data are from the Homeland Security Infrastructure Program (HSIP) database.
"This new research helps us better understand the scale of earthquake hazards and ultimately strengthen the nation's ability to protect Americans against future events," Kishor Jaiswal, a USGS research structural engineer as well as the lead author of the study, said. "Of particular concern is the significant amount of critical infrastructure located in high earthquake-hazard areas, ranging from private and public schools to health care facilities and fire stations."