An estimated 159.2 million Americans live in areas exposed to "very strong" earthquake shaking capable of damaging buildings, according to a new study using the latest US Geological Survey (USGS) hazard model. That figure is up from 111.8 million under the agency's previous model.
The assessment places 47.3 million homes and $25.6 trillion worth of buildings in potential danger. At the lower, but still potentially destructive, level of "strong" shaking, the number of Americans exposed rises to approximately 234 million, with about 73 million homes and $38 trillion in buildings within the affected areas.
California remains one of the country's most notorious earthquake hotspots, but the study found the threat extends far beyond the West Coast. Danger has increased around the New Madrid Seismic Zone in the central United States, and heavily traveled bridges make New York, Atlanta and St Louis unexpectedly vulnerable to severe disruption. Researchers warned that a damaged crossing could cut neighborhoods off from hospitals, delay emergency crews and paralyze traffic long after shaking stops.
The figures do not mean earthquakes have become more frequent. They show that improved scientific models have revealed far more people, homes and infrastructure may be exposed than previously understood.
How the risk was measured
Researchers measured danger using a benchmark with a two percent chance of being exceeded within 50 years, roughly the lifespan of a building or major piece of infrastructure. That does not mean a devastating earthquake is expected everywhere within the next five decades. Instead, it lets scientists and engineers compare the potential intensity of rare but destructive shaking across the country.
The dramatic rise in estimated exposure largely reflects advances in understanding of how earthquakes begin, how faults can rupture together and how seismic waves travel through different types of ground. For the first time, the 2023 forecast updated hazard calculations for all 50 states simultaneously, incorporating revised earthquake records, improved computer models and new information about faults and underground sediment. Deep layers of sediment matter because they can amplify seismic waves, making the ground shake more strongly than surrounding areas.
The newest forecast does not raise the estimated danger everywhere. Compared with earlier models, the predicted probability of damaging shaking fell across much of the western United States but increased around the New Madrid Seismic Zone, helping explain why earthquake preparedness is not only a California concern.
Bridges and infrastructure at risk
The consequences of shaking depend on more than an earthquake's strength. The number of people, buildings and vital services in its path, and how well they can withstand the movement, determine the ultimate scale of a disaster. A lower-hazard city filled with vulnerable buildings and heavily used infrastructure can face greater disruption than a more earthquake-prone place that has invested in stronger construction.
To illustrate that, the researchers examined more than 600,000 highway bridges with a combined replacement value exceeding $1.5 trillion. Together those bridges carry the capacity for more than 4.5 billion individual vehicle trips each day. The team assessed risk partly by calculating the average annual number of vehicles that could lose access to a bridge after earthquake damage, a measure that accounts for how busy each crossing is and how long it could remain out of service, rather than repair costs alone.
Many bridges facing the greatest physical hazard were concentrated around San Francisco, Los Angeles and the New Madrid region. But the list of bridges posing the greatest disruption risk spread much farther, encompassing the Seattle, Portland, Salt Lake City, St Louis, Atlanta, Charleston and New York metropolitan areas. Some of those cities have lower earthquake hazard, but their heavily traveled crossings and aging infrastructure could magnify the impact of damaging shaking.
The roughly 6,000 bridges ranked in the highest one percent for hazard had an estimated replacement value of $31 billion. Those ranked in the highest one percent for risk were worth approximately $74 billion, a gap the researchers said shows why a map of earthquake probability alone gives an incomplete picture of the danger facing a community. A damaged bridge can sever access to hospitals, prevent emergency crews from reaching neighborhoods and force thousands of motorists onto congested alternative routes, with disruption that can continue long after the shaking stops. The researchers said similar concerns apply to water and gas pipelines, power stations, wastewater facilities and communications towers.
How estimates have shifted over time
The study found that estimates can change substantially as the science improves. Using fixed modern population and infrastructure data, the researchers calculated that the 1996 model would have identified about 10,900 high-hazard bridges, compared with approximately 7,300 under the 2023 forecast.
They cautioned that this should not be read as evidence the country has become safer. Development has continued, buildings have aged and communities have expanded into areas where earthquakes could cause serious losses. Instead, the researchers said, the changes show how new information can reshape understanding of where the strongest shaking may occur. They argued that future planning must weigh vulnerability and exposure alongside earthquake probability, or authorities could overlook places where a comparatively rare event would produce enormous human and economic consequences.
