Nearly half of all Americans could face violent earthquake shaking capable of damaging buildings, according to an alarming new study.
Researchers found that 159.2 million people live in areas exposed to ‘very strong’ shaking under the latest US Geological Survey (USGS) hazard model, up from 111.8 million in its previous version.
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 soars to approximately 234 million.
While California remains one of the country’s most notorious earthquake hotspots, the study found that the threat extends far beyond the West Coast.
Danger has increased around the New Madrid Seismic Zone in the central US, while 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 the shaking stops.
The figures do not mean earthquakes have suddenly become more frequent, but show that improved scientific models have revealed far more people, homes and vital infrastructure may be exposed than previously understood.
Researchers found that 159.2 million people live in areas exposed to ‘very strong’ shaking under the latest US Geological Survey (USGS) hazard model. Above is an AI-generated stock image
The researchers measured the 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 allows scientists and engineers to compare the potential intensity of rare but destructive shaking across the country.
The latest model indicates that 234 million people could be exposed to at least ‘strong’ shaking, while approximately 73 million homes and $38 trillion in buildings stand within the affected areas.
At the more dangerous ‘very strong’ level, shaking can become powerful enough to damage buildings, particularly those that are older or were not designed with earthquakes in mind.
The dramatic rise in estimated exposure largely reflects advances in scientists’ 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 the hazard calculations for all 50 states simultaneously. Researchers incorporated revised earthquake records, improved computer models and new information about faults and underground sediment.
Deep layers of sediment are important because they can amplify seismic waves, causing the ground to shake more strongly than surrounding areas.
Danger has increased around the New Madrid Seismic Zone (center, right) in the central US, while heavily traveled bridges make New York, Atlanta and St Louis unexpectedly vulnerable to severe disruption
The study found that the newest forecast does not increase the estimated danger everywhere.
Compared with some earlier models, the predicted probability of damaging shaking fell across much of the western US but increased around the New Madrid Seismic Zone in the central US.
This helps explain why earthquake preparedness is not only a California concern.
The consequences of shaking depend on far more than the strength of an earthquake.
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 therefore face greater disruption than a more earthquake-prone place that has invested in stronger construction.
To demonstrate that difference, the researchers examined more than 600,000 highway bridges with a combined replacement value exceeding $1.5 trillion.
Together, those bridges have the capacity to carry more than 4.5 billion individual vehicle trips each day, making them essential to ordinary travel and emergency response.
The team assessed risk partly by calculating the average annual number of vehicles that could no longer use a bridge following earthquake damage.
The map shows how estimated earthquake danger has changed across six versions of the National Seismic Hazard Model. Red areas have remained at high risk or become more hazardous, blue areas have remained at low risk or become less hazardous, and gray areas show no clear change
That measure considers how busy each crossing is and how long it could remain out of service, rather than focusing solely on repair costs.
Many bridges facing the greatest physical hazard were concentrated around San Francisco, Los Angeles and the New Madrid region.
However, 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 experience lower levels of earthquake hazard, but their heavily traveled crossings and aging infrastructure could magnify the impact if damaging shaking occurred.
The roughly 6,000 bridges ranked within the highest one percent for hazard had an estimated replacement value of $31 billion.
By contrast, those ranked within the highest one percent for risk were worth approximately $74 billion.
That gap shows why simply looking at a map of earthquake probability can provide 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 already congested alternative routes. The resulting disruption may continue long after the shaking stops.
The researchers said similar concerns apply to other systems needed for daily life, including water and gas pipelines, power stations, wastewater facilities and communications towers.
The study also 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 interpreted as evidence that 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 changes demonstrate how new information can reshape scientists’ understanding of where the strongest shaking may occur.
The researchers argued that future planning must consider vulnerability and exposure alongside earthquake probability.
Otherwise, authorities could overlook places where a comparatively rare event would produce enormous human and economic consequences.
