Nearly half of all Americans, approximately 159.2 million people, now live in areas exposed to 'very strong' earthquake shaking capable of damaging buildings, according to an updated hazard model from the US Geological Survey (USGS). This represents a substantial increase from the previous assessment, which estimated 111.8 million people were at such risk.
The latest findings also indicate that 47.3 million homes and buildings valued at $25.6 trillion are located in these high-risk zones. If considering 'strong' shaking, which can still cause damage, the number of exposed Americans rises to an estimated 234 million.
While California remains a prominent earthquake hotspot, the expanded threat is not confined to the West Coast. The study highlights increased danger around the New Madrid Seismic Zone in the central United States. Furthermore, heavily trafficked bridges in metropolitan areas such as New York, Atlanta, and St. Louis are identified as unexpectedly vulnerable to severe disruption, even if the shaking intensity is not the highest.
Researchers emphasized that these figures do not suggest earthquakes are occurring more frequently. Instead, the dramatic rise in estimated exposure is attributed to significant advancements in scientific modeling. These improvements provide a more comprehensive understanding of how earthquakes originate, how fault lines can rupture collectively, and how seismic waves propagate through various geological formations, including deep layers of sediment that can amplify shaking.
The benchmark used for this assessment is a two percent chance of being exceeded within 50 years, a timeframe roughly equivalent to the lifespan of infrastructure. This allows for the comparison of potential shaking intensity across different regions, even for rare but destructive seismic events.
Shifting Hazard Landscape
The updated 2023 forecast, which simultaneously updated hazard calculations for all 50 states, incorporates revised earthquake records, enhanced computer models, and new data on faults and underground sediment. While the predicted probability of damaging shaking has decreased in some parts of the western U.S. compared to earlier models, it has notably increased around the New Madrid Seismic Zone.
This shift underscores that earthquake preparedness is a concern extending beyond traditional seismic zones. The potential consequences of shaking are determined not only by the earthquake's magnitude but also by the number of people, buildings, and critical services in its path, and their resilience.
Infrastructure Vulnerability
The study also examined the vulnerability of the nation's infrastructure, particularly its highway bridges. More than 600,000 highway bridges, collectively valued at over $1.5 trillion, were analyzed. These bridges support billions of daily vehicle trips and are crucial for transportation and emergency response.
The research assessed risk by calculating the average annual number of vehicles that could be immobilized by earthquake damage to a bridge. This metric considers the bridge's usage and potential downtime, not just repair costs. Bridges facing the greatest physical hazard were concentrated in areas like San Francisco, Los Angeles, and the New Madrid region. However, bridges posing the greatest disruption risk, due to high traffic volume and potential extended closures, were spread more widely. This list includes bridges in the Seattle, Portland, Salt Lake City, St. Louis, Atlanta, Charleston, and New York metropolitan areas.
In these cities, even moderate shaking could have amplified consequences due to heavily traveled crossings and potentially aging infrastructure. The research highlighted that a damaged bridge could cut off access to hospitals, impede emergency services, and redirect traffic onto already congested routes, leading to prolonged disruption.
Similar concerns were raised regarding other essential systems, including water and gas pipelines, power plants, wastewater facilities, and communication networks. The researchers cautioned that while the science has improved, leading to refined hazard estimates, communities continue to develop and expand, potentially into areas with new or previously underestimated seismic risks. They advocate for future planning to integrate vulnerability and exposure assessments alongside earthquake probability to better understand and mitigate potential disaster impacts.