Earthquake risk measures the potential for economic, human, and structural loss resulting from seismic activity. This risk is not evenly distributed across the United States, but it is also not confined only to the West Coast. Seismic activity affects nearly every state to some degree, meaning the potential for damaging ground shaking is a widespread geographic concern. Understanding which areas face the highest risk requires examining the underlying geological structures and the vulnerability of the built environment.
Defining and Measuring Seismic Risk
Scientists distinguish between seismic hazard and seismic risk to accurately map vulnerability across the country. Seismic hazard refers solely to the probability of the ground shaking at a certain intensity within a specific timeframe. Seismic risk, however, combines this hazard with the exposure of people and property in that area, giving a truer picture of the potential for damage and loss.
A primary quantifiable metric used in hazard maps is Peak Ground Acceleration (PGA), which measures the maximum horizontal acceleration experienced at a site during an earthquake. PGA is expressed as a percentage of gravity and directly informs building codes because it indicates the force structures must withstand. Determining the likelihood of a high-PGA event involves calculating the recurrence interval, which is the average time between earthquakes of a specified magnitude or shaking severity in a particular region.
The recurrence interval is determined through historical records, instrumental seismicity data, and paleoseismology. For instance, a return period of 475 years is often used in engineering to define a ground motion level with a 10% chance of being exceeded in 50 years. The high seismic hazard in an area like California translates to a higher risk because of its dense population and extensive infrastructure, requiring more stringent building standards informed by these metrics.
The Primary High-Risk Seismic Zones
The highest earthquake risk in the United States is concentrated in three major zones: the Pacific Coast, the Intermountain West, and a significant zone in the central and eastern US.
Pacific Coast
California is home to the iconic and frequently active San Andreas Fault, which runs approximately 800 miles and marks the boundary between the Pacific and North American tectonic plates. This massive strike-slip fault system, along with parallel structures, is responsible for the state’s high frequency of seismicity, threatening major metropolitan areas from San Diego to the San Francisco Bay Area.
North of California, Washington and Oregon are dominated by the Cascadia Subduction Zone, a megathrust fault stretching offshore. This zone is capable of producing magnitude 9.0+ earthquakes, an event that last occurred in 1700, and is a major source of high-risk potential for the entire region. The proximity of the fault to major cities like Seattle and Portland means significant strain is continuously building.
Intermountain West
The Intermountain West has a major seismic source in the Wasatch Fault Zone in Utah. This fault runs directly beneath a densely populated corridor that includes Salt Lake City and its surrounding communities. While less frequent than California’s events, a major rupture on the Wasatch Fault would have a catastrophic impact on the region’s population and infrastructure.
Central and Eastern US
A significant high-risk area exists far from a plate boundary in the Central United States: the New Madrid Seismic Zone (NMSZ). This zone affects Missouri, Tennessee, Kentucky, Arkansas, and parts of Illinois, and produced a series of powerful magnitude 7.5 to 8.1 earthquakes in 1811–1812. The risk in this region is compounded because the seismic waves travel more efficiently through the hard, dense rock of the central North American plate, potentially causing damage over an area up to 20 times larger than a comparable earthquake in the West.
Alaska and Hawaii
Beyond the contiguous states, Alaska consistently experiences the highest number of large earthquakes annually due to the Alaska-Aleutian Megathrust and the Denali Fault. Hawaii also maintains a high level of seismic activity, though its earthquakes are primarily driven by volcanic and magmatic movements. The high frequency and magnitude of events in Alaska and the potential for large, destructive events in Hawaii place both states among the highest-risk areas nationally.
Secondary Hazards and Regional Impacts
Beyond the immediate ground shaking, earthquakes often trigger secondary hazards that can be even more destructive. One of the most significant secondary hazards is liquefaction, which occurs when intense shaking causes loose, water-saturated soil to temporarily lose its strength and behave like a liquid. This phenomenon is particularly dangerous in coastal areas, river floodplains, and reclaimed land, where it can cause buildings to sink, foundations to tilt, and underground utility lines to rupture.
Coastal regions, especially the Pacific Northwest, Alaska, and Hawaii, face the threat of tsunamis generated by large offshore earthquakes, particularly those along subduction zones. These powerful waves can inundate the shoreline within minutes of the initial shaking, causing devastating flooding and destruction in low-lying areas. The Cascadia Subduction Zone is a prime example, where a massive rupture could send a series of large waves toward the coasts of Washington, Oregon, and northern California.
In mountainous, seismically active areas, the ground shaking often destabilizes slopes, leading to landslides and rockfalls. This hazard is prevalent in states with rugged terrain, such as California, Utah, and Alaska, where the dislodged material can block roads, damage structures, and pose a direct threat to life.
Steps for Personal and Structural Preparedness
For individuals living in high-risk zones, personal preparation begins with securing the interior of the home to prevent injury from falling objects. This involves fastening heavy furniture, such as bookcases and china cabinets, securely to the walls using straps or anchors. Water heaters should also be strapped and bolted to the floor to prevent gas or water line ruptures, which can lead to fire or flooding.
A comprehensive emergency kit, often called a “Go-Kit,” should be assembled with enough food, water, and necessary medications for at least three days. Structural preparedness for older homes involves seismic retrofitting, which strengthens the connection between the structure and its foundation, making it more resistant to intense shaking. This structural upgrade can significantly reduce the risk of major damage and collapse.