Seismic activity, commonly understood as earthquakes, represents a complex risk. The level of danger a region faces is a combination of the likelihood and intensity of ground shaking (the hazard) and the population and infrastructure exposed to it (vulnerability). Understanding which areas of the country are most at risk requires examining the underlying tectonic forces that drive earthquakes across the United States. This seismic landscape is defined by dramatically different geological settings, ranging from plate boundaries on the West Coast to ancient fault systems deep within the continental interior.
The Western Edge: Interplate Boundary Risks
The highest frequency of large earthquakes occurs along the Pacific coast, where the North American continent interacts directly with oceanic plates. This interplate seismicity is characterized by relentless tectonic movement and the potential for the nation’s most powerful seismic events. The San Andreas Fault system in California is the most well-known feature of this boundary, marking where the Pacific Plate slides past the North American Plate.
This transform boundary involves two massive crustal blocks moving horizontally past one another, primarily manifesting as a right-lateral strike-slip fault. The fault system extends over 800 miles. Friction generated by the Pacific Plate moving northwest relative to the continent builds enormous strain, which is periodically released in major events, such as the estimated magnitude 7.9 San Francisco earthquake in 1906.
North of the San Andreas system, the Cascadia Subduction Zone poses a threat of a potentially larger magnitude event. This boundary, stretching from Northern California up to British Columbia, is a megathrust fault where the Juan de Fuca Plate is diving beneath the North American Plate. Because the plates are locked, strain is accumulating along the entire thousand-kilometer-long interface.
The Cascadia Subduction Zone is forecast to produce a magnitude 9.0 or greater earthquake, an event known to have occurred historically, most recently in 1700. Such a megathrust quake would generate intense, prolonged ground shaking across the Pacific Northwest, followed by a destructive tsunami. The resulting wave would affect the shorelines of Washington, Oregon, and Northern California, increasing the hazard profile for these coastal communities.
Alaska represents the area of highest seismic activity in the country, driven by the convergence of the Pacific and North American plates along the Alaska-Aleutian subduction zone. The Pacific Plate subducts beneath the North American Plate at up to 75 millimeters per year, fueling both deep and shallow earthquakes. This plate interaction was responsible for the 1964 Great Alaska Earthquake, which registered a magnitude of 9.2, the largest recorded in North American history.
The Aleutian Trench region continues to be a source of frequent and powerful earthquakes, posing tsunami threats to Alaska and the entire Pacific rim. This tectonic collision generates thrust faulting along the plate interface and intermediate-depth seismicity within the subducting slab. The size and frequency of these events solidify Alaska’s position as the most seismically active state, where major events occur multiple times per decade.
The Central and Eastern Threat: Intraplate Seismic Zones
While the West Coast is defined by plate boundaries, significant risk exists hundreds of miles inland within the stable North American Plate. These intraplate seismic zones involve the reactivation of ancient faults under stress transmitted across the continent. The New Madrid Seismic Zone (NMSZ) is the most prominent of these zones, affecting parts of Missouri, Arkansas, Tennessee, Kentucky, and Illinois.
The NMSZ is situated along the Reelfoot Rift, a failed rift valley that formed more than 500 million years ago. The faults within this rift are now under compressive stress from the continuous motion of the North American Plate, causing them to periodically slip. This zone was the source of a series of three massive earthquakes in the winter of 1811–1812, with estimated magnitudes of 7.0 or greater.
Historical accounts from the 1811–1812 sequence describe widespread damage and even temporarily reversing the flow of the Mississippi River. The recurrence interval for such large-scale events is estimated to be approximately 500 years. The risk is magnified because the ancient, dense crust of the Eastern United States transmits seismic waves far more efficiently than the fractured crust of the West.
Earthquakes of the same magnitude in the East are felt over significantly larger geographic areas, sometimes spanning hundreds of miles, increasing the exposure. For instance, a magnitude 5.8 earthquake in Virginia in 2011 was felt by tens of millions of people across the eastern seaboard. This occurs because the older, colder crust absorbs less seismic wave energy, allowing the shaking to propagate over greater distances.
Another historical source of intraplate risk is the Charleston Seismic Zone in South Carolina, the location of the destructive 1886 earthquake. This event, with an estimated magnitude of 6.8 to 7.2, caused widespread destruction in Charleston and was felt across much of the Eastern United States. The mechanism is thought to involve localized stress concentration at the intersection of deeply buried faults within a pre-existing zone of weakness.
Geological Anomalies and Human-Induced Activity
Beyond traditional tectonic zones, unique geological stressors and human actions have created localized areas of seismic risk. Hawaii’s seismicity is almost entirely volcanic, driven by the movement of magma and the instability of the island’s flanks. Earthquakes here are common, often occurring in swarms related to magmatic intrusions or the movement of the volcano’s southern flank toward the sea.
The seaward sliding of the massive Kīlauea and Mauna Loa volcanoes along major rift zones can release strain in powerful, non-volcanic events, such as the magnitude 7.9 earthquake that struck in 1868. These events reflect the internal stresses of a continually growing and shifting volcanic landmass. Frequent, smaller earthquakes are also caused by the pressurization of magma systems beneath the surface.
In the central United States, a highly localized seismic threat has emerged in the form of induced seismicity, particularly in Oklahoma, Kansas, and Texas. This activity is directly linked to industrial processes, primarily the deep underground injection of wastewater produced during oil and gas extraction. The injection of massive volumes of fluid increases the pore pressure within the Earth’s crust, effectively lubricating dormant faults and allowing them to slip.
Oklahoma experienced a dramatic increase in earthquake frequency starting around 2009, surpassing California in the number of magnitude 3.0-plus events. The increased pressure from the disposal of produced water into deep formations, such as the Arbuckle formation, can trigger faults that were previously stable. Injection depth, especially near the crystalline basement, is a strong factor in determining the risk of triggering these events.
Areas with geothermal stress, such as the Yellowstone Caldera in Wyoming, also exhibit unique seismic patterns. The caldera, a massive volcanic system, experiences frequent earthquake swarms, accounting for over half of its seismic activity. These swarms are caused by the movement of hydrothermal fluids and magma beneath the surface, interacting with a complex network of faults.