Earthquakes are sudden movements of the Earth’s crust, occurring when two blocks of the planet’s rigid outer layer slip past one another along a fault. The United States experiences a notable amount of seismic activity due to its geological setting. This article explores the specific areas in the U.S. where these events are most common.
Primary Earthquake Regions in the US
California is known for frequent seismic activity, largely due to the San Andreas Fault system, a major fault stretching approximately 750 miles through the state. This fault forms a boundary between the Pacific and North American tectonic plates, producing significant earthquakes, such as the 1906 San Francisco earthquake, estimated at magnitude 7.8. The U.S. Geological Survey (USGS) indicates that an earthquake of magnitude 6.7 or greater occurs about every 6.7 years statewide.
Alaska is the most seismically active state, experiencing numerous earthquakes annually, including the 1964 magnitude 9.2 Good Friday Earthquake, the most powerful recorded in U.S. history. The primary cause is the Alaska-Aleutian subduction system, where the Pacific Plate is forced beneath the North American Plate. This subduction zone produces very large earthquakes, making it one of the most active globally.
The Pacific Northwest, including Oregon and Washington, faces potential for very large earthquakes from the Cascadia Subduction Zone. This 700-mile long convergent plate boundary lies offshore and can generate earthquakes of magnitude 9.0 or greater, along with tsunamis. The last known great earthquake in this zone occurred in 1700, with geological evidence suggesting a recurrence interval of about 500 years for major events.
The Intermountain West, including Nevada, Utah, Idaho, and Montana, experiences regular seismic activity. This region is part of the Basin and Range Province, characterized by numerous normal faults. Nevada ranks high among states for earthquake magnitude.
Hawaii’s seismic activity is unique, driven by volcanic processes rather than plate boundary interactions. The Hawaiian Islands are formed over a hotspot, a plume of hot rock rising from deep within the Earth’s mantle. Earthquakes occur as magma moves within and beneath active volcanoes like Kilauea, Mauna Loa, and Hualalai, and due to strain released along their flanks.
Geological Explanations for US Seismic Activity
The movement and interaction of tectonic plates drive most seismic activity across the United States. These plates are constantly moving, and where their edges meet, pressure builds until it is released as an earthquake. The Pacific Plate’s northwestward motion relative to the North American Plate creates significant stress along the western coast.
Major fault types reflect how these plates interact. Strike-slip faults involve plates sliding horizontally past each other. Subduction zones, like those off Alaska and the Pacific Northwest, occur where one plate dives beneath another, leading to powerful megathrust earthquakes. In the Intermountain West, normal faults are prevalent, where the Earth’s crust is being stretched and thinned, causing blocks of land to move downward.
Hawaii’s earthquakes stem from hotspot activity. A mantle plume brings hot material to the surface far from plate edges, creating volcanoes. Magma movement within these volcanoes, along with the stress exerted by their immense weight on the Pacific Plate, generates frequent seismic events.
Emerging and Less Common Seismic Areas
Beyond the well-known western regions, other areas in the U.S. experience significant seismic events.
The New Madrid Seismic Zone (NMSZ) in the central United States, spanning parts of Missouri, Arkansas, Kentucky, Tennessee, and Illinois, is a notable example. This intraplate seismic zone was responsible for a series of very large earthquakes in 1811-1812, estimated between magnitude 7 and 8, felt across much of the eastern U.S.. The NMSZ continues to be active, with hundreds of smaller earthquakes occurring annually, though most are too small to be felt.
Oklahoma and Texas have seen a dramatic increase in earthquake activity in recent years, primarily due to induced seismicity. This phenomenon is linked to human activities, specifically the deep injection of wastewater from oil and gas operations into underground disposal wells. Thousands of earthquakes have occurred in this region since 2009, including some exceeding magnitude 5.
The Eastern U.S. generally has lower seismic activity compared to the West, but significant historical earthquakes have occurred. The 1886 Charleston earthquake in South Carolina was one of the largest historic earthquakes in eastern North America, estimated at magnitude 6.9-7.3. This event caused considerable damage and fatalities in Charleston and was felt over a wide area. While less frequent, earthquakes in the East can affect a larger area due to the older, more rigid crust.