Earthquakes are sudden shakings of the Earth’s surface, caused by the rapid release of energy within the Earth’s crust. This energy travels in waves, leading to the ground movement we experience. The United States experiences significant seismic activity annually. Understanding where and why these events occur aids preparedness.
The Primary Earthquake Hotspot: The West Coast
The West Coast of the United States, particularly California and the Pacific Northwest, experiences the highest frequency and intensity of earthquakes in the U.S. California is active due to the San Andreas Fault, which extends approximately 750 miles. This fault is a right-lateral fault where the Pacific and North American Plates slide past each other. The southern segment, near Los Angeles, poses a risk as it has not ruptured in over 300 years, allowing stress to accumulate. Other faults like the Hayward and San Jacinto also contribute to California’s seismic activity.
Further north, the Pacific Northwest faces a threat from the Cascadia Subduction Zone, a long convergent plate boundary off the coast from northern California to British Columbia. This zone can produce powerful megathrust earthquakes of magnitude 9.0 or greater. The last event occurred in January 1700, and scientists estimate a 37% chance of a magnitude 7.1 or greater earthquake in this zone within the next 50 years.
Unseen Threats: Other Notable Seismic Zones Across the US
Beyond the West Coast, other U.S. regions also experience significant seismic activity, though often less frequently or intensely. The New Madrid Seismic Zone (NMSZ) in the central United States, extending through parts of Arkansas, Missouri, Tennessee, Kentucky, and Illinois, is one example. This zone was the site of powerful earthquakes in 1811-1812, estimated between magnitudes 7 and 8, felt across much of the eastern U.S. While large events are less frequent, the NMSZ experiences about 200 small earthquakes annually, and historical studies suggest magnitude 7-8 earthquakes occur roughly every 500 years.
Alaska is the most seismically active state in the U.S., experiencing 11% of the world’s recorded earthquakes. It averages one magnitude 8 or larger earthquake every 13 years and a magnitude 7 to 8 earthquake annually. The 1964 Great Alaska Earthquake, a magnitude 9.2 event, remains the second largest ever recorded globally. Its activity stems from multiple tectonic plate boundaries, including subduction zones.
The Intermountain West, including Utah and Nevada, also experiences regular seismic activity. This region features active fault systems, such as Utah’s Wasatch Fault Zone. Though less frequent than on the West Coast, earthquakes here can still pose risks. Even the Eastern United States, despite lower activity, has historical earthquake zones like Charleston, South Carolina, and eastern Massachusetts, demonstrating that seismic events can occur far from major plate boundaries.
Understanding the “Why”: Geological Drivers of US Earthquakes
Earthquakes stem from the movement of Earth’s tectonic plates, which are large, rigid sections of the planet’s outermost layer. The crust is fractured into these plates, and their slow motion causes stress to build up at their boundaries. When this accumulated stress exceeds rock strength, they suddenly break and slip along fault lines, releasing energy as seismic waves.
In the U.S., much seismic activity relates to the interaction between the North American Plate and other plates. Along the West Coast, the Pacific Plate slides horizontally past the North American Plate along the San Andreas Fault system, creating strike-slip faults, where blocks of crust move past each other horizontally. In the Pacific Northwest, the Juan de Fuca Plate dives beneath the North American Plate at the Cascadia Subduction Zone, generating thrust faults, where one block of crust is pushed up and over another. In contrast, in regions like the Intermountain West, normal faults are common, formed where crustal blocks pull apart. These different fault movements dictate earthquake characteristics in various regions.
Quantifying Risk: How Scientists Measure Earthquake Probabilities
Scientists cannot predict the exact time, location, and magnitude of future earthquakes. However, they can assess earthquake probabilities, describing the likelihood of an event of a certain magnitude within a specific region over a given timeframe. These probabilities are calculated using historical earthquake data, including seismograph records and geological studies. Geological models and fault slip rate measurements also contribute, helping scientists understand stress accumulation. The U.S. Geological Survey (USGS) develops seismic hazard maps, which are important tools for public awareness, informing building codes, and guiding preparedness planning.