Alaska is one of the most seismically active places on Earth, experiencing more earthquakes than any other U.S. state. The region sees a significant number of seismic events annually, including an average of one earthquake with a magnitude of 8 or greater approximately every 13 years. Alaska also experiences a magnitude 7 to 8 earthquake about every two years, and around six earthquakes between magnitudes 6 and 7 each year. The frequent ground shaking results from complex geological forces.
Understanding Plate Tectonics
Earth’s outermost layer, the lithosphere, is not a single solid shell but is broken into numerous large pieces called tectonic plates. These immense plates are in constant, slow motion across the planet’s surface, driven by heat generated deep within the Earth’s mantle. Their interactions are responsible for most of the world’s seismic activity, with earthquakes common where plates meet.
Tectonic plates can interact in several ways, each capable of generating seismic events. Some plates pull apart at divergent boundaries, while others slide past each other horizontally at transform boundaries. The most significant seismic activity often occurs at convergent boundaries, where plates collide. These collisions build stress in the Earth’s crust, released as earthquakes.
The Aleutian Subduction Zone
The primary reason for Alaska’s high earthquake frequency is the Aleutian Subduction Zone, a major convergent plate boundary stretching approximately 2,500 miles (4,000 kilometers) from the Alaska Range to the Kamchatka Peninsula. At this boundary, the dense oceanic Pacific Plate subducts beneath the lighter North American Plate. This process causes stress and friction as the plates grind.
The Pacific Plate moves northwestward relative to the North American Plate, subducting at a rate that varies from about 5.1 to 7.5 centimeters (2 to 3 inches) per year from east to west along the zone. This motion accumulates elastic strain in the Earth’s crust. When stress exceeds rock strength, it is suddenly released, resulting in earthquakes. These events can include megathrust earthquakes, which are the largest type of earthquakes globally.
The Aleutian Subduction Zone has been the source of some of the largest earthquakes. For instance, the 1964 Great Alaska Earthquake, with a magnitude of 9.2, was the second largest worldwide. Another significant event was the 1965 Rat Islands earthquake, which registered a magnitude of 8.7. These megathrust events occur along the interface where the two plates are locked.
Other Major Fault Systems
While subduction is the dominant force, Alaska’s complex geology also includes other significant fault systems that contribute to its seismic activity. These secondary faults often accommodate stresses that arise from the broader interactions between the Pacific and North American plates. They represent areas where the overriding plate’s crust deforms in response to subduction.
One prominent example is the Denali Fault, a major intracontinental right-lateral strike-slip fault that extends through much of southern Alaska. This fault accommodates some of the motion and compression within the North American Plate caused by the Pacific Plate’s ongoing movement. The 2002 Denali Fault earthquake, a magnitude 7.9 event, was the strongest recorded in interior Alaska, showing these internal fault systems can produce large earthquakes.
Another active system is the Fairweather/Queen Charlotte Fault system, located in Southeast Alaska. This fault is primarily a strike-slip boundary, similar to California’s San Andreas Fault, where the Pacific Plate slides horizontally past the North American Plate. It has produced multiple earthquakes exceeding magnitude 7, including the 1958 magnitude 7.8 Fairweather earthquake. These crustal faults add to the overall seismic hazard across the state, generating earthquakes distinct from those on the subduction zone interface.
Types of Earthquakes in Alaska
Diverse tectonic forces in Alaska result in different earthquake types, each with unique origins. Megathrust earthquakes occur directly along the interface between the subducting Pacific Plate and the overriding North American Plate. These events typically involve large areas of the plate boundary slipping, leading to the largest magnitudes and often generating tsunamis. They are responsible for significant seismic events in Alaska’s history.
Shallow crustal earthquakes occur within the upper 40 kilometers of the Earth’s crust, primarily within the North American Plate. These earthquakes are often associated with the Denali, Fairweather, and other regional fault systems that accommodate internal deformation and stress. They can cause significant shaking at the surface, especially if they occur close to populated areas. Such earthquakes are a consequence of the accumulated strain in the brittle upper crust.
Deep-focus earthquakes, also known as intraslab earthquakes, originate within the subducting Pacific Plate as it plunges into the Earth’s mantle. These events can occur at depths exceeding 200 kilometers, far below the immediate plate boundary. They are caused by the stresses and phase changes within the cold, descending oceanic slab. While often felt over a wide area due to their depth, the distance to the surface can sometimes reduce the intensity of ground shaking compared to shallower events of similar magnitude.