Why Earthquakes Happen in Alaska

From the dramatic Southeast coast to the heights of the Alaska Range and the volcanic islands of the Aleutians, earthquakes build the landscapes that drive Alaska’s rivers, glaciers, and even climate zones. Most of these earthquakes—and all major earthquakes—can be traced to the movement of tectonic plates.

The landmass beneath the Pacific Ocean is one of a few dozen tectonic plates that make up the earth’s crust. Each year, the Pacific Plate pushes a couple of inches towards Alaska, which is part of the North American Plate. Where these two plates meet, the dense oceanic rocks of the Pacific thrust under the more buoyant continental rocks of Alaska. This process is called subduction.

Subduction zone earthquakes follow the descent of the Pacific plate down to 200 km (~125 miles) or more. Alaska’s largest earthquakes, exceeding magnitude 8 and even 9, occur primarily in the shallow part of the subduction zone, where the crust of the Pacific Plate sticks and slips past the overlying crust.

Examples of this type of earthquake include the 1964 M9.2 Good Friday Earthquake and the 1965 M8.7 Rat Islands Earthquake, the second and eighth largest earthquakes ever recorded worldwide. Recently, Alaska experienced the 2020 M7.8 Simeonof Earthquake and the 2021 M8.2 Chignik Earthquake.

Animation showing how the Pacific Plate subducts beneath Alaska

Tectonics in Southeast Alaska are also driven by the movement of the Pacific Plate, but in a different way. As the plate inches toward the northwest, it grinds past Southeast Alaska and British Columbia. Unlike the subduction zone, these faults slip primarily in a side-to-side motion, with a different tectonic plate on each side. The earthquakes caused by this movement are shallow and occur within the crust of the earth. This is a well-developed fault system that has been active for tens of millions of years. In the past, it has hosted numerous earthquakes approaching magnitude 8. The 2014 M6.0 Palma Bay Earthquake and the 2013 magnitude 7.5 Craig Earthquake are recent examples.

Animation showing how the Pacific Plate subduction drives seismicity of mainland Alaska

Meanwhile, this northwest motion of the Pacific Plate exerts tremendous force on Alaska, compressing the land in a north-south direction and shearing, or tugging, southern Alaska to the west. The Alaska mainland is crosscut by numerous fault systems that accommodate this compression and shearing.

The vigor of these faults, and our knowledge of them, generally decreases toward the north, although the impact of this compression can be traced all the way into the Arctic Ocean.

The resulting seismicity is remarkable for its variety and geographic reach: events like the 2002 M7.9 Denali Fault Earthquake, and the 1958 magnitude 7.3 Huslia earthquake, the 2018 M6.4 Kaktovik Earthquake, and several seismic zones in the Interior including the Minto Flats Seismic Zone, all result from this powerful compressional force.

However, Alaska can also produce earthquake swarms, or a series of earthquakes that is not similar to a typical mainshock-aftershock sequence. It is still uncertain as to exactly why swarms occur, but they occur around the state and can vary in magnitude, quantity, and duration. In 2014, a swarm near Noatak rattled residents with five magnitude 5.3-5.7 earthquakes spread out over two months. In 2015, a swarm off St. George Island shook the normally quiet Pribilofs. In 2018, a swarm in the eastern Brooks Range began and accounted for more than 2,000 of the year's record 55,000 earthquakes in Alaska. In 2019, another vigorous swarm sequence began in the Purcell Mountains. More than 9,000 earthquakes have been recorded as part of this swarm, including five earthquakes with magnitudes larger than 5.

How Many and How Large?

The Earthquake Center detects an earthquake every ten minutes, on average. As our monitoring network improves, we report more earthquakes because we are able to detect smaller earthquakes across more of the state. As the USArray project added new seismic stations, beginning in 2014, in previously unmonitored areas we noticed an upward trend of detectable earthquakes from around the state. In 2018, we reported an all-time high of nearly 55,000 earthquakes in Alaska, followed by roughly 50,000 events per year since. During 2019-2020, the Alaska Earthquake Center underwent an unprecedented expansion of our permanent seismic monitoring network. We were able to achieve it by acquiring 96 of 158 temporary USArray sites in Alaska.

The subduction zone produces very large earthquakes—as large as anywhere in the world—including three of the twelve largest earthquakes ever recorded. We reported nearly 250,000 earthquakes in Alaska over the last five years. Including some of the largest worldwide. The largest United States earthquake in the last 50 years was the 2021 M8.2 Chignik Earthquake. Seventy-five percent of all earthquakes in the United States with magnitudes larger than five happen in Alaska and magnitude six and seven earthquakes can happen nearly anywhere in Alaska.

Poster of earthquakes in Alaska with factors that drive seismicity. A full-size version of this poster and additional resources are available here.

What We Do About Earthquakes

All Alaskans live with earthquake hazards. The Alaska Earthquake Center exists to minimize our risks by understanding where earthquakes occur and why. Tracking the earthquakes that occur each day provides clues about the earthquakes that are likely in the future.

When earthquakes do occur, rapid reporting allows emergency managers to assess the potential impacts. By measuring the shaking across the region, and even in buildings, we are able to assess the potential impacts and determine what type of emergency response is likely to be most effective. Rapid reporting on our website and social media platforms provides residents with context around what they felt.

The breadth of our network—with monitoring stations located in cities, villages, and critical infrastructure ranging from Southeast to the North Slope to the Bering Strait—demonstrates the reach of earthquake hazards as well as our commitment to minimizing those hazards for all Alaskans.