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 generally considered to be 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 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 magnitude 9.2 Good Friday Earthquake and the 1965 magnitude 8.7 Rat Islands Earthquake, the second and eighth largest earthquakes ever recorded worldwide.

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 magnitude 7.9 Denali Fault Earthquake, and the 1958 magnitude 7.3 Huslia earthquake, as well as the highly active Minto Flats Seismic Zone, all result from this powerful compressional force.

How many and how large?

The Earthquake Center detects an earthquake every fifteen minutes, on average. In 2014, we reported an all-time high of over 40,000 earthquakes in Alaska. As our monitoring network expands we report more earthquakes because we are able to detect smaller earthquakes across more of the state. Thus, we expect to report more and more earthquakes as the Transportable Array project adds new seismic stations in previously unmonitored areas.

The subduction zone produces very large earthquakes—as large as anywhere in the world—including three of the twelve largest earthquakes ever recorded. Magnitude six and seven earthquakes can nearly happen anywhere in Alaska.

We reported over 150,000 earthquakes in Alaska over the last five years. Thirty-one of those had magnitudes of 6 or greater, and four had magnitudes of at least 7. Seventy-five percent of all earthquakes in the United States with magnitudes larger than five happen in Alaska.

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 the public and 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.

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