Why are Alaska quakes so big?

On the night of July 28, 2021 a massive 8.2 magnitude quake rocked Alaska. It isn't the first such mega-quake, and it certainly won't be the last. Alaska is home to some of the world's biggest quakes, including an epic 9.2 quake that struck in 1964. What is happening up there? 

A massive Magnitude 8.2 quake rocked Alaska, with shaking in many areas lasting for more than a full minute.

Highlights

By Marshall Connolly (NEWS CONSORTIUM)
7/29/2021 (3 years ago)

Published in Green

Keywords: Alaska, earthquake, size, big, large

LOS ANGELES, CA (California Network) - Californians are used to earthquakes. They always catch you by surprise, but they are usually mild, almost fun events, and they typically only last a few seconds. But in those few seconds, a lot of damage can be done. That's because everything moves in an earthquake, even things that were never supposed to move in the first place. And when the movement is sudden and sharp, it can cause store shelves to empty, pictures to fall, and windows to shatter. 

Earthquakes are caused by a variety of factors, but a common cause is the movement of tectonic plates. Earth's crust is broken into many massive plates which are sliding around on top of the Earth's molten mantle. Each plate is bordered by others and where they touch, something happens. They usually crumple and form mountain ranges. But they can also grind past one another and lock up, that means, they get stuck. Over time, the pressure builds up until one plate fractures. That causes vibrations in the crust, which we experience as an earthquake. 

Off the coast of Alaska, there is a massive subduction zone, which is another kind of fault. The Pacific Plate is sliding under the North American plate. This sliding isn't constant, moving at around two inches per year, on average. But often, the subduction stops in places where the Pacific Plate snags on the North American Plate. Again, the pressure builds until the crust fractures. 

Often, when plates fracture in subduction zones, the subducting (diving) plate will snap upwards. This displaces the water directly above, pushing it up and creating a wave known as a tsunami. Tsunamis can travel across oceans and even around the world, in the most extreme cases. 

More fracturing means more intense shaking that lasts longer. It is not just the shaking itself, but the duration of the shaking that makes quakes destructive. A good jolt will rattle nerves, but a sustained, rolling quake can start to clear shelves and damage infrastructure if it continues long enough.

In Alaska, the quakes are caused by interactions along multiple faults, some of which are extremely long, such as where the Pacific Plate subducts. In Alaska, that fault is called the "Aleutian Megathrust." 

While this fault produces some of the world's largest quakes, they are often harmless, relatively speaking. That's because they are usually quite remote, occurring in sparsely populated areas, usually far out to sea. So the quakes can be massive, but have a minimal impact on human life. Of course, they can cause serious damage and harm, and the tsunamis they might spawn can kill people even thousands of miles away, many hours later. 

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In Alaska, there is an average of a magnitude 7 quake about every 1-2 years, and a magnitude 8 every dozen years or so. The long subduction zone is the culprit. 

Regions like California have a different kind of fault, the strike-slip kind described above where two plates slide past each other and lock. These systems can produce large quakes too, but they usually do not cause tsunamis because they don't often displace water. An exception is when they cause a submarine landslide, but this is rare. 

Most Californians have never faced a significant tsunami event, and have only heard their coastal sirens during tests. Few would know exactly what to do. However, in places like Alaska, where monster quakes are more common, the people understand when they hear the alarms, they cannot wait, and move to higher ground immediately, no matter how late, dark, or cold it may be. They know their lives depends on it. 

The most recent event did not cost any lives. While a tsunami warning was issued, and some waves did register, the warning was canceled. For now, the people of Alaska are lucky, and they only have to clean up shelves and some glass. Let's hope nothing worse happens for a very long time to come, if ever.