Why Did Alaska’s Big Quake Lead to a Tiny Tsunami?

Geophysics, plate tectonics and the vast ocean all determine how severe a tsunami may be

Alaska Quake
The epicenter of last night's earthquake in Alaska USGS

Last night, around 12:30 AM local time, a 7.9 magnitude earthquake shook southern Alaska. Centered roughly 170 miles off the coast, the temblor sparked a series of public alarms and cell phones alerts all warning of possible tsunami. Many residents in coastal cities, including Kodiak and Sitka, were told to seek shelter in higher grounds, reports The New York Times

By the early morning hours, warnings for all these regions had been canceled. But the quake didn't go without a tsunami: The tiny wave that materialized stood somewhere between 1 inch and 8 inches, according to various sources.

As Alessandra Potenza at The Verge reports, earthquakes tend to happen at plate boundaries where Earth’s tectonic plates meet up. These boundaries, along with other fractures in the Earth's surface, are known as faults. Sometimes the two plates grind smoothly against one another, but sometimes they get stuck, gradually building up energy. The sudden unsticking of the plates releases that pent-up energy in an earthquake.

As Potenza reports, south of Alaska the Pacific plate is being shoved under the North American plate in what is known as a subduction zone. Because of this movement, Alaska very seismically active. In 1964, a 9.2 magnitude quake—the second largest earthquake ever recorded—shook the region. It was followed by a tsunami that killed 131 people and caused $2.3 billion in property damage.

So why did Alaska's latest temblor not cause a massive wave? It all has to do with the geology of the region and the type of earthquake that took place. 

There are three general types of faults. Normal faults occur where pieces of earth’s crust are pulling apart, creating valleys. Strike-slip faults are areas where the two pieces of crust are slipping past each other horizontally. Reverse faults, or thrust faults, occur when one chunk of crust slips underneath another.

Peter J. Haeussler, a research geologist with U.S. Geological Survey in Anchorage tells The NYTimes that last night's earthquake likely occurred at a strike-slip fault away from the main subduction zone. These faults are less likely to produce tsunami waves because they result in primarily horizontal, rather than vertical, movement. While a strike-slip fault like California’s San Andreas fault can cause havoc on land, in the deep ocean they don’ t tend to create big tsunamis.

“To get a tsunami, you have to have substantial vertical movement on the seabed,” USGS geophysicist Don Blakeman tells Potenza. The quakes that produced the 2004 tsunami that decimated Indonesia and the 2011 tsunami that led to the Fukushima nuclear disaster were both created by more vertical thrust faults.

Even so, the early alarms were necessary. As Maddie Stone at Earther explains, while scientists are pretty good at detecting earthquakes and timing tsunamis using modeling software, they just don’t have the monitoring firepower to know how large that potential wave may be.

“It’s easy to predict when a tsunami will arrive, it is very difficult to predict how big it is going to be,” geophysicist Mika McKinnon tells Stone.  “There is a tsunami right now, it just happens to be under a foot tall.”

In essence, the energy from an earthquake displaces a certain amount of water, creating a ripple. But how large that ripple actually is very hard to determine. Out in the open ocean, it may be just inches high; it's not until the wave begins to bunch up as it approaches the shore does its size becomes apparent.

As Stone reports, once a tsunami gets close to shore, tide gauges begin to give researchers some idea of its magnitude, and human observers also begin sharing data. But by that time, it’s too late to begin an evacuation. So the rule of thumb is better safe than sorry when it comes to warning the public.

It seems many people in Alaska heeded that warning, especially since it was preceded by an earthquake that lasted 90 seconds in some areas, reports the Associated Press. Some shelters were full to capacity. Keith Perkins, who lives in Sitka, says his cellphone alerted him to the tsunami warning. And town sirens went off soon after. Even though he believes his home is in a spot high enough to avoid a tsunami, he decided to stull go to a local school designated as a shelter and evacuation center. “I figured I’d probably just better play it safe,” he says.

So far, the Alaska Earthquake Center at the University of Alaska Fairbanks reports there have been 17 aftershocks in the 4 to 5 magnitude range and expects more. However, officials say it's unlikely any will be of the same magnitude as last night’s quake—and it's unlikely any would produce more tsunamis.

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