One Year Later: As the earth breaks

 At the base of Hatcher Pass, the Castle Mountain fault zone is visible from the road.
At the base of Hatcher Pass, the Castle Mountain fault zone is visible from the road. (KTUU)
Published: Nov. 11, 2019 at 9:57 AM AKST
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With an earthquake recorded on average every 15 minutes and 143 active faults across the state, all Alaskans live with some level of earthquake hazard.

Near the base of Hatcher Pass, one of those 143 faults is visible from the road. The Castle Mountain fault runs from near Sutton, along the Talkeetna Mountains to the Houston area.

"The Castle Mountain is often seen as one of the most important active faults in all of Southcentral Alaska," said Peter Haeussler, a research geologist with the U.S. Geological Survey during a recent visit to the fault. "We know starting from the region near Houston and westward, there's a really nice surface expression of this fault trace and we think it could cause something like a magnitude 7 earthquake if it were to go."

While it has the potential for a large earthquake, the Castle Mountain fault was not the source of last year's 7.1 quake

In southern Alaska, there are three main sources for earthquakes. Two of them relate to the subduction of the Pacific plate under the North American plate.

"The pacific plate is sliding down underneath our feet, and right here," said Haeussler, "if we were able to drill a hole, it might be 30, 35 miles where we run into that. It's sort of like a giant conveyor belt going down into the earth's mantel."

Big earthquakes like the 9.2 Good Friday Earthquake in 1964 occur in the shallow space where two plates come together. "Those are what we call megathrust earthquakes," Haeussler described. "It's this giant thrust, kind of fault movement, one plate, one chunk of rock sliding against the other."

But as one plate bends, deeper in the subduction zone, the earth's crust continues to break and those fractures can also cause earthquakes.

"Actually last year's Anchorage earthquake was one of those kinds of earthquakes..." said Haeussler. "related to bending of this subducting plate, far beneath our feet, 30 miles or so beneath our feet so when that broke, that's what we felt at the surface.

"There's no surface expression really that you could see of an earthquake like that because it was so down deep."

By contrast the Castle Mountain fault is a crustal fault, right at the earth's surface. The most famous of these kinds of faults is probably the San Andreas in California.

Studying Alaska's faults and knowing how often earthquakes have occurred in the past can help plan for the future.

Haeussler said, "We're probably not going to have a '64-like earthquake in the near future. Our evidence for the recurrence for those kinds of earthquakes suggests that they may happen every 3 to 600 years."

On the the other hand... "We could have another Anchorage earthquake, one of these deep ones that doesn't come to the surface, probably almost at any time," said Haeussler.

While researchers can't predict the hour or even the month an earthquake might occur, their work goes a long way to make life in Alaska a little safer.

"Knowing something about the history of these faults, putting them into what eventually becomes seismic hazard maps and eventually building codes and assuming places adopt building codes, that can be a really good way to make this a safer place to live," said Haeussler. "Even if we can't predict the hour and the day that these earthquakes will occur."

And about that theory that smaller earthquakes relieve the pressure in a fault so a large earthquake doesn't occur, Haeussler says it doesn't work that way.

"Not at all," he said. "The little earthquakes are signs that maybe stress is building, but they don't relieve enough stress to do it. For example, I calculated once we would have to have something like a magnitude 5 earthquake, I forget, every 5 minutes or so, in perpetuity to relieve the stresses the occurred from the 1964 earthquake."

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