The Fault in the Facts: Did Alaska have a quake larger than 9.2 before modern recordings started?

ANCHORAGE, Alaska (KTUU) - The magnitude 9.2 Great Alaska earthquake that struck Prince William Sound on March 27, 1964, is known as the largest to hit Alaska and the second-largest ever recorded in the world.

But, has there been a quake larger than that in Alaska before modern recording stated?

USGS Research Geologist Rob Witter said the science behind the answer is changing.

“About seven years ago, there was some thought that there might have been an earthquake larger than the 1964 earthquake,” he said. “But today, new data that was found on Kodiak Island suggests that the magnitude 9.2 might be one of the bigger ones, if not the biggest one in the recent past. And by recent past, remember, I’m talking about thousands of years.”

He said there are past earthquakes that rival the size of the 1964 quake, but scientists can’t precisely measure how big they were.

“The interval between quakes this size is about 600 years,” he said. “However, in the rest of the world along subduction zones, like the one that we live in, it’s been observed that earthquakes are unpredictable.”

Scientists can’t predict earthquakes. Witter said they don’t know when the next giant quake might hit Southcentral Alaska.

“Over the past 4,000 years, there’s been about six other earthquakes in the Prince William Sound region that were rivals to the 1964 earthquake,” he said.

But how do scientists know if a quake that rivals the size of the 1964 earthquake happened?

“We have evidence, mostly along the coast in Turnagain Arm at Girdwood and in parts of Prince William Sound, including the Copper River Delta, where the most recent earthquake prior to the 1964 earthquake happened about 800 to 850 years ago,” Witter said.

“It may have rivaled the size of the 1964 earthquake, but there’s some evidence from Kodiak Island that suggests that that earthquake may not have involved the fault underneath Kodiak Island,” he said.

That might make the pre-historic quake a little bit smaller, but still maybe close to a magnitude 9, he said.

“We’re talking about giants here. And the geological evidence that we use to make these judgments is pretty crude. So, there’s a lot of uncertainty,” he said.

To help figure out a timeframe of when these quakes hit, scientists did radiocarbon analyses on the trees and soil.

“The age estimates may span 100 years or more,” he said. “So, we have kind of some vague ideas of when they happen, but we certainly know that they happened.”

To find earthquakes like these, Witter said scientists are doing the dirty work.

“We dig in the dirt along the coast. We go out onto the bluffs, like the bluff that the Girdwood ghost forest, and we use shovels and other equipment to dig into the fossil record of earthquakes,” he said. “We also use survey devices to measure the elevations of these different buried soils or other forms of evidence and usually measure it with respect to sea level to understand how much the earthquake caused the site to go up or down.”

Witter said if you go to coastal towns like Whittier or Seward, it’s important to be familiar with local evacuations plans. Those are available online.

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