What sets this week’s earthquake part from the 1964 earthquake and tsunami

Alaska 1964 Good Friday earthquake and tsunami damage. (Courtesy of NOAA’s Historic Coast & Geodetic Survey Collection)

Kayla Desroches/KMXT

Kodiak could have been the first coastal community hit had there been a large tsunami early Tuesday morning.

A little after 12:30 a.m., the Alaska & West Coast Tsunami Warning Center reported a magnitude 7.9 earthquake roughly 175 miles southeast of the City of Kodiak.

Estimated time of arrival for the first wave was initially 1:45 a.m., but waves reached less than a foot along the coast. And it looks like the aftershocks will remain mild.

KMXT checked in to find out why coastal Alaska missed the worst-case scenario.

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Get to high ground. That’s what officials told Kodiak residents early Tuesday morning. But how high?

Dan Belanger, earthquake & tsunami program manager with the Alaska Division of Homeland Security and Emergency Management, says state officials generally say at least 100 feet.

“That’s just kind of a cover-all phrase that we like to say because we’re hoping to get people out of the coastal zone areas or the inundation areas and 100 feet seems to be a good blanket line to cover.”

He says the number is higher or lower depending on the area.

But for Kodiak, that minimum still applies under new tsunami inundation maps the state released in September 2017.

That’s except for some minor redraws since the last update in 2003.

“You do have historical data. You guys have been impacted before, so you know where the wave went before and how it hit. However, Kodiak has also changed as far as your infrastructure and your utilities and stuff over the years, so things might have altered a little bit from what was there before, so they try to look at best case scenarios, largest case scenarios and determine out what the possible impact for the community will be.”

The historical data is from the 1964 earthquake and tsunami which decimated

downtown Kodiak, and that was a 9.2 magnitude earthquake, says Belanger.

This a less drastic situation.

Barrett Salisbury is an earthquake geologist with the Alaska Division of Geological & Geophysical Surveys and was involved in the publication of Kodiak’s new tsunami inundation maps.

He says, first off, the location of this quake was different than in 1964. He says this one happened within the Pacific Plate.

“So, this was slightly removed from where the 1964 earthquake came from, which was basically the literal boundary between the North American and the Pacific Plates.”

He says the 1964 event was called a megathrust earthquake, where one tectonic plate pushes beneath the other and disturbs the sea floor. Salisbury says this week’s incident was a strike-slip earthquake.

“It just means crust is passed by one another. We didn’t lift up the column of ocean that gave rise to all the energy in the tsunami. There was a small tsunami. I think it was several inches, maybe 10 inches, but it’s not what we would expect from a major earthquake on the megathrust boundary.”

He says there have been multiple minor aftershocks since the morning’s earthquake.

“What we can expect is several more earthquakes for tens of years to be occurring in this area. What we’ll probably expect is that the earthquakes will be of similar style, so they will be strike slip earthquakes within the Pacific Plate.”

Salisbury says, for now, they do not think that poses a risk for tsunami generation, but he cautions Alaskans to take this as a wake-up call and to be prepared in the case of a more serious event.

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