Science Behind the Tsunami: Why It Didn't Happen on Oregon Coast

Published 01/24/2018 at 4:45 PM PDT
By Oregon Coast Beach Connection staff

(Oregon Coast) – This week's tsunami watch along the Oregon coast, due to a quake up in Alaska, was a stern reminder of the precariousness of life on this gorgeous shoreline, and it was a bit of a wake up call to many first responder agencies. About 2 a.m. on Tuesday morning, officials issued a tsunami watch for the entire Oregon coast, as that earthquake just off the Alaskan coast had at least a slight chance of creating a sizable wave event as far south as California.

About 4:15 a.m., the watch was canceled to the relief of the entire state – at least to those who weren't still asleep. Even without any order to do so, some coastal residents fled to higher ground.

The magnitude 7.9 quake off Alaska's shores caused actual evacuations there, with sirens firing off in the middle of the night and citizens heading at least 100 feet above sea level. Even British Columbia was on the receiving end of a tsunami warning.

Lucky for everyone, all alerts and watches were canceled – although a six-inch wave was recorded in one part of Alaska's shoreline.

It all hearkened back to the great quake of 1964, which killed over 100 in Alaska. This one also affected the Oregon coast: causing a tsunami wave which inundated Seaside, killed four children on a beach in Newport, and washed out the bridge at Cannon Beach (ironically this caused the creation of the now famed Cannon Beach Sandcastle Festival).

So why did no actual tsunami come of this quake?

The science behind it is interesting. The USGS and geologists on the Oregon coast – including Seaside's Tom Horning – have the answer.

Unlike that quake in '64, or the big one expected off the Oregon coast someday, this was a kind of underwater earthquake that simply does not displace much water. This was a strike-slip fault, meaning the two chunks of landmass slide against each other horizontally instead of vertically.

The big nasty coming to this coastline is part of a fault system that will cause a subduction quake – where two plates pop against each other suddenly in an up and down direction, instead of side to side. It's also known as a thrust quake, according to scientists.

This one was side to side – of the strike-slip variety.

“Usually, little vertical displacement takes place, so there isn't a tsunami,” Horning said.

Still, Horning said, these kinds of quakes are capable of creating tsunami chaos. If a submarine strike-slip rattler causes massive underwater landslides, shaking up debris from the continental shelf, then tsunamis can occur. This was just the situation from another Alaska-centered shaker in 1946 that caused extensive damage in Hawaii.

If this sort of quake were to occur on land – as it does with the San Andreas fault in California – it can bend train tracks and tear fences apart.

In a subduction zone, like that off the Oregon coast, it is the result of one continental plate shoving itself underneath another. When that tension releases, one plate drastically moves up or down. This will cause substantial movement of ocean water, resulting in a tsunami.

It takes a 7.0 magnitude quake or higher to generate a tsunami. Small quakes around magnitude 2 to 3.5 occur about 20 times a year off the Oregon coast.

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