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Why Those Quake Swarms Off Oregon Coast? Geologists Provide Answers

Published 3/29/24 at 4:35 a.m.
y Andre' Hagestedt / Oregon Coast Beach Connection

(Bandon, Oregon) – From Monday through late Tuesday, a swarm of about 10 underwater quakes hit off the south Oregon coast, wriggling beneath the surface from 24 miles outside of Bandon to a little over 100 miles off Port Orford. It ramped up slowly starting late on Monday, and then on Tuesday seven of them hit within minutes of each other, trailing off again with one final shaker around 1 a.m. on Wednesday morning. (Photo Oregon King Tides / Paul O'Donovan: You can see evidence of how faultlines lift up things at the diagonal shapes of Shore Acres near Coos Bay.)

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Most were rather small, at a M 1.8 all the way up to a medium M 5.7 quake (the latter of which some reported feeling as far away as the Willamette Valley).

The majority took place around the Gorda Ridge and the Blanco Fracture Zone, which is where these south coast quakes often hit, multiple times per year. All of it is about 200 miles from the massive fault of the Cascadia Subduction Zone – which is the biggie everyone worries about. Quakes in this area don't have any relation to what's happening there.

Every time some of the little quakes pull a Taylor Swift and “shakes it off,” people get a little alarmed.

“It is not portending the next big one,” said geologist Tom Horning, who is based out of Seaside on the north coast.

Swarms like this aren't anything that geologists worry about, though they don't happen often – at least statistically. They are unusual in that they aren't common, though this area fires a single quake off every month or so. This cluster of 10 quakes was nothing, however, compared to the 77 or so that hit the southern waters within a couple days back in December of 2021.

So, why do these swarms happen?

First, you have to understand the action here: the Blanco Fracture Zone is where other plates are grinding up against each other, slipping up and down.

Part of this, say some geologists, is that the Blanco zone can break more easily into smaller events because the crust here is younger and warmer.

“Other sides of the plate are where the smaller quakes are occurring with regularity,” Horning said. “However, these other parts of the plate boundary are having quakes regularly because of local geologic conditions that favor frequent events. Their locked interfaces stand vertically and are locked for only a few miles underground. It is easy for the plate movements to overwhelm these smaller locked zones, so frequent small quakes take place.”

Really, it comes down to "stuff happens," in the geologic sense.

The quakes here usually occur about 10 km deep – or six miles.

The Gorda Ridge is one area where they happen a lot, which is where this swarm took place.

One curiosity of this swarm was that two – the very first and last – happened just 24 miles off Bandon / Port Orford. That's a little closer than usual. It's east of the Cascadia Basin boundary, which is an area that contains these fracture zones.

Graphic courtesy USGS - Tuesday's quake swarms

What is this area right near Bandon called? Neither Horning nor Dr. Scott Burns from PSU knew, and there may not be a name for it.

“I would call it the eastern part of the Gorda Plate,” Burns told Oregon Coast Beach Connection. “You have three ocean bottoms in the graphic: Eastern part of the Juan de Fuca plate (north of the Blanco Fault zone); south of the Blanco Fault zone you have the eastern and west parts of the Gorda Plate and the Gorda Ridge between them.”

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Both Burns and Horning said this is simply similar territory to the Blanco area.

“ I think that those two quakes are really related to the Blanco Fault zone which is between the Gorda and Juan de Fuca plates,” Burns said. “They line up with the fault zone! It is interesting why the swarm occurred where it did on the Gorda Plate.”

Horning added more about the small quakes close to shore – and a rather surprising fact the big, famous headland just north of Port Orford.

“It aligns with Cape Blanco and the Gorda Transform Fault, so it is my guess that the two faults moved due to strain release along the spreading center farther to the south,” he said. “It may not be as active as the Gorda Ridge, but it is lifting Cape Blanco fast enough to create the cape in the first place.”

Geology of this coastline is not just a trip into the past – it's quite trippy at times.

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Bandon, courtesy Manuela Durson Fine Arts 

Cape Blanco

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Andre' GW Hagestedt is editor, owner and primary photographer / videographer of Oregon Coast Beach Connection, an online publication that sees over 1 million pageviews per month. He is also author of several books about the coast.

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