So Far, 74 Quakes Off Oregon Coast In Just Over 24 Hours

Updated 12/08/21 at 1:55 PM PST
By Oregon Coast Beach Connection staff

(Yachats, Oregon) – [UPDATED NEW NUMBERS] What seems like an unprecedented number of small to sizable quakes off the Oregon coast have rattled underwater starting early Tuesday morning, with the number still rising 36 hours later. As of 2 p.m. Wednesday, some 74 undersea quakes have hit about 300 miles directly west of Yachats in just over 24 hours. (Graphic courtesy USGS)

There are no tsunami alerts associated with any of these quakes and they are too far offshore to result in any shaking on land.

Experts say it's nothing to worry about because it has nothing to do with the larger Cascadia Subduction zone that also exists off the Oregon coast, which will one day create a massive quake and subsequent tsunami. In fact, the swarms are normal, except these were farther offshore than usual

The quake swarm is unusual, as that area generally receives about 20 to 30 quakes per year. This area is part of the Blanco Fracture Zone, a faultine separate from the larger Cascadia fault. Here, it is simply two plates rubbing up against each other and occasionally releasing that pressure.

Back in April of this year, another swarm of about seven quakes happened along the Blanco Fracture Zone.

This rather striking set of quakes began about 5 a.m. on Tuesday, with a magnitude 4.2. The small rattlers kept occurring about every hour or less, and by 12 hours later they had piled up to about 30. Now, as of 4 a.m. on Wednesday, a total of 49 quakes have occurred in that section.

All were about 6 miles below the ocean floor, and in a range of nearly 100 miles long from west to east, and from north to south. The range is close to where the Juan de Fuca Ridge and the Blanco faultline meet.

Geologist Scott Burns of Portland State University called the swarm normal, but the numbers and other aspects were surprising.

"It is just rarer because it was out at the west end of the Blanco Fault which gets less activity but when it has swarms, the magnitudes are greater," he said. "There was a 5.8 which is huge! It took a long time for that one to build up. During the year we get lots of quakes on the Blanco Fault (fault between the Juan de Fuca and Gorda plates) and most are closer in. Yesterday's swarm was farther out and bigger and less common."

Most were in the 4.0 to 4.8 magnitude range, but at least ten were over 5.0, which is sizable. It takes a magnitude 7.0 or higher to generate a tsunami. Two quakes were magnitude 5.8, and a few in the last 24 hours set off quake alert apps, to the chagrin of numerous recipients. Some on social media expressed worry about these alerts, but the larger quakes were nowhere near population centers.

The USGS did get some reports of people having felt these quakes.

See in real time Oregon, Coast Earthquake Updates - Live Data Oregon, Washington, California, Tsunamis

According to geology experts like Burns, this area is not a threat to population centers, and it will not help trigger the larger Cascadia quake.

The Blanco Fracture does intersect with some of the larger faultlines in various ways, however.

The Cascadia Subduction Zone is the large one that the Oregon and Washington coast have to worry about, where the two plates of the Pacific Ocean and North America are butting up directly, with one sliding under the other.

There are smaller sections that grind up against each other, such as the Blanco area – but here the plates are sliding past each other.

Seaside geologist Tom Horning has spoken to Oregon Coast Beach Connection about this area in the past.

“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.”

The Cascadia Subduction Zone is the large one, however.

“The main subduction zone interface is locked to a depth of about 40 miles along a gently inclined ramp, so it is locked for nearly 150 miles along an interface that extends for 600 miles north south,” Horning said. “When it breaks, we will have a M9 quake and the killer tsunami.”

Quakes such as these swarms at the Blanco area have nothing to do with the larger faultine, and contrary to popular belief they are not “releasing” pressure for that area and are not lessening the chances of the big one.

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Photos above courtesy Seaside Aquarium

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