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Two Sizable Earthquakes Off S. Oregon Coast Within Minutes of Each Other

Published 02/20/21 at 7:26 PM PDT
By Oregon Coast Beach Connection staff

Two Sizable Earthquakes Off S. Oregon Coast Within Minutes of Each Other

(Bandon, Oregon) – Two sizable offshore quakes happened Saturday afternoon off the southern Oregon coast, both within minutes of each other.

According to the USGS, the first was a magnitude 5.1 that hit at 1:15 p.m., about 175 miles west of Bandon. It was at a depth of 6.2 miles.

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The second hit several miles away at a magnitude 4.9 at 1:22 p.m. It also was at the same depth as the first quake.

The USGS is reporting at least two people felt both of the quakes.

There was no tsunami alert. It takes at least a M7 quake to initiate a tsunami.

These often come from a series of underwater faults in and surrounding the Cascadia Subduction Zone, although they are separate faultlines. Today’s two quakes off the south Oregon coast took place along the Blanco Fracture, which intersects with other areas of the greater faultlines and the two big continetnal plates in various ways.

The Cascadia Subduction Zone is part of the boundary of the Juan de Fuca plate, and parts of that plate contain the smaller areas that grind up against each other, according to Seaside geologist Tom Horning.

It’s these spots along the Blanco Fracture that often create these smaller offshore quakes, and they do so maybe 20 times a year or more.

“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.”

It’s the larger section of the Cascadia Subduction Zone we have to worry about.

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“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.”

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