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6.3 Earthquake Hits West Of Coos Bay

This morning (8/29) just after 8am when a 6.3 earthquake struck near the coast of Oregon East of Coos Bay. According to the USGS website, there were more than 350 reports of people feeling shaking from the coast, inland to Eugene, even all the way inland to Bend According to USGS the quake occurred as the result of strike-slip faulting along the Blanco Fracture Zone, a transform fault marking the boundary between the Pacific Plate to the southwest and the Juan de Fuca Plate to the northeast.

Moment tensor solutions show faulting occurred as a result of slip on a steeply dipping fault, either from left-lateral slip on a northeast striking fault or right-lateral slip on a southeast striking fault. Given the moment tensor and orientation of the fracture zone, this event occurred on a steeply dipping, right-lateral, southeast striking fault. At this location, the Juan de Fuca Plate slides past the Pacific Plate at a rate of 49 mm a year.

The Juan de Fuca plate ultimately subducts beneath North America along the Cascadia subduction zone about 200 km to the east of today’s event; hence, this earthquake did not occur on the subduction zone, but rather is the result of Pacific:Juan de Fuca plate boundary interactions farther west. The Pacific-Juan de Fuca Plate boundary offshore of northern California, Oregon, and Washington routinely host moderate size earthquakes.

Within 250 km of the August 29th event, there were 21 other earthquakes of magnitude 6 and larger over the preceding 100 years. The largest of these was an M 6.9 earthquake that occurred on July 13th, 1991 that was felt throughout much of western Oregon and northern California. The 1991 event ruptured southeast of the Blanco Fracture Zone, within the Juan de Fuca Plate closer to the coasts of Oregon and northern California. One year prior to this August 29, 2019 earthquake, a similar earthquake in size, location, and mechanism, occurred on August 22, 2018 and was felt along the Oregon coast.

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