M 5.5 - off the coast of Oregon

Tectonic Summary

An earthquake swarm started December 7, 2021 approximately 400 km offshore from the Oregon coast. The swarm occurred along the Blanco Fracture Zone, a right-lateral transform fault zone marking the boundary between the Pacific Plate to the southwest and the Juan de Fuca Plate to the northeast. In the first 12 hours of the swarm, the USGS recorded 37 earthquakes of M3.4 or greater, including nine earthquakes of M5 or greater. In this time range, the two largest earthquakes in the swarm were M5.8. There were sparse reports of light felt shaking associated with these earthquakes along the Pacific Northwest coast.

Moment tensor solutions for the larger events in the swarm primarily show faulting occurring as a result of strike-slip motion 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 tensors and orientation of the fracture zone, these events likely 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/year along an azimuth of 110 degrees from north. The Juan de Fuca plate ultimately subducts beneath North America along the Cascadia subduction zone about 360 km (220 miles) to the east of this swarm; 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.

Events the size that have so far occurred in this swarm will not generate noticeable tsunamis. The main tsunami threat for the Pacific Northwest is the Cascadia subduction zone that lies ~360km (220 miles) to the east of this sequence. The Cascadia subduction zone has not shown any indication of increased seismicity during this sequence. Additionally, previous recorded swarms in this region have not triggered significant earthquakes in the Cascadia subduction zone.

The Blanco Fracture Zone has previously hosted similar swarms to the December 2021 swarm. Since 1995, the largest earthquake within 50 km of the December 2021 swarm was M6.3 in January 2003. The January 2003 earthquake was part of a swarm that occurred over a roughly 24-hour period and included four other earthquakes of M5.1-M5.7. In June and July 2008, this region hosted a swarm with 24 earthquakes larger than M4 and 7 earthquakes spanning M5.0 to M5.7. A smaller swarm in this region occurred in August 2009 and included four earthquakes of M4.3 or larger, including an M5.1 and M5.0 that occurred 13 minutes apart. Within 100 km of the recent swarm, there were at least 100 earthquakes of magnitude 5 and larger over the preceding 100 years.

Key Points

• This earthquake swarm started 12/07/2021 at 13:21 UTC (08:21 EST, 05:21 PST) with a magnitude 4.2 earthquake.

• By 18:00 UTC (13:00 EST, 10:00 PST) on Wednesday Dec 8, roughly 29 hours after the start of the sequence, the USGS had located 57 earthquakes M3.5+ in the swarm region, including 14 earthquake of M5 or larger. The two largest earthquakes in the sequence over this time span were M5.8.

• The swarm is located within the Blanco Fracture Zone, a strike-slip fault system located on one edge of the Juan de Fuca plate about 400 km (260 miles) offshore.

• The swarm is about 325km (200 miles) to the west of the Cascadia Trench where the Juan de Fuca plate begins to subduct beneath the North American plate.

• Events this size are very unlikely to generate tsunamis. The main tsunami threat for the Pacific Northwest is the Cascadia subduction zone that lies ~325km (200 miles) to the east of this sequence. The Cascadia subduction zone has not shown any indication of increased seismicity during this sequence. Additionally, previous recorded swarms in this region have not triggered significant earthquakes in the Cascadia subduction zone.

• The earthquakes are located offshore and have only caused sparse reports of light shaking. “Did You Feel It?” reports from the largest earthquake (M5.8) indicate light felt shaking in Oregon.

• The fault plane solutions for the larger earthquakes are all strike-slip, consistent with the right-lateral strike slip nature of the plate boundary.

• There is precedence for swarms like this one in this plate boundary zone; thus, the ongoing swarm does not broadly reflect abnormal seismic activity.

• The largest event within 50 km of this swarm since 1995 was of M6.3 in January 2003. That earthquake was also part of a swarm that predominantly occurred over a roughly 24-hour period and included four other earthquakes of M5.1-M5.7.

• Previous swarms in this region have not triggered significant earthquakes in the Cascadia subduction zone.

• This series is currently very active, the behavior can continue for days to weeks, or it can stop suddenly any time