Strong earthquake swarm continues off the coast of Oregon

The Pacific Northwest was in the spotlight on Wednesday, when a series of powerful earthquakes off the coast of Oregon raised fears of a major earthquake.

Written by Jochen Braunmiller, Ph.D., University of South Florida

Quote: Brownmiller, C, 2021, Strong earthquake swarm continues off the coast of Oregon, Temblor, http://doi.org/10.32858/temblor222

More than a dozen -5 magnitude earthquakes have struck off the coast of Oregon since the intense seismic swarm began Tuesday, December 7, with a magnitude of 4.2, according to the US Geological Survey (USGS). As of 11 a.m. PT Thursday, the two largest earthquakes measured 5.8 on the Richter scale. The swarm continues and other strong or even larger events may continue.

Immediately after a strong earthquake strikes off the coast of Oregon, concerns usually arise about what this means for the Cascadia Subduction Zone. Is ‘the big one’ going to happen soon? It is a common refrain. The short answer is that the current swarm is not directly related to the Cascadia subduction zone.

Oregon coast. Credit: dumplings, Pixabay

An area prepared for a major earthquake

The Cascadia subduction zone is the plate boundary where the Juan de Fuca plate sinks below the North American plate a few centimeters per year. This subduction zone, which is currently closed, could produce the “big quake” – an earthquake that could devastate a roughly 1,000-kilometre fault stretching from Cape Mendocino in California to Vancouver Island in British Columbia. The last such event occurred in January 1700 (Atwater et al., 2005). We do not know when the next event will occur, but we do know that it will (see Goldfinger et al. 2012 for a history of past events derived from aker).

Map of the boundaries of the Cascadia subduction zone plate. Credit: Alicia.iverson, CC BY-SA 4.0, via Commons Wikimedia

The Blanco Transform Fault (BTF) is part of the plate boundary between the Pacific and Juan de Fuca plates. Its eastern end lies about 100 miles (150 km) west of Cape Blanco, Oregon, and extends for 200 miles (350 km) in a west-northwest direction, connecting two oceanic ridges (Embley and Wilson, 1992). The two plates on either side of the fault move horizontally with respect to each other, known as a “slip-slide” motion—like the motion along the San Andreas Fault in California. USGS fault plane solutions—commonly referred to as beach balls—for larger earthquakes in the series indicate right-sided slip motion, meaning that opposing crust masses move to the right relative to each other. Because the fault sides move horizontally, tsunamis, caused by vertical movement of the sea floor (or landslides in the water), did not occur even for the largest BTF earthquakes.

Recent seismic activity on the Blanco transform fault is relatively far from the Cascadia subduction zone. Credit: J. Braunmiller

Differences in wrong behavior

BTF is divided into four sections. The current swarm occurs along a wide area along and perpendicular to the main fault, according to USGS data. Some distribution at the sites, especially orthogonal to the fault, is likely due to difficulties in accurately and accurately locating marine earthquakes. Earthquakes are ripping west-northwest directional faults, according to USGS beachball data, and most earthquakes likely line up along known fault segments. USGS data indicate that the main site of swarm activity may have migrated several tens of kilometers to the east since its inception. The lack of ocean floor seismic stations near the swarm makes precise positioning difficult, but we know from the temporary deployment of ocean floor seismic sensors that earthquakes along the BTF generally follow linear fault traces (Kuna et al., 2019). There is no such network at present and we will know more about the actual distribution of events only once they are transmitted relative to each other.

USGS aircraft malfunction solutions, otherwise known as beach balls, to larger swarm events. Credit: data from the USGS, image by J. Braunmiller

An interesting observation about seismic activity along the BTF is that the eastern and western parts behave differently. The largest historical BTF earthquakes (magnitude 6.5) occurred along the eastern part and are usually aftershock sequences, with the largest aftershocks being at least one unit smaller than the main shock. The eastern part is also one of the few regions in the world where large recurring seismic patches have been found and the recurrence interval was on the order of 15 years (Boettcher and McGuire, 2009).

In contrast, earthquake activity along the western region of the BTF often occurs in swarms (Dziak et al., 1996; 2003; Merle et al., 2008)—a series of earthquakes without a distinct main shock. The current swarm is no exception because the two largest events both reached 5.8 degrees and many others are in the 5.3-5.5 range. Seismologists are not sure why the swarms occur here, but they are likely related to the complex geometry of the faults and more widely distributed earthquakes—compared to the eastern BTF—that indicate activity on nearly parallel faults (Braunmiller and Nabelek, 2008). Active faults in the western part of the BTF are younger, less developed, and probably composed of shorter segments, which favor earthquake swarms over main-shock-aftershock sequences.

continuous swarm

It is likely that the swarm will rush in for a few more days or perhaps weeks. The seismic stations used by the USGS are on Earth and because of their distance from earthquake epicenters, they can only detect larger events. Stations will miss the large number of expected smaller events.

It is highly unlikely that a BTF earthquake was large enough to cause damage on the ground or to generate a tsunami, but its frequent activity is intriguing to seismologists.

references

Atwater, B.F., S. Musumi-Rokkaku, K. Satake, Y. Tsuji, K. Ueda, and D. K. Yamaguchi, The 1700’s Orphan Tsunami – Japanese Evidence for the Original North American Earthquake, US Jeol. Investigative Survey Paper 1707, doi: 10.3133/pp1707, 2005.

Boettcher, MS, J. J. McGuire, Scaling the relationships of seismic cycles on mid-ocean ridge shift faults, Geophysics. Precision. I wish. , 2009.

Braunmiller, J, and J. Nabelek, Segmentation of the Blanco Fault Zone Transformation from Earthquake Analysis: Complex Tectonics of Ocean Conversion Fault, J. Geophys. Res, 113, B07108, doi: 10.1029/2007JB005213, 2008.

Dziak, RB, CG Fox, RW Empley, GE Lupton, JC Johnson, WW Chadwick, and R. Kosky, Detection and Response of a Possible Volcanic Wave Event Swarm over the Western Transformation Fault Zone, Geophysics and Response. Precision. Lett., 23, 873-876, 1996.

Dziak, R. P., W. W. Chadwick, C. G. Fox, and R. W. Embley, Water temperature changes in the southern Juan de Fuca Ridge associated with the 6.2 Mw Blanco transformation earthquake, Geology, 31, 119-122, 2003.

Embley, RW, DS Wilson, Morphology of the Blanco Transform Fault Zone, Northeastern Pacific: Implications for its Tectonic Evolution, Marine Geophysical Research, 14, 25-45, 1992.

Goldfinger, C., C. H. Nelson, A. E. Morey, J. E. Johnson, J. R. Patton, E. B. Karabanov, J. Gutierrez-Pastor, A. T. Eriksson, E. Gracia, G. Dunhill, R. J. Enkin, A. Dallimore, and T. Vallier, Turbidite Event History – Methods and Implications for the Archaic Holocene in the Cascadia Subduction Zone, Jeol USA. Investigative Survey Paper 1661-F, doi: 103133/pp1661F, 2012.

Kuna, VM, J.L. Nabelek, and J. Braunmiller, Mode of Interactive Slide and crust-mantle at oceanic transformation faults, Nature Geoscience, 12, 138-142, doi: 10.1038/s41561-018-0287-1, 2019.

Merle, S.G., RP Dziak, R.W. Embley, J.E. Lupton, RR Greene, W.W. Chadwick, M. Lilley, DR Bohnenstiel, J. Braunmiller, M. Fowler, and J. Resing, Preliminary analysis of multiple beam, seafloor and water column data collected from the Guan de Fuca plate and Gorda Ridge earthquake swarm sites, March–April 2008, Eos Trans. AGU, 89 (52), Fall Meet. Supplement, Summary T23B-2025, 2008.

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