The Pacific Northwest was jolted into the spotlight on Wednesday, when a series of strong earthquakes off the Oregon coast awakened fears of a big one.
By Jochen Braunmiller, Ph.D., University of South Florida
Citation: Braunmiller, J., 2021, Strong earthquake swarm ongoing off Oregon coast, Temblor, http://doi.org/10.32858/temblor.222
More than a dozen magnitude-5 earthquakes have struck off the Oregon coast since a seismic swarm started in earnest on Tuesday, December 7, with a magnitude-4.2 shock, according to the U.S. Geological Survey (USGS). As of 11 am PST on Thursday, the two largest earthquakes reached magnitude-5.8. The swarm is ongoing and other strong or even larger events might still happen.
Immediately after a strong earthquake off the Oregon coast, fears of what it means for the Cascadia subduction zone usually surface. “Will a ‘Big One’ happen soon?” is a common refrain. The short answer is that the current swarm is not directly related to the Cascadia subduction zone.
A zone primed for a major quake
The Cascadia subduction zone is the plate boundary where the Juan de Fuca plate dives beneath the North American plate at a few centimeters per year. This subduction zone, which is currently locked, could produce the ‘Big One’ — an earthquake that could rupture a ~1,000 kilometer-long fault that runs 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 one will occur, but we do know that it will (see Goldfinger et al. 2012 for a history of past events derived from turbidites).
The Blanco Transform Fault (Blanco transform) is a section of the plate boundary between the Pacific and Juan de Fuca plates. Its eastern end is approximately 100 miles (150 kilometers) west of Cape Blanco, Oregon and it runs for about 200 miles (350 kilometers) 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 “strike-slip” motion — like movement along the San Andreas Fault in California. USGS fault plane solutions — commonly referred to as beachballs — for the larger earthquakes in the series indicate right-lateral strike-slip motion, meaning opposing blocks of crust move to the right relative to each other. Because the sides of the fault are moving horizontally, tsunamis, which are the result of vertical movement of the seafloor (or landslides into water), have not occurred, even for the largest Blanco transform earthquakes.
Differences in fault behavior
The Blanco Transform Fault is segmented into four sections. The current swarm is occurring along a wide zone along and perpendicular to the main fault, according to USGS data. Some of the distribution in locations, particularly perpendicular to the fault, is likely due to difficulties accurately and precisely locating offshore earthquakes. The earthquakes are rupturing west-northwest trending faults, according to USGS beachball data and it is likely that most earthquakes line-up along known fault segments. The USGS data suggest the main locus of swarm activity may have migrated several tens of kilometers to the east since its start. The lack of seismic stations on the ocean floor close to the swarm makes accurate locations difficult but we know from a temporary deployment of seismic sensors on the ocean floor that earthquakes along the Blanco transform generally follow linear fault traces (Kuna et al., 2019). No such network is currently in place and we will know more about the actual distribution of the events only once they have been relocated relative to each other.
An interesting observation about seismicity along the Blanco transform is that the eastern and western parts behave differently. The largest historic Blano transform earthquakes (magnitude-6.5) occurred along the eastern part and are usually mainshock-aftershock sequences, where the largest aftershock is roughly at least one magnitude unit smaller than the mainshock. The eastern part is also one of the few regions in the world where large repeating earthquake patches have been found and the repeat interval is on the order of 15 years (Boettcher and McGuire, 2009).
In contrast, earthquake activity along the western Blanco transform often occurs in swarms (Dziak et al., 1996; 2003; Merle et al., 2008) — a sequence of earthquakes without a distinct mainshock. The current swarm is no exception because the two largest events both reached magnitude-5.8 and several more are in the magnitude-5.3-5.5 range. Seismologists are not sure why swarms happen here, but they are likely related to the complex geometry of the faults and more widely distributed seismicity — compared to the eastern Blanco transform — that suggests activity on roughly parallel faults (Braunmiller and Nabelek, 2008). The active faults in the western Blanco transform are younger, less-well developed, and probably consist of shorter segments, favoring earthquake swarms over mainshock-aftershock sequences.
It is likely that the swarm will rumble on for another few days or perhaps weeks. The seismic stations used by the USGS are on land and because of their distance from the epicenters, can detect only larger events. The stations will miss the large number of smaller events that are expected.
It is highly unlikely that a Blanco transform earthquake will ever be large enough to cause damage on land or to generate a tsunami, but its frequent activity is interesting for seismologists.
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