M6.0 offshore Oregon lightly felt onshore

Not in any way related to the Cascadia subduction zone

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A magnitude 6.0 earthquake struck offshore the coast of Oregon on October 30th, 2024. Because the earthquake occurred more than 250 km away from land, it caused very limited shaking onshore: although 285 people reported their experience to the USGS, more than a quarter indicated that they did not feel the quake, and the remainder reported only weak shaking.

Figure 1: October 30, 2024 M6 earthquake offshore Oregon. Square show reports of felt shaking from the USGS. White = did not feel it. Blue = weak shaking. Contours around the star show estimated shaking intensities. Source: https://earthquake.usgs.gov/earthquakes/eventpage/us7000np33/map

Why would people who didn’t feel an earthquake bother reporting that to the USGS? First, let’s note that it is very helpful when people do this: if the only reports are from people who did feel shaking, then a handful of people who were in the right place at the right time can expand the apparently impacted region significantly. Negative reports are useful! However, they are also pretty rare, because in most cases people who didn’t feel an earthquake don’t have a reason to react.

Perhaps some people along the Oregon coast are more attentive than most to offshore earthquakes. That would make sense: the coastline of Oregon (and Washington to the north, and the northern bit of California to the south) is paralleled by the extremely hazardous Cascadia subduction zone. Here, the Juan de Fuca plate is sinking eastward beneath North America.

This plate boundary is famous in geological circles: while modern geophysical observations detect only very low levels of seismicity, geological evidence demonstrates that slip on this fault caused a great earthquake in 1700 CE (M8.7-9.2). Here is a scenario map from the USGS illustrating what shaking probably looked like in that event:

Figure 2: Scenario map of the 1700 CE Cascadia earthquake from the USGS. Source: https://earthquake.usgs.gov/scenarios/eventpage/cszm9ensemble_se/shakemap/intensity

Despite the fact that there are no written records of the shaking in 1700, geologists have managed to pinpoint not just the day (January 26) but even the hour (~9:00 PM, Pacific Time) of that rupture. The evidence is broad and diverse: coastal forests that were killed as the land sank during the rupture itself, with the last growth ring dating to 1699; layers of sand deposited above tidal muds, carried in by the subsequent tsunami; cracks filled with sand in subsided ground, indicators of shaking-induced liquefaction; and critically, a 2-5 meter-tall “orphan tsunami” that arrived at the same time at two locations 30 km apart in Japan with no precursory shaking. Oral histories of native people living in the area also tell of the shaking and the subsequent tsunami and associated landslides, which together killed hundreds of people. Further exploration has indicated that the earthquake also triggered underwater landslides, depositing layers know as “turbidites.” Below the most recent turbidite lie older, similar layers: clues about more distant earthquakes. In all, the evidence indicates that a ~M8-9 earthquake ruptures the Cascadia subduction zone every few hundred years. How many hundreds of years depends on who you talk to: some scientists argue for a recurrence interval of 300 years; others 500. This is an area of active research, and the specific number depends on which part of the fault is under investigation. However, all scientists agree that the earthquakes are irregular: some earthquakes occur only decades apart, while others are separated by centuries.

Either way, the next Cascadia earthquake is coming. For those interested in learning more about the impacts of such an event, we recommend this article by Kathryn Schulz at The New Yorker. The Pacific Northwest Seismic Network has specific recommendations for how to prepare.

So: like most subduction zones, Cascadia has a geological record of great earthquakes. However, it stands out among subduction zones for a different reason: where most are illuminated by seismicity, Cascadia is not. In other locations, we can track subducting slabs into the mantle by the earthquakes generated either on the subduction boundary, or within the sinking slab. This pattern is so consistent that it even has a special name: a Wadati-Benioff zone, named after the two seismologists who independently discovered it. However, Cascadia is different.

Below you can compare “silent” Cascadia with a much noisier part of the South American subduction zone. (The Nankai subduction zone, offshore southern Japan, shares this dubious distinction of silence with Cascadia, and is also known to produce great earthquakes.)

Figure 3: Comparison of seismicity at the silent Cascadia subduction zone (left) with part of the noisier South American subduction zone (right). Earthquakes are colored by depth.

Why is the Cascadia subduction zone so seismically quiet? It probably has something to do with the character of the subducting plate. Because there is an active spreading ridge located right offshore, the oceanic crust of the Juan de Fuca Plate is extremely young. That means that the plate is still quite hot, and the lithosphere is thin. Thus, there is less commotion as the plate bends downward and subducts.

While earthquakes do occur regularly offshore of Oregon, seismicity there is almost entirely limited to a completely different tectonic system: the transform plate boundaries that separate the Juan de Fuca plate from the Pacific Plate. That plate boundary, of course, is the one that ruptured on October 30th, 2024 — specifically, the Blanco Fracture Zone: a right-lateral transform fault zone, ~350 km long, that slips about 2 centimeters per year. The focal mechanism of today’s earthquake confirms that this is the feature that slipped: a fault oriented WNW-ESE, slipping in right-lateral motion.

Figure 4: Focal mechanism of the 2024-10-30 M6 earthquake offshore Oregon. Source: https://earthquake.usgs.gov/earthquakes/eventpage/us7000np33/moment-tensor

If you are worried that this M6 earthquake on the Blanco Fracture Zone might somehow trigger the next great earthquake at Cascadia, don’t be. First of all, only a minority of earthquakes end up being foreshocks of anything. Second, the earthquake occurred nearly 200 km away from the subduction zone, and was too small to cause stress changes that far away. While aftershocks should be expected within the Blanco Fracture Zone itself, the USGS aftershock forecast indicates that they should be minor, and likely not felt on land — only a 3% chance of an event above M5 over the next year.

Figure 5: Seismicity since 1980 recorded by the USGS. Earthquakes above M6 are labelled.

In fact, this earthquake seems like a particularly run-of-the-mill event. The USGS reports that, since 1980, ten M6+ earthquakes have occurred on the Blanco Fracture Zone — on average, one every 4.5 years; of those, six were right around the location of the recent M6. The largest was a M6.5 in 1985.

So, this earthquake is right on target: it’s been about five years since the last M6+ event on this fracture zone. So, why did we write about it? Well, we figured that this would be a good opportunity to talk about Cascadia — because if we waited for a notable earthquake to occur on the silent subduction zone itself, we might have to wait for a long time.

References

Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuji, Y., Ueda, K. and Yamaguchi, D.K., 2016. The orphan tsunami of 1700: Japanese clues to a parent earthquake in North America. University of Washington Press. https://doi.org/10.3133/pp1707

Bradley, K., Hubbard, J., 2024. M7.1 earthquake strikes southern Japan; megaquake advisory issued. Earthquake Insights, https://doi.org/10.62481/cea4a692

Goldfinger, C., Nelson, C.H., Morey, A.E., Johnson, J.E., Patton, J.R., Karabanov, E.B., Gutierrez-Pastor, J., Eriksson, A.T., Gracia, E., Dunhill, G. and Enkin, R.J., 2012. Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone (No. 1661-F). US Geological Survey. https://doi.org/10.3133/pp1661F

Ludwin, R.S., Dennis, R., Carver, D., McMillan, A.D., Losey, R., Clague, J., Jonientz-Trisler, C., Bowechop, J., Wray, J. and James, K., 2005. Dating the 1700 Cascadia earthquake: Great coastal earthquakes in native stories. Seismological Research Letters, 76(2), pp.140-148. http://dx.doi.org/10.1785/gssrl.76.2.140

Missett, K., 2023, Don’t get scared, get prepared! PNSN blog, https://www.pnsn.org/blog/2023/09/14/don-t-get-scared-get-prepared

Nelson, A.R., DuRoss, C.B., Witter, R.C., Kelsey, H.M., Engelhart, S.E., Mahan, S.A., Gray, H.J., Hawkes, A.D., Horton, B.P. and Padgett, J.S., 2021. A maximum rupture model for the central and southern Cascadia subduction zone—reassessing ages for coastal evidence of megathrust earthquakes and tsunamis. Quaternary Science Reviews, 261, p.106922. https://doi.org/10.1016/j.quascirev.2021.106922

Schulz, K., 2015. The Really Big One. The New Yorker, https://www.newyorker.com/magazine/2015/07/20/the-really-big-one