Magnitude 7.0 earthquake strikes offshore Cape Mendocino, California

Reports of shaking >1,000 km away.

Citation: Bradley, K., Hubbard, J., 2024. Magnitude 7.0 earthquake strikes offshore Cape Mendocino, California. Earthquake Insights, https://doi.org/10.62481/a3404512

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A magnitude 7 earthquake struck ~55 km offshore northern California at 10:44 AM local time on December 5, 2024. This is the ninth earthquake of 2024 to reach magnitude 7, across the globe — a little low compared to the annual average of about 14 events per year since 1980 (although this average does include aftershocks of really big events).

The earthquake caused widespread shaking, reaching intensity ~VII (very strong) on the coast closest to the epicenter. Here’s a map showing the estimated shaking intensity, which is routinely produced by the USGS National Earthquake Information Center for most moderate to large earthquakes around the globe:

Figure 1: Early aftershocks and estimated shaking intensity for the Mw7.0 earthquake.

So far, more than 15,000 people have reported their experience to the USGS. You can add your own felt report here. Fortunately, the maximum shaking intensity was (relatively) limited, because the earthquake occurred pretty far offshore. The rugged coastline of Cape Mendocino is relatively sparsely populated. Minor property damage has been reported, but no injuries.

Figure 2: Felt shaking. reported to the USGS. Source: https://earthquake.usgs.gov/earthquakes/eventpage/nc75095651/dyfi/intensity

The quake triggered a ShakeAlert early warning, which was distributed to people who were expected to experience at least intensity III or IV (depending on the settings of the warning system). For small earthquakes, relatively few people can benefit from ShakeAlert, because in the time it takes to detect and characterize the earthquake, most of the shaking has already occurred. However, for this larger event, some people were notified tens of seconds in advance — for instance, people in Fort Bragg, 146 km to the southeast, could have received up to 26 seconds of advance notice, depending on how long it took for the alert to be delivered. To receive ShakeAlert warnings onto your phone, follow the instructions here. To learn what to do if you receive an alert, click here. Note that the protocol varies depending on where you are and what you are doing (e.g. at home vs. driving), so it is a good idea to review the instructions while thinking about your daily routine! Chances are you won’t want to try to look this stuff up on the spot.

(2024-12-06, 20:53: Please note that the paragraph above was edited slightly for clarity following feedback from a scientist who works on earthquake early warning.)

A tsunami warning was also temporarily issued after the earthquake, but was later cancelled. Because the earthquake was strike-slip, causing lateral rather than vertical motion of the seafloor, it was not expected to trigger a tsunami. However, it is always better to be safe than sorry: even strike-slip earthquakes can cause underwater landslides, which can create tsunamis. Because underwater landslides are difficult to detect (much more difficult than earthquakes), the only way to be sure this did not happen is to wait and see. When in doubt, check tsunami.gov for the latest information.

Interestingly, about three minutes after the M7.0 mainshock, a second large earthquake was reported ~250 km to the southeast. The initial reported magnitude of this second earthquake was M5.8; however, it was soon downgraded to M4.1. More recently, it was upgraded slightly to M4.3. We suspect that this event was triggered by the waves of the first earthquake, but was mischaracterized because of interference with the shaking from the more distant, larger earthquake. Determining the true magnitude seems to be a bit of a challenge. It is truly fortunate that we have real live seismologists around to work through the data rather than relying only on automated systems!

The images below show two seismographs, with vertical lines marking the arrival of the P and S waves of the triggered M4.3 event, which arrived while the stations were still experiencing the effects of the larger M7.0. The upper seismograph is from station GCVB, about 18 km away from the triggered earthquake. The lower one is from stations SKGS, about 27 km away. Only stations within ~20 km of the triggered event were able to record it effectively.

Figure 3: Seismographs from two stations near the M4.3 triggered earthquake, illustrating how the signal was only clear near the epicenter. Source: https://ds.iris.edu/wilber3/find_stations/11909389

Tectonic setting

Why is Cape Mendocino such a seismically active area? Let’s take a look at some data, and also examine some previous large earthquakes. We have actually written about earthquakes around Cape Mendocino twice before — once after a M4.5 earthquake on March 21, 2023, and again after a M4.7 earthquake on September 30, 2023. So you can see that earthquakes are not unexpected here, although of course the recent M7.0 is much larger than either of those events.

There are basically four potential sources of large earthquakes in the Cape Mendocino region, each of which represents a different part of the plate tectonic system. We like the look of this simple map, taken from a report on the 1994 Mendocino Fault earthquake (discussed later):

Figure 4: A map of the Mendocino Triple Junction, from Dengler et al. (1995).

The tectonic plates in this area are the North America Plate, the Pacific Plate, and the Gorda Plate.

The Gorda Plate is basically the southern part of the larger Juan de Fuca Plate, and is entirely oceanic crust, produced by spreading along the Gorda Ridge. If you’re wondering (we were), the Gorda Plate is named after the Gorda Basin, an area so rich in fishing that Portuguese fishermen describe it as fat (gorda) in the 19th century.

The North America Plate is all continental crust in this particular area, and is therefore mostly above sea level. North of Cape Mendocino, the western edge of the North America Plate dives underwater, descending to the Cascadia Trench, where it meets the Pacific Plate.

The Pacific Plate is mostly oceanic crust in this area, but farther south from Cape Mendocino it is also decorated by a narrow band of continental crust containing the California coastline. The Pacific-North America plate boundary is notoriously wide and complex, and gives California much of its seismic character.

Cape Mendocino is the nexus of action, because the three plates meet at one point — called a triple junction. The relative motions of these three tectonic plates are what causes most earthquakes in this area.

Like the four cardinal directions on a compass, there are basically four cardinal fault systems that surround Cape Mendocino. You can explore a nice map of California faults using this interactive map from the California Department of Conservation.

To the west of Cape Mendocino, the offshore Mendocino Fault is a seismically active strike-slip fault that connects active oceanic spreading along the western edge of the Juan de Fuca Plate to the strike-slip and subduction faults to the east. This fault is also (perhaps more commonly?) known as the Mendocino Fracture Zone. However, the term fracture zone indicates seismically dead features that were once transform faults. In contrast, the Mendocino Fault is technically an active oceanic transform fault — a fracture zone in the making. In fact, the true Mendocino Fracture Zone is over 4,000 kilometers long, stretching across much of the Pacific Ocean basin. Thus, we prefer the Mendocino Fault terminology for this much shorter (about 270 km long) and much more active fault.

Anyway, today’s M7 event is one of only two clear examples of a truly large strike-slip rupture of the fault, the other being the 1994 M7.0 Mendocino Fault earthquake, which again we will discuss later. Similar-sized earthquakes did occur offshore Cape Mendocino in 1878 and 1923, but to our own knowledge they have not been conclusively linked with rupture of the Mendocino Fault. Thus, today’s earthquake is an exciting event that will probably teach us some important lessons about this plate boundary fault.

To the north of Cape Mendocino, the Cascadia subduction zone quietly lurks. This super-silent subduction zone produces very few small earthquakes each year, instead preferring to undergo periods of slow, aseismic slip. However, geology and historical records prove that this subduction zone does occasionally produce a mega-quake by wholesale rupture of a large part of the subduction megathrust (the inevitable Cascadia earthquake is one of the famous “big ones” we are all waiting for). We wrote about the Cascadia subduction zone on October 31, 2024 following an unrelated M6.0 earthquake offshore, so feel free to check that out.

The vow of seismic silence taken by the Cascadia subduction zone, which is probably due to the geological youth (and thus high temperature) of the subducting lithosphere, is broken near the Cape. There, internal deformation along the edge of the subducting slab produces numerous earthquakes at depths between 15 and 30 kilometers. The 2022 Mw6.4 Ferndale earthquake is a recent, and unusually large, example of this kind of deformation.

Some moderate to large earthquakes have also been recorded in the upper plate, above the subducting slab. For instance, the 1992 Mw7.2 Cape Mendocino (a.k.a. Petrolia) earthquake was a thrust-type event that lifted the coast of the Cape upward by about a meter. While the thrusting in this earthquake is clearly related to the plate convergence, the 1992 earthquake seems too shallow to actually have ruptured the subduction plate interface.

To the south of Cape Mendocino, the San Andreas Fault cuts along the rugged California coast. (Recall that we were enumerating the four cardinal faults of Cape Mendocino; we got a little side-tracked.) It is well known that this famous strike-slip fault system can produce large earthquakes anywhere along its entire length. In 1906 a huge rupture almost 300 kilometers long originated near San Francisco, propagating southward to San Juan Bautista and northward to Cape Mendocino. This rupture produced the devastating M7.9 San Francisco earthquake that helped set much of modern earthquake science in motion. However, since 1906 there has been little significant seismicity along the northernmost San Andreas Fault itself, and most active seismicity occurs on other fault strands within the coastal ranges. For more discussion about the faults of the northern San Andreas, check out our June 2024 post about a M4.3 earthquake on the Maacama Fault.

Finally, offshore to the northwest of Cape Mendocino there is a region of surprising seismicity, surprising mainly because it occurs far from any actual plate boundary. As far as cardinal directions, this one is perhaps a bit poorly defined, but it will have to do. The Gorda Plate is apparently stuck in a difficult position. The oceanic spreading along the Gorda Ridge to the west drives the Gorda Plate southeastward toward land and into the Cascadia subduction zone. However, at the same time, the Mendocino Fault enforces purely east-west motion along the southern boundary of the plate. Unable to obey both masters, the plate has to instead break apart and deform. Large earthquakes within the Gorda Plate tend to be strike-slip events, and can achieve magnitudes above 7. Luckily, their distance from shore limits their shaking potential, and their strike-slip nature limits their ability to raise tsunamis.

Each of these tectonic systems has big faults that can produce big earthquakes. Interactions of and feedbacks between these faults cause even further seismic chaos, reflected in the large number of earthquakes that occur across and around the Cape, from the shallowest crust down into the uppermost mantle.

Of course, the question always arises: could earthquakes on the Mendocino Fault — like the recent M7.0 — somehow trigger, accelerate, or otherwise increase the likelihood of the big Cascadia subduction earthquake, or other faults nearby? We can’t give any kind of answer to that ourselves, but we are confident that experts will be running the numbers soon.

This kind of question has been asked, and partially answered, before. A study in 2010 found that it was likely that earthquakes on the Mendocino Fault were indeed promoting rupture of faults within the Gorda Plate, and vice versa (Rollins and Stein, 2010).

That’s a lot to take in! Ultimately, the complexity of faulting in the Cape Mendocino area is apparent in a map of overall recorded seismicity of the region, colored by time (events prior to 1976 are black):

Figure 5: Map of recorded seismicity along and around the Mendocino Fault. Earthquakes are colored by time, and also projected to the right onto a timeline.

As promised several times, let’s now return to the most similar previous earthquake. The 1994 Mendocino Fault earthquake originated near 126° west longitude, farther west than today’s mainshock origin. Aftershocks of the 1994 earthquake eventually illuminated the entire section of the fault between the mainshock and land, suggesting that the rupture propagated all the way to the coast. GPS displacements recorded landward movements of the crust to the north of the Mendocino Fault by up to ~25 millimeters, consistent with the expected right-lateral strike slip motion. This was an early example of a large earthquake monitored by a GPS network.

Figure 6: Figure 2 of Dengler et al., 1995, showing the mainshock and aftershocks of the 1994 Mendocino Fault earthquake.

This scenario is pretty similar to the 2024 earthquake sequence, at least thus far.

We plotted up the recorded earthquakes one day after the 1994 mainshock (shown in red) and thus far after today’s mainshock (shown in blue). The plate boundaries (black lines) are highly schematic in this plot. It is clear that the 2024 event started farther east, but it seems to have ruptured both west and east of the origin (note to the cognoscenti: we are weirdos who usually plot focal mechanisms at their origin locations, and not their centroid locations — and when we don’t, we try to make a note of it).

Figure 7: One day of aftershocks of the 1994 earthquake (red), compared to the aftershocks thus far from the 2024 earthquake (blue).

An interesting question is whether the 2024 earthquake rupture overlaps with the 1994 rupture, or whether it has ruptured a more easterly part of the fault that did not really break in 1994. Certainly the epicenters of the earthquakes are not the same. A M7 rupture should be ~50 km long, and the two earthquake epicenters are ~50 km apart, so it is possible that the ruptures could overlap in between. Plate motion models imply up to 50 mm/yr of slip on the fault, although it is poorly constrained, so it possible that ~1-1.5 m of strain might have “recharged” in the 30 years since the 1994 earthquake.

At first glance, there is apparently some overlap between the easternmost 1994 aftershocks and the westernmost 2024 aftershocks. The aftershock zone of the 2024 earthquake will progressively fill in over time, as faults surrounding the mainshock continue to exceed their stress limits. Aftershocks tend to outline fault rupture zones, illuminating parts of the fault system that were stressed by the original rupture, but did not necessarily slip themselves. We will learn more about those stressed areas in the coming days and months, as more earthquakes occur, and as seismologists revisit the original data to more precisely locate events.

References

Dengler, L., K. Moley, R. McPherson, M. Pasyanos, J. W. Dewey, M. Murray, 1995. The September 1, 1994 Mendocino Fault earthquake. California Geology. https://www.conservation.ca.gov/cgs/Documents/Publications/California-Geology-Magazine/CG_1995-03-04.pdf

Hubbard, J. and Bradley, K., 2024. M4.3 earthquake on Maacama Fault rattles Northern California. Earthquake Insights, https://doi.org/10.62481/6d56e269

Hubbard, J. and Bradley, K., 2023. M4.6 earthquake in California triggers ShakeAlert - March 21, 2023. Earthquake Insights. https://earthquakeinsights.substack.com/p/m46-earthquake-in-california-triggers

Hubbard, J. and Bradley, K., 2024. M6.0 offshore Oregon lightly felt onshore. Earthquake Insights. https://doi.org/10.62481/c41be517

Hubbard, J. and Bradley, K., 2023. Magnitude 4.7 earthquake jolts northern California. Earthquake Insights, https://doi.org/10.62481/ac6116bf

Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault. Journal of Geophysical Research: Solid Earth, 115(B12). https://doi.org/10.1029/2009JB007117

Yoon, C.E. and Shelly, D.R., 2024. Distinct Yet Adjacent Earthquake Sequences near the Mendocino Triple Junction: 20 December 2021 M w 6.1 and 6.0 Petrolia, and 20 December 2022 M w 6.4 Ferndale. The Seismic Record, 4(1), pp.81-92. https://doi.org/10.1785/0320230053