Did the 2024 magnitude 7.0 Cape Mendocino earthquake trigger aftershocks on the San Andreas?

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Coulomb stress calculations and the distribution of aftershocks indicate that the early December magnitude 7.0 earthquake likely stressed the northern end of the San Andreas, near the end of the 1906 San Francisco earthquake rupture.
 

By Volkan Sevilgen, Temblor, Inc., Shinji Toda , IRIDeS, Tōhoku University, Hector Gonzalez-Huizar, CICESE, and Ross S. Stein, Temblor, Inc.
 

Citation: Sevilgen, V., Toda, S., Gonzalez-Huizar, H., and Stein, R.S., 2024, Did the 2024 magnitude 7.0 Cape Mendocino earthquake trigger aftershocks on the San Andreas?, Temblor, http://doi.org/10.32858/temblor.355
 

On the morning of Thursday, Dec. 5, at 10:44 a.m. local time (Dec. 5, 18:44 UTC), a magnitude 7.0 earthquake struck offshore Northern California, about 100 kilometers (about 60 miles) west of the town of Ferndale, where a magnitude 6.4 event struck in 2022. According to the U.S. Geological Survey (USGS), the magnitude 7.0 strike-slip event occurred at a depth of about 10 kilometers (6 miles).

The National Weather Service issued a tsunami warning for parts of coastal Oregon and California, likely because of the initial magnitude of 7.3 (soon revised down to 7.0 as more data came in), shallow depth, and location of the event. The warning was broadcast as far south as Davenport, California (near Santa Cruz) and northward into Oregon, but was lifted about an hour later, when it became evident that no destructive tsunami was forthcoming.

Although the tsunami warning may have been a surprise, the earthquake itself was not. This particular region is among the most seismically active parts of the United States. In this preliminary analysis, we consider what happened and why and explore implications for future events. We also present evidence of dynamic triggering — a phenomenon by which seismic waves from one earthquake cause faults at a distance from the mainshock to rupture.
 

The collision of three tectonic plates

Just east of the earthquake’s epicenter is what’s called the Mendocino Triple Junction, where the Pacific, Juan de Fuca and North American plates — and the faults separating them — all meet in a chaotic crunch.
 

Figure 1. Aftershocks of the mainshock reveal rupture on the Mendocino Fault, which connects to the San Andreas and Cascadia megathrust at the tectonic triple junction. The earthquake epicenters were obtained from the USGS catalog at 9:00 a.m. Pacific time on Dec. 7. The inset includes red stars locating the magnitude 7 epicenter and the magnitude 5.8 Yerington earthquake that struck Nevada several days later. Credit: Temblor, CC BY-NC-ND 4.0
Figure 1. Aftershocks of the mainshock reveal rupture on the Mendocino Fault, which connects to the San Andreas and Cascadia megathrust at the tectonic triple junction. The earthquake epicenters were obtained from the USGS catalog at 9:00 a.m. Pacific time on Dec. 7. The inset includes red stars locating the magnitude 7 epicenter and the magnitude 5.8 Yerington earthquake that struck Nevada several days later. Credit: Temblor, CC BY-NC-ND 4.0

 

The San Andreas Fault, which separates the Pacific and North American plates through California, arrives at the Mendocino Triple Junction from the southeast. It takes a sharp left turn to meet the Mendocino Fault, a transform fault that stretches thousands of kilometers west, separating the Pacific Plate to the south from the Juan de Fuca Plate to the north. To the north, the Cascadia Subduction Zone accommodates the Juan de Fuca Plate sliding beneath North America, mostly in very large earthquakes every few hundred years (although just how Cascadia ruptures is still an active question for earthquake scientists).
 


 

At the Mendocino Triple Junction, the plates get distorted and misaligned, causing them to fracture, tear and deform in frequent earthquakes. Since 1980, this region has experienced six earthquakes of magnitude 7 or larger, including Thursday’s 7.0 shock. (For comparison, the destructive 1989 Loma Prieta earthquake was a magnitude 6.9 event.) Aftershocks and seismologic constraints suggest that Thursday’s quake occurred on the Mendocino Fault (Figure 1).
 

Seismic gap

A seismic gap is a section of an active fault that hasn’t experienced a significant earthquake in recent history, in between segments that have. Rollins and Stein (2010) identified a gap along the Mendocino Fault in between a magnitude 7.0 event in 1994 and a magnitude 6.1 event in 1983 (Figure 2). The December 2024 earthquake’s aftershock distribution suggests that the mainshock occurred in this gap, though some aftershocks extended beyond the previously identified gap boundaries toward the San Andreas Fault, possibly indicating stress transfer to adjacent fault segments (discussed further below). 

Although the epicenter was about 50 kilometers offshore, aftershocks within the first 24 hours extended eastward to within a few kilometers from the coast. This suggests that the rupture propagated eastward from the epicenter toward the coastline (Figure 1).
 

Figure 2. The likely rupture area of the Dec. 5, 2024, earthquake is consistent with a gap in the region that has been building at least since 1976. Both the O (in 2000) and T (in 2010) earthquakes, which were magnitude 5.9 events, were too small to accommodate the slip deficit in the likely rupture area. Credit: modified from Rollins and Stein, (2010)
Figure 2. The likely rupture area of the Dec. 5, 2024, earthquake is consistent with a gap in the region that has been building at least since 1976. Both the O (in 2000) and T (in 2010) earthquakes, which were magnitude 5.9 events, were too small to accommodate the slip deficit in the likely rupture area. Credit: modified from Rollins and Stein, (2010)

 

Ground-shaking models

Because of the large magnitude and relatively shallow depth, the earthquake was felt as far north as Salem, Oregon, and throughout much of northern California, including the San Francisco Bay Area and Central Valley. As of this writing, more than 16,000 people had submitted shaking reports through the USGS Did You Feel It? questionnaire.

Cape Mendocino — in line with the eastward-propagating rupture — recorded the highest ground shaking measured by seismic sensors, approximately 0.5 g, or 0.5 times the force of gravity. (This level of acceleration is similar to what passengers experience during emergency braking in an automobile.) Elsewhere, instruments and people alike reported mostly light shaking.

These shaking data provide an opportunity to assess how well Temblor’s global ground motion model (which uses location, magnitude, depth, and Temblor SiteAmp Vs30 data at 100-meter worldwide resolution) can predict shaking levels from new earthquakes. We directly compare the Temblor ground motion model predictions to observations — both felt reports and instrumental measurements (Figure 3). To do so, we downloaded the USGS Did You Feel It? report locations and corresponding shaking intensities. We also downloaded seismic station coordinates and measured shaking (Peak Ground Acceleration in g).

The Temblor ground motion model successfully predicts much of the observed shaking distribution of last Thursday’s earthquake. The Temblor ground motion model might slightly overestimate shaking at levels that won’t cause any damage far from the epicenter. We also note that at these distances, some of the observations are higher than our model. On balance, we consider our ground motion model to be consistent with observed shaking, especially at close distances to the rupture — where it matters the most.
 

Figure 3. Temblor's ground motion model (right panel) is consistent with the shaking reported by the USGS via both instruments and felt reports (left panel). Credit: Temblor, CC BY-NC-ND 4.0
Figure 3. Temblor’s ground motion model (right panel) is consistent with the shaking reported by the USGS via both instruments and felt reports (left panel). Credit: Temblor, CC BY-NC-ND 4.0

 

Earthquake contagion unlikely

Out of anywhere in California, Cape Mendocino is the place that is forecast to experience the largest earthquake in any given 100-year period, per Temblor’s earthquake occurrence model (Figure 4). Looking at a specific magnitude, the model indicates that a magnitude 7.2 shock has a 1% per year likelihood, which means that in an average human lifetime, the occurrence of a magnitude 7.2 event has better than even chances — higher than 50%. In fact, this region was rocked by earthquakes in 1922, 1980, 1992 and 2005 that were magnitude 7.2 or larger, and several other recorded earthquakes have come close.
 

Figure 4. Earthquake likelihood as shown in the Temblor App. Credit: Temblor, CC BY-NC-ND 4.0
Figure 4. Earthquake likelihood as shown in the Temblor App. Credit: Temblor, CC BY-NC-ND 4.0

 

The infamous 1906 San Francisco earthquake ruptured 477 kilometers (296 miles), from San Juan Bautista, which is southeast of the San Francisco Bay Area, to the Mendocino Triple Junction (see Prentice et al., 1999). An important question is how much stress was transferred to the northern San Andreas by the December 5 event.

We show several key calculations (Figure 5 and 6) of the stress transferred by the magnitude 7.0 shock to surrounding faults. The first calculation tells us that the rupture brought the northernmost 50 kilometers (about 30 miles) of the 1906 rupture on the San Andreas Fault about 0.25 bar closer to failure (Figure 5a). The magnitude 7.0 event also brought the southern tip of the Cascadia megathrust fault 0.25 bar closer to failure, although over a more restricted section of the fault (Figure 5b).
 


Figure 5. Calculations of the Coulomb stress imparted by the magnitude 7.0 Cape Mendocino quake to major surrounding faults. (a) Stress transferred to the northernmost San Andreas, which last ruptured in 1906. (b) Stress transferred to the southernmost Cascadia megathrust, which likely last ruptured in 1700. Credit: Temblor, CC BY-NC-ND 4.0
Figure 5. Calculations of the Coulomb stress imparted by the magnitude 7.0 Cape Mendocino quake to major surrounding faults. (a) Stress transferred to the northernmost San Andreas, which last ruptured in 1906. (b) Stress transferred to the southernmost Cascadia megathrust, which likely last ruptured in 1700. Credit: Temblor, CC BY-NC-ND 4.0

 

Are these Coulomb stress changes large enough to promote another mainshock? Probably not, but they are large enough to cause seismicity rate increases in the red zones. There does seem to be a migration of seismicity onto the northern San Andreas (evident in Figure 1), but as of this writing, those shocks are currently of relatively small magnitude (events of about magnitude 3.3 or smaller). There are several similarly small shocks on the southernmost Cascadia megathrust surface since the magnitude 7.0 shock struck, but it is unclear if those are thrust events.

Following the magnitude 7.0 mainshock, we observed a notable increase in seismic activity inland, broadly along the aftershock zones of the 2021 magnitude 6.2 Petrolia and the 2022 magnitude 6.4 Ferndale earthquakes. This heightened seismicity aligns well with our calculated Coulomb stress increases in these areas, suggesting a relationship between stress transfer and subsequent earthquake activity (Figure 6).
 

Figure 6. Seismic activity in Northern California. Each panel represents a two-day window of seismic events. The top left shows the magnitude 5.7 and magnitude 6.2 earthquakes near Petrolia, California, on December 20, 2021, with 2 days of aftershocks. The top right illustrates the magnitude 6.4 event near Ferndale, California on December 20, 2022 with two days of aftershocks. THe bottom panels depict pre- and post-seismic activity for the magnitude 7.0 Cape Mendocino, California earthquake on December 5, 2024. The bottom panels highlight the relative seismic quiescence before the mainshock, and the uptick in seismicity after. Earthquakes plotted from USGS. Credit: Temblor, CC BY-NC-ND 4.0
Figure 6. Seismic activity in Northern California. Each panel represents a two-day window of seismic events. The top left shows the magnitude 5.7 and magnitude 6.2 earthquakes near Petrolia, California, on December 20, 2021, with 2 days of aftershocks. The top right illustrates the magnitude 6.4 event near Ferndale, California on December 20, 2022 with two days of aftershocks. THe bottom panels depict pre- and post-seismic activity for the magnitude 7.0 Cape Mendocino, California earthquake on December 5, 2024. The bottom panels highlight the relative seismic quiescence before the mainshock, and the uptick in seismicity after. Earthquakes plotted from USGS. Credit: Temblor, CC BY-NC-ND 4.0

 

Another way to assess the stress transfer from the magnitude 7.0 earthquake is to calculate the stress on surrounding focal mechanisms (the “beachballs”) for all available shocks since 1976 (Figure 7). Those mechanisms give the actual orientation of the faults that receive stress. If they turn red, slip on those faults has been promoted; if they turn blue, slip on those faults has been inhibited by the magnitude 7.0 mainshock.

We see that beachballs along the Mendocino Fault and extending to and onto the San Andreas Fault turned red, indicating promotion (Figure 7a). In contrast, no thrust mechanisms exist on the southern Cascadia megathrust, and so earthquake promotion there is equivocal. We made the same kind of calculation after the nearby 2022 magnitude 6.4 Ferndale earthquake (Stein et al., 2022), which is shown in Figure 7b. We also conducted a joint Coulomb stress change analysis of the two earthquakes (Figure 7c).
 

Figure 7. (a) Coulomb stress transferred to the focal mechanisms of background (before the Dec. 5, 2024, shock) earthquakes. Stress increases on the northern tip of the San Andreas, but almost no thrust mechanisms exist for the Cascadia Fault, so no conclusions can be drawn there. (b) The 2022 magnitude 6.4 Ferndale earthquake promoted earthquakes where the San Andreas enters the Mendocino Triple Junction, but inhibited quakes farther east, onshore. (c) The combined impact of the two quakes appears to promote earthquakes where the San Andreas merges into the Mendocino Fault. Credit: Temblor, CC BY-NC-ND 4.0
Figure 7. (a) Coulomb stress transferred to the focal mechanisms of background (before the Dec. 5, 2024, shock) earthquakes. Stress increases on the northern tip of the San Andreas, but almost no thrust mechanisms exist for the Cascadia Fault, so no conclusions can be drawn there. (b) The 2022 magnitude 6.4 Ferndale earthquake promoted earthquakes where the San Andreas enters the Mendocino Triple Junction, but inhibited quakes farther east, onshore. (c) The combined impact of the two quakes appears to promote earthquakes where the San Andreas merges into the Mendocino Fault. Credit: Temblor, CC BY-NC-ND 4.0

 

The comparison of the two quakes (Figure 7a and 6b) reveals two insights: First, the magnitude 6.4 shock did not bring the epicenter of the magnitude 7.0 shock closer to failure, so we see no simple chain reaction between these two shocks. Second, the magnitude 6.4 and magnitude 7.0 shocks each brought the northwest tip of the San Andreas Fault closer to failure by about 0.25 bar. Per Figure 7c, we see that a 40-kilometer-long (about 25-mile-long) fault section, where the San Andreas merges into the Mendocino, is 0.25-0.5 bar closer to failure than it was before 2022 — a significant amount, but certainly not so large as to suggest a San Andreas event is imminent.

Interestingly, the magnitude 6.4 shock also inhibited failure by about 0.25 bar on the northernmost part of the San Andreas Fault that remains onshore. This section of the fault may have returned to its stress state prior to the Ferndale earthquake.
 

Dynamically triggered seismicity?

Dynamic triggering occurs when seismic waves from a large earthquake temporarily alter stress conditions on distant faults, causing them to fail. Unlike aftershocks, which occur near the mainshock, dynamically triggered events can occur hundreds of kilometers away and are identified by their timing relative to the passing seismic waves and statistical comparison with background seismicity rates. 

The seismic waves from the magnitude 7.0 earthquake appear to have triggered other small earthquakes far from its epicenter (Hubbard and Bradley, 2024). We observed at least three earthquakes — with epicenters between 253 and 314 kilometers (157 and 195 miles) to the southeast of the mainshock — that occurred a few minutes after the mainshock (Figure 8).
 

Figure 8. The map at left shows all recorded earthquakes in Northern California starting 120 hours prior to and continuing through Dec. 5, 2024, per the IRIS IEB platform. Arrows point to the epicenters of the magnitude 7.0 earthquake and three other distant earthquakes that may have been triggered by seismic waves from this event. The time elapsed between the magnitude 7.0 earthquake and each of the triggered earthquakes is shown in parentheses. On the right, the upper graph shows cumulative seismicity within the small red square, with hours on the x-axis beginning 120 hours prior to the magnitude 7.0 event. Notice that at about the time of the Cape Mendocino earthquake, the seismicity rate jumps. The lower graph is the time series of the earthquakes within the red square. Credit: Temblor, CC BY-NC-ND 4.0
Figure 8. The map at left shows all recorded earthquakes in Northern California starting 120 hours prior to and continuing through Dec. 5, 2024, per the IRIS IEB platform. Arrows point to the epicenters of the magnitude 7.0 earthquake and three other distant earthquakes that may have been triggered by seismic waves from this event. The time elapsed between the magnitude 7.0 earthquake and each of the triggered earthquakes is shown in parentheses. On the right, the upper graph shows cumulative seismicity within the small red square, with hours on the x-axis beginning 120 hours prior to the magnitude 7.0 event. Notice that at about the time of the Cape Mendocino earthquake, the seismicity rate jumps. The lower graph is the time series of the earthquakes within the red square. Credit: Temblor, CC BY-NC-ND 4.0

 

About 250 kilometers (155 miles) southeast, near Clear Lake, California, a magnitude 4.3 earthquake occurred 3 minutes after the magnitude 7.0 event (Figure 8). The region in which this event occurred has been seismically active for several years. However, an earthquake of this size has not occurred in nearly three months. This earthquake apparently triggered its own aftershocks, resulting in an evident increase in the seismicity rate in this region (outlined by the red-colored square in Figure 8).

The other two possibly triggered events occurred 9 and 13 minutes after the magnitude 7.0 earthquake, and both had a magnitude of 1.9. No previous events were reported for almost two days in the vicinity of these events. The most recent event was a magnitude 1.3 event that occurred 47 hours and 11 minutes before the magnitude 7.0 mainshock.

Although these (likely) triggered events are relatively small in magnitude, understanding and reporting such cases is important for scientists who are trying to understand this process. Indeed, dynamic triggering has been well-documented elsewhere (Gonzalez-Huizar, 2012; Gonzalez-Huizar and Toda, 2021). In particular, the greatest quakes have dynamically triggered large earthquakes around the world (Pollitz et al., 2012; Sevilgen et al., 2012).
 

A curious quake

At 3:08 p.m. local time (23:08 UTC) on Dec. 9, 2024, a magnitude 5.8 earthquake struck near Yerington, Nevada (Figure 1). Approximately 500 kilometers (310 miles) from the magnitude 7 mainshock and 70 kilometers (44 miles) east of Lake Tahoe, two notable seismic events occurred before the magnitude 5.8 Yerington earthquake. A magnitude 3.8 earthquake occurred 24 hours before the magnitude 7.0 Cape Mendocino earthquake. After the Cape Mendocino earthquake, a magnitude 2.8 event occurred in the same place just 44 minutes later.

Is there some relationship between the magnitude 5.8 Yerington earthquake that shook this part of Nevada on Dec. 9 and and the Cape Mendocino earthquake that struck on Dec. 5? The temporal and spatial clustering of these relatively uncommon events suggests possible delayed dynamic triggering, a phenomenon where seismic waves from a large earthquake can influence fault systems at considerable distances. Such a mechanism could initiate seismic activity hours or even days after the mainshock (Castro et al., 2015). However, such a relationship has not been demonstrated convincingly thus far.
 

A final word

The Dec. 5, 2024, magnitude 7.0 Cape Mendocino earthquake occurred in one of California’s most seismically active regions, and so, this event wasn’t a surprise. Thankfully, the earthquake’s impact on populated areas was relatively low due to its remote location. The event likely brought the northern terminus of the 1906 San Francisco earthquake slightly closer to failure.
 

Science editor: Dr. Alka Tripathy-Lang, Ph.D.
Reviewer: Chris Rollins, Ph.D.
 

References

Castro, R. R., González‐Huízar, H., Ramón Zúñiga, F., Wong, V. M., & Velasco, A. A. (2015). Delayed dynamic triggered seismicity in northern Baja California, México caused by large and remote earthquakes. Bulletin of the Seismological Society of America, 105(4), 1825-1835. doi:10.1785/0120140310.

Gonzalez-Huizar, H., Velasco, A. A., Peng, Z., & Castro, R. R. (2012). Remote triggered seismicity caused by the 2011, M9.0 Tohoku-Oki, Japan earthquake. Geophysical Research Letters, 39(10), L10302. https://doi.org/10.1029/2012GL051015

Gonzalez-Huizar, H., & Toda, S. (2021). New Zealand sees exotic earthquake sequence. Temblor. http://doi.org/10.32858/temblor.160

Hubbard and Bradley (2024), Magnitude 7.0 earthquake strikes offshore Cape Mendocino, California Earthquake Insights.

Pollitz, F. F., Stein, R. S., Sevilgen, V. & Bürgmann, R. (2012). The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide. Nature 490, 250–253.

Rollins, J. C., & 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, 115(B12), B12306. https://doi.org/10.1029/2009JB007117

Sevilgen, V., Stein, R. S. & Pollitz, F. F. Stress imparted by the great 2004 Sumatra earthquake shut down transforms and activated rifts up to 400 km away in the Andaman Sea. Proc. Natl. Acad. Sci. U.S.A. (2012).

 

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