Mw5.8 earthquake shakes western Nevada

A Walker Lane strike-slip event

Citation: Bradley, K., Hubbard, J., 2024. Mw5.8 earthquake shakes western Nevada. Earthquake Insights, https://doi.org/10.62481/7bb099ad

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A magnitude 5.8 earthquake struck western Nevada at 3:08 PM local time on December 9, 2024. Maximum shaking reached intensity ~VI (strong), but because the earthquake occurred in a pretty empty part of Nevada, few people felt shaking this strong. Nearby cities like Carson City and Reno experienced intensity III-IV (weak to light).

The shaking intensity contours on the map below are wild and wiggly. This is because shaking intensity can vary significantly depending on the geology: seismic waves typically slow down and amplify in soft sediments. The shaking model shown on the map takes into account these variations across the basin-and-range geology of western Nevada, projecting that the basins (filled with sediment) likely shook more strongly than the ranges dividing them.

Figure 1: The 2024-12-09 M5.8 earthquake in western Nevada and its aftershocks. Colored lines on the map are contours of shaking intensity estimated by the USGS.

People who felt the earthquake described it to the EMSC as: “a slow rolling,” “a slight rocking,” “a slight rumbling,” and “like a big rig pass[ing] by.” One person commented the swaying of their car made them feel a little nauseated; another that the movement on the 3rd floor was making them nauseous. This inspired us to look up which word usage was correct. The winner? Actually, both! Merriam Webster informs us that:

Though many people feel strongly that nauseous can only be used to mean “causing nausea” or “nauseating” (as in “a nauseous smell”), its use to mean “affected with nausea” or “nauseated” (as in “a smell that made me nauseous”) is well established and in widespread use. -Merriam Webster, https://www.merriam-webster.com/grammar/can-you-feel-nauseous-or-nauseated

Useful information for your next earthquake report. In any case, whatever your opinions are of the word usage, it seems that the earthquake was generally experienced as an unexpected and disquieting phenomenon, but not one of great concern. Nevertheless, the earthquake was indeed widely felt, and in fact more than 10,000 people have reported to the USGS so far, including people as far away as San Francisco. You can add your own report here, and if you also want to describe it in creative language, submit your comments to the EMSC here.

Figure 2: Map of felt reports to the USGS. Source: https://earthquake.usgs.gov/earthquakes/eventpage/nn00888580/dyfi/intensity

Aftershocks can be expected — and indeed, thirteen events above M2.5 have already been recorded, with the largest reaching M4.2. The USGS currently estimates a 14% chance of an earthquake above M5 in the next week, and a 6% chance that this earthquake will be followed by a larger one. The aftershock forecast is periodically updated, so check their website for the latest information.

Figure 3: Aftershock forecast for the M5.8 earthquake, last updated 2024-12-10 00:08:38 (UTC). Source: https://earthquake.usgs.gov/earthquakes/eventpage/nn00888580/oaf/overview

Tectonic setting

Why did this earthquake happen, and how does it fit into the spectrum of earthquakes that have been recorded here? Let’s take a look at the active tectonics in this area.

Figure 4: Recorded earthquakes in the vicinity of the recent M5.8. Black lines are faults from the US Quaternary Fault map, plus one state boundary that is not a fault.

The Basin and Range of Nevada is a classic geological setting: here, huge normal faults have lifted up long, blocky mountain ranges and thrown down deep valleys in between, which have filled up with the eroded debris of the mountains.

Near the origin of today’s earthquake, the most prominent fault of this type is the Wassuk Range Fault Zone, which cuts along the eastern side of the, you guessed it, Wassuk Range. It was these kinds of spectacularly exposed mountain-edge faults that led geological superhero G.K. Gilbert to propose in 1875 the revolutionary idea that crustal extension can lift up great mountains. If we were guided solely by the faults we can easily see, we would expect most earthquakes in Nevada to be of normal-faulting type.

However, there is another type of fault that cuts through the deserts of eastern California and western Nevada. These are strike-slip faults, which, though abundant, are much less apparent at the surface, because they simply displace the landscape sideways, without forming large mountains or valleys. These faults run mostly northwest-southeast, chopping up the typically northeast-southwest oriented block mountains that so fascinated Gilbert.

Although it is hard to see these faults directly, we can rely on the seismology of recent earthquakes to see what is going on. We plotted all focal mechanisms from the various standard catalogs, without scaling the symbols by magnitude. We colored thrust events blue, strike-slip events black, and normal events red. Behind the focal mechanisms, we also plotted GPS velocities relative to stable North America (green arrows).

Figure 5: Earthquake focal mechanisms, colored by type, and GPS vectors (green arrows). The Walker Lane and Eastern California Shear Zone are outlined.

There is a clear area of mixed red (normal) and black (strike-slip) focal mechanisms following the California-Nevada border, which we have outlined as a long blob. This is called the Walker Lane (and in fact, Walker Lake is located just to the southeast of today’s earthquake, and the town of Walker is to the southwest. From which, if either, does the name Walker Lane come?). Compared to eastern Nevada, the Sierra Nevada mountains and the Central Valley of California are being dragged the Northwest along with the Pacific Plate at a rate of about 10-11 millimeters per year, which has to be accommodated within the Walker Lane. We wrote about the Walker Lane back in May 2023, when a M5.5 earthquake occurred at its northwestern edge.

In Southern California, similar strike-slip faults crossing the desert make up the Eastern California Shear Zone, which we have outlined as a smaller blob on the map above. The active tectonics of this area are very similar to those of the Walker Lane. The reason why the Walker Lane and the Eastern California Shear Zone have different names is that they are separated by the NE-SW Garlock Fault, an apparently continuous feature that cuts off the southern end of the Sierra Nevada Mountains. Exactly how the Eastern California Shear Zone and Walker Lane connect across the Garlock Fault is an enduring tectonic and geological puzzle.

Several prominent Californian earthquakes over the last decades have arisen from strike-slip ruptures of complex faults in the Eastern California Shear Zone and the Walker Lane. These include the 1992 Mw7.3 Landers, 1992 Mw6.5 Big Bear, and 2019 Mw7.1 Ridgecrest earthquakes. Even larger was the 1872 Owens Valley earthquake (Mw7.4), which was a classic Walker Lane strike-slip event. These earthquakes clearly showed that seismic hazard in the western USA is not limited to the much more famous San Andreas Fault. (We look forward to a Hollywood movie in which California breaks off of the North American continent - along the Walker Lane. We volunteer to be science advisors for the film.)

So, how does the recent Mw5.8 earthquake fit into this tectonic puzzle — i.e., what fault ruptured? Well, we can only make educated guesses at this point.

The strike-slip faults in the Walker Lane system mostly trend northwest-southeast, mimicking the overall blob axis, and they have a right-lateral sense of slip. That is exactly what we see in the focal mechanism from today’s Mw5.8 mainshock: one of the nodal planes is indeed oriented northwest-south, with right-lateral slip.

Figure 6: Focal mechanism for the recent earthquake. Source: USGS, https://earthquake.usgs.gov/earthquakes/eventpage/nn00888580/moment-tensor

So, we might expect that the northwest-southeast plane is the actual fault, and that the rupture was right-lateral.

However, things aren’t always that easy. The focal mechanism actually gives two options for the fault plane, and doesn’t prefer either. So it is also possible that there is a northeast-southwest oriented fault that slipped. This might not be parallel to the overall shear zone, but that isn’t actually much of a problem. So, how can we tell which nodal plane represents the true fault orientation?

There are clues that may point to one plane over the other. Usually, most of the aftershocks of a moderate-sized earthquake will happen on the same fault that ruptured. So we can look at a map view of the aftershocks, and if they line up with one of the possible fault planes, we can propose that one as the actual fault. This tends to work particularly well with vertical strike-slip faults. Here’s a map of the aftershocks:

Figure 8: December 9 earthquake and its aftershocks, showing NE-SW alignment. Black lines are faults from the US Quaternary Faults database.

In our opinion, the pattern of aftershocks seems to indicate rupture of a northeast-southwest oriented fault.

This isn’t actually so unexpected. Geologists have already identified several areas within the Walker Lane where the faults have this unusual orientation, including in the area of today’s earthquake! A beautiful 2017 paper in Geosphere by Li et al. examined these somewhat obscure faults in detail, using field studies and remote sensing.

Figure 9: Map of major faults in the central Walker Lane. Figure 1 of Li et al. (2017); red star added to show the location of the recent earthquake.

Based on the maps shown in that paper, today’s event seems to have happened along the Wabuska lineament, south of Silver Springs. A lineament is not really a single fault, but rather an alignment of linear features, possibly including fault traces, that together indicate the presence of one or more major fault systems. Identification of lineaments is a first step in understanding fault systems that aren’t otherwise obvious at the surface.

If this earthquake really did arise from faulting along the Wabuska lineament, then it provides an excellent validation of the geological work that has already been done in this area, which predicted left-lateral strike-slip faulting from several lines of evidence.

What does it all mean? Well, the pieces of the Earth Puzzle do have to fit together in the end. Li et al. proposed that inside the Walker Lane, small pieces of the crust are moving in different ways, but always consistent with the overall stress and strain:

Figure 10: Schematic model illustrating how NE-SW strike-slip faults and N-S basins might develop within the overall NW-SE Walker Lane. Source: Figure 13b of Li et al. (2017).

This looks like a giant example of the kind of complex deformation that we often see in small brittle shear zones exposed in outcrops. While each small part of the shearing area may move differently, the overall picture is a coherent deformation. We just get a momentary glimpse of the overall geological motion, which is ultimately guided by the boundary conditions of the shear zone. For those who get all riled up from this kind of tectonic study, Pierce et al. (2021) is another open-access paper that looks at the larger Walker Lane using a similar geological approach.

References:

Hubbard, J. and Bradley, K., 2023. M5.5 earthquake in California occurred at northwestern edge of Walker Lane Shear Zone. Earthquake Insights, https://doi.org/10.62481/81f43bc2

Li, X., Huang, W., Pierce, I.K., Angster, S.J. and Wesnousky, S.G., 2017. Characterizing the Quaternary expression of active faulting along the Olinghouse, Carson, and Wabuska lineaments of the Walker Lane. Geosphere, 13(6), pp.2119-2136. https://doi.org/10.1130/GES01483.1

Pierce, I.K., Wesnousky, S.G., Owen, L.A., Bormann, J.M., Li, X. and Caffee, M., 2021. Accommodation of plate motion in an incipient strike‐slip system: The central Walker Lane. Tectonics, 40(2), p.e2019TC005612. https://doi.org/10.1029/2019TC005612