Tectonic uncertainty swirls as earthquake rocks California and Nevada borders

On July 8, 2021, a 6.0-magnitude earthquake struck the Central Walker Lane Fault Zone, California and Nevada. Walker Lane offers some puzzles.

Written by Ian Pearce, Ph.D., currently at the University of Oxford, UK (neotectonic)

Quote: Pierce, Eye, 2021, Tectonic uncertainty spins as California and Nevada border earthquake shakes, Temblor, http://doi.org/10.32858/temblor.187

On the afternoon of July 8, 2021, a 6.0-magnitude earthquake shook most of central California and western Nevada. Severe shaking that lasted for up to 20 seconds was reported 80 miles (~130 kilometers) away in Reno, while the Bay Area, Central Valley, Southern California and even Las Vegas shook as well. Damage reports are slowly pouring in from many of the remote communities closest to the vibration, and videos of car-sized boulders have spread on a major highway. Less than 24 hours ago (at the time of publication), the area shook more than 100 aftershocks above 2.5 degrees. Over 25,600 people have reported feeling shivering to the USGS, did you feel it? website. If you feel the main shock or any of the aftershocks – or if you don’t feel them – report them.

A map showing the earthquakes that have rocked the Antelope Valley, south of Lake Tahoe, since yesterday afternoon.

This earthquake originated in seismically busy Central Walker Lane, between Yosemite National Park in California and Carson City, Nevada, in the southern part of the Antelope Valley. In the past three years, four major earthquakes – greater than 6.0 – have hit the Walker Lane region. What is driving all this seismic activity and what does it mean for earthquake hazards away from the famous San Andreas Fault?

Busy walker lane

Walker Lane is a pervasive area of ​​normal malfunctions and slip faults. It follows a 60-mile (100-kilometre) trail along the eastern Sierra and the California/Nevada border, down from Death Valley and the Garlock Fault in the south to the north of the Honey Lake Valley region. The Walker Lane Fault absorbs approximately 20 percent of the 2-inch (50 mm) right lateral shear annually between the Pacific and North American plates, while the remaining 80 percent is absorbed along the more famous San Andreas Fault.

Annotated map showing the locations of earthquakes greater than or equal to 6 in the Walker Lane region.

The Walker Lane has been getting a lot of attention lately, and for good reason. In the past three years, more moderate earthquakes have occurred along the Walker Lane fault system than any other fault system in the western United States. This earthquake is the fourth event greater than 6.0 on the Richter scale since the 2019 Ridgecrest earthquake sequence that included earthquakes of 6.4 and 7.1 on the Richter scale, and the 6.5 Monte Cristo extent earthquake in 2020. In 2021 alone, yesterday’s earthquake joins To a series of other widely felt events ranging from 4 to 5 degrees near Lake Tahoe and the Northern Sierra.

However, Walker Lane did not start showing up in events in 2019. In 2016, the Nine Mile Ranch sequence spawned three events of ~5.5 magnitude near Hawthorne, Nevada, in less than an hour, following a similar sequence from 2011. In In 2008, the Mogul series struck Reno, Nevada, with a magnitude 4.7 earthquake. The largest known event at Walker Lane was the 1872 Owens Valley earthquake measuring 7.4 on the Richter scale, roughly the maximum we would expect for an earthquake along any single fault in this system.

Decades of paleo and tectonic studies have revealed dozens of faults throughout Walker Lane, some with evidence of large prehistoric earthquakes likely exceeding 7.0. For example, the most recent earthquake occurred along the Genoa fault near the city of Carson about 450 years ago (Ramelli et al., 1999). The fault extending down the western shore of Lake Tahoe recently ruptured about 5,500 years ago (Pierce et al., 2017).

tectonic puzzle

Turning attention to tectonics at Central Walker Lane where this latest magnitude 6.0 event occurred, an unsolved tectonic mystery swirls. Using high-resolution fixed GPS stations, we can observe – in real time – the Sierra Nevada’s summit slide northwestward at a rate of about 0.3 inches (7 mm) per year relative to Fallon, Nevada, as it cuts off the stretch of antelope, Smith, and Mason Valley. However, geologically speaking, we cannot find enough slip error to explain this observed shear.

Overview of the study area of ​​Pierce et al. 2020, including Antelope Valley. Shadows of dark gray ridges indicate lidar data. The red lines show certain errors. Bold black lines show fault connections between bedrock and placer deposits. The thin black lines come from the USGS quadruple error and folded database. Credit: Pierce et al., 2020, CC BY 4.0

While a number of hypotheses have been proposed to explain this mismatch, we expect that future large earthquakes will exhibit complex, transient, and discontinuous deformation, in contrast to long-standing fault filaments that produce faults, large modifications of features in the landscape, and other geomorphological features that we may need to measure rates of Long-term error (Pierce et al., 2020). This more subtle and less dramatic pattern of deformation was seen in the 2020 Monte Cristo Range earthquake, as well as other previous Locker Lane events. Other explanations for the measurable missing shear that we would expect based on GPS data include clockwise basin rotations and surrounding natural faults, and indirect slip faults linking mountain range interfaces in this region.

The fault responsible for the 6.0-magnitude July 8 event produced a normal torque tensor striking north – which seismologists use in the beach ball diagram to determine the fault’s direction and way in which it is moving. Natural rifting corresponds to local rifting patterns in Antelope Valley. In other words, the earthquake occurred on a north-striking fault moving due to the east-west oriented extension, similar to what we would expect for a north-oriented Antelope Valley fault bounding the range. This fault does not have a significant slant component, as one might expect if this range accommodates the right lateral shear – movement recorded by the San Andreas Fault, and overall movement of Walker Lane as recorded by GPS. This supports our conclusion in our 2020 paper that this range does not show slashes – it is mostly normal.

An image of the frontal fault in the Antelope Valley region. Credit: Ian Pearce

Unappreciated risk

Finally, eastern California and western Nevada have a lot of underappreciated earthquake risk. While these risks are well known to the seismic science community, the public seems to view these moderate-magnitude 5 to 6 events as “big events” when in reality, the inevitable magnitude 7 event that will occur in one is rapid Development in the region, the loss of life and property will be devastating.

Everyone who lives in this area should not only keep an earthquake kit and have a plan for the inevitable upcoming earthquakes, but should also make sure that their homes are earthquake ready. There are a number of cost-effective measures that homeowners can easily implement to mitigate damage, such as securing hot water heaters to walls and securing foundations. Finally, these tempering events should serve as a warning to our community leaders, and communities should encourage business owners to retrofit the relatively large number of unsupported building structures in many historic areas of Reno, Carson Valley, Troquet and Tahoe.

Remember, if you feel shaky, the USGS says to drop, cover, and wait.

references

Pierce, I.K., Wisnosky, S.G., & Owen, L.A. (2017). Cosmic-terrestrial surface exposure dating to moraines in Lake Tahoe in Sierra Nevada, California and estimation of the slip rate of the West Tahoe fault. Geomorphology, 298, 63-71.

Pierce, I.K., Wesnousky, S.G., Owen, L.A., Bormann, J.M., Li, X., & Caffee, M. (2020). Adapting board motion in a sliding system. Elementary batting: The Central Walker Lane. Tectonics, 40(2), e2019TC005612. doi: 10.1029/2019TC005612

Ramelli, A. R., Bell, J. W., Depolo, C. M., & Yount, J. C. (1999). Large, late Holocene earthquakes on the Genoa fault, western central Nevada, and eastern California. Bulletin of the American Seismological Society, 89(6), 1458-1472.

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