Magnitude-6.5 earthquake rattles Nevada and California

By Alka Tripathy-Lang, Ph.D (@DrAlkaTrip)
 

A shallow earthquake struck near the California-Nevada border in the early morning hours on May 15, 2020, waking people as far away as the Bay Area and Las Vegas.
 

Citation Tripathy-Lang, Alka, (2020), Magnitude-6.5 earthquake rattles Nevada and California, Temblor, http://doi.org/10.32858/temblor.090
 

A magnitude-6.5 quake struck a remote part of Nevada today (May 15, 2020), but was felt in the San Francisco Bay area, Bakersfield, and Las Vegas. Based on its aftershocks and focal mechanism, the event probably struck on an unnamed left-lateral fault. Credit: Temblor
A magnitude-6.5 quake struck a remote part of Nevada today (May 15, 2020), but was felt in the San Francisco Bay area, Bakersfield, and Las Vegas. Based on its aftershocks and focal mechanism, the event probably struck on an unnamed left-lateral fault. Credit: Temblor

 

On May 15, 2020, at 4:03 a.m. local time, the desert area west of Tonopah, Nev., was rattled awake by a widely felt magnitude-6.5 earthquake. Nucleating at a depth of 1.7 miles (2.8 kilometers), this shallow temblor occurred on a nearly vertical fault surface where no matter which side of the fault you’re on, the other side moved to the left. Called a left-lateral strike-slip fault, it is similar to the fault that ruptured during the magnitude-6.4 Ridgecrest foreshock that struck approximately 170 miles (270 kilometers) to the south less than a year ago.

Damage appeared to be minimal, with the Nevada Department of Transportation reporting minor pavement damage to a half-mile section of U.S. Highway 95.

 


 

Earthquakes east of the Sierra Nevada

As the Pacific Plate moves northwest relative to North America, much of that motion occurs on the famed San Andreas Fault. However, a significant component of the movement between these two tectonic plates, almost 20-25 percent of the total motion, shows up several hundred miles to the east, in the Walker Lane Belt, says Ian Pierce, a postdoctoral researcher at Oxford University who studies active faults. The Walker Lane Belt runs roughly parallel to the California-Nevada border, east of the Sierra Nevada. Like the notorious San Andreas, Walker Lane is a right-lateral fault zone, meaning whichever side you are on, the other side moves to the right.

Spanning 500 miles (800 kilometers) between near Ridgecrest, Calif., at its southern extent into the northern Sierra Nevada, the Walker Lane Belt comprises many smaller zones of right-lateral faulting that are linked by small left-lateral faults, says Pierce. “It looks like this earthquake was one of those left-lateral faults rupturing,” he says. “As far as the tectonic setting,” he continues, “it’s basically the same as Ridgecrest last year.”

 

Similarities to Ridgecrest

On July 4, 2019, a magnitude-6.4 foreshock rattled Ridgecrest’s residents, but that was just the opening act to the magnitude-7.1 mainshock, which occurred 34 hours later. Pierce compares the Tonopah earthquake with the Ridgecrest foreshock, and points out that aside from their similar magnitudes, “they both occurred on left-lateral faults with small surface ruptures on fairly short—maybe 20-kilometer—fault[s].”

 


 

At Ridgecrest, the magnitude-7.1 mainshock ripped through the foreshock’s fault surface at a right angle, and propagated many miles to the south. Such orthogonal ruptures, where faults break at right angles to one another, are very common in the Walker Lane Belt, according to Pierce.

 

Map of the southern section of the Walker Lane Belt around surrounding regions showing the the past 30 days of earthquake activity. The three stars indicate important quakes—the July 2019 Ridgecrest magnitude-6.4 foreshock, the July 2019 Ridgecrest magnitude-7.1 mainshock, and the Tonopah magnitude-6.5 event of May 15, 2020. Stars are scaled to correspond with magnitude. Credit: Temblor
Map of the southern section of the Walker Lane Belt around surrounding regions showing the the past 30 days of earthquake activity. The three stars indicate important quakes—the July 2019 Ridgecrest magnitude-6.4 foreshock, the July 2019 Ridgecrest magnitude-7.1 mainshock, and the Tonopah magnitude-6.5 event of May 15, 2020. Stars are scaled to correspond with magnitude. Credit: Temblor

 

However, Pierce says, although Ridgecrest started with a big quake and was followed by an even larger one the next day, “we probably won’t have a magnitude-7.1 tomorrow.”

 

Aftershock forecasts

The U.S. Geological Survey (USGS) issues aftershock forecasts, which can be found here. Over the course of the next week, the chance of an aftershock with magnitude-7.0 or higher is 1 percent, indicating that it’s certainly possible, but with very low probability. On the other hand, the chance of a magnitude-3.0 aftershock or higher is greater than 99 percent. As of this writing, at least 12 aftershocks greater than magnitude-4.0 have been reported, including magnitude-4.9 and magnitude-5.1 shocks that occurred less than an hour after the mainshock.

“People don’t think of Nevada as being very active, but it really is,” says Kathleen Hodgkinson, a geophysicist at UNAVCO.

 

Felt on the other side of the mountains

As of this writing, more than 21,000 people have reported feeling (or not feeling) the event, according to the USGS “Did you Feel It?” citizen science initiative. People felt the distinctive shaking associated with earthquakes in Las Vegas, about 170 miles (280 kilometers) to the southeast, all the way to the California Bay Area, about 280 miles (450 kilometers) west. Austin Elliot, a research geologist at the USGS Earthquake Science Center in the Bay Area, described waking up to “the seemingly ceaseless thumping of the closet doors” on twitter. He also pointed out that “building height amplified the otherwise maybe imperceptible ground motions,” referencing the fact that the higher up you are, the more likely you are to feel the swaying as seismic waves pass by.

 


 

Wendy Bohon, an earthquake scientist at IRIS, reminds everyone that “aftershocks will continue to cause shaking, so if you feel the jolt of an earthquake, remember to drop, cover and hold on!”

If you felt this earthquake, let the USGS know here. As always, for the latest information about ongoing seismicity related to this earthquake, check the USGS event page.

 
 

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Alka Tripathy-Lang, PhD

Alka Tripathy-Lang is a freelance science writer based in Chandler, Arizona, and holds a Ph.D. in geoscience.
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  • Bill Korbholz

    Those pictures sure make it look like a right-lateral fault, yet it is described as left-lateral. What am I missing?

  • Patrick McClellan

    How is the left-lateral fault slip reported in this article reconciled with the right-lateral pavement damage across US 95 shown in the NV DOT photos?

  • Alka Tripathy-Lang

    Hi Bill! Great question. Those are pictures from NDOT documenting damage, not necessarily sense of motion. Austin Elliot responded to this same question on twitter, and says he thinks “the cracking there is from differential shaking of the alluvial fans and the fine playa sediment.” He’s among the teams of geologists out there right now looking for surface rupture. Ian Pierce, also interviewed for this article, has posted photos of left-lateral cracks near the epicenter that are not on the highways, but rather in the dirt.

  • Caroline

    I live 25 miles from where the 6.5 shook the other morning and let me tell you, I have slept through a domestic dispute turned murder, I can sleep through a plane landing, but I was awake and out the damn door faster than fast! Feels more like I’m on a boat ever since. 300 aftershocks and counting.

  • Half of the epicenter cluster lies in the Monte Cristo Range which does not have any active ENE-striking faults and is instead characterized by NNW and NS striking faults as Mina deflection structure merge with faults of the central Walker Lane. Therefore a good number of the after shocks (and maybe even the 6.5M EQ itself) occurred on the NS and NNW striking faults in addition to some occurring on ENE faults along the NE and north side of the Columbus Salt Marsh. Check out mapping by Oldow and Cland (2017, 2018).

    edit: adding citation

  • See my comment above. In short a good port of these events likely occurred on north-south and north-northwest striking faults. Any fault movement on NS or NNW striking faults would be right-lateral and any movement on ENE faults would be left-lateral. I did fault mapping out there for years and its a very complex area. As for an offset as small as on the center line, Alka’s answer below covers that.

  • Alka Tripathy-Lang

    Hi @disqus_Z9SVUJbdAv:disqus, please see the response Bill Korbholz below! In short, the pavement photos show damage, not sense of motion.

  • Alka Tripathy-Lang

    Thanks for sharing your experience! Make sure to tell the USGS about what you’re feeling here: https://earthquake.usgs.gov/earthquakes/eventpage/nn00725272/tellus.

    Also, remember to drop, cover and hold on! For more information about what to do during an earthquake depending on where you are (inside, outside, in bed, etc), check out this great resource: https://www.earthquakecountry.org/step5/

    Glad you’re safe, and try not to get seasick (earthsick?) 🙂

  • Mike Power

    The moment tensor to me indicates either left lateral strike slip on a NNW striking fault or right lateral displacement on an ENE striking fault. The aftershock pattern strikes ENE. The main shock epicenter is at the east end of the ENE trend and the later aftershocks trend west. This suggests to me that this is right-lateral strike slip displacement on the Mina Deflection. This contradicts the prevailing wisdom that displacement on the Mina Deflection is left lateral, accommodating extensional displacement. Am I missing something here?

  • Ross Stein

    Here are the USGS nodal planes for strike/dip/rake: (1) 73°/78°/-24°; (2) 168°/67°/-167°. So, that is right-lateral/normal on a NNE-striking plane, or left-lateral/normal on a WSW-plane. Plane (2) is parallel to the aftershocks. It is possible that slip began on plane 1 and jumped to plane 2. In general, the strike-slip/normal mechanism is typical of the Walker Lane and Basin & Range faulting.

  • Ross Stein

    Take a look at today’s article, in which we have benefitted from satellite radar imagery and a lot more aftershocks to make inferences.