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
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.
U.S. Highway 95 between the U.S. Route 6 junction and U.S. Highway 360, will remain closed until 5 pm today for earthquake related inspections and repairs. https://t.co/AQdAa4GkP9 @EsmeraldaCounty @TonopahNevada @pvtimes @ClarkCountyNV @NHPSouthernComm @NevadaDPS @goldfieldnevada pic.twitter.com/8dH7HYq0Ie
— Nevada DOT (@nevadadot) May 15, 2020
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].”
— KSNV News 3 (@News3LV) May 15, 2020
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.
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.”
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.
WELL. That explains the seemingly ceaseless thumping of the closet doors. May have been more than 40s worth after all, like I thought.
400km distant; now I see what a big L.A. quake would feel like up here.
Super clear @raspishake record of the Nevada quake from my living room: https://t.co/LB2bhdR1bw pic.twitter.com/YUnDy7Q7j3
— Austin Elliott (@TTremblingEarth) May 15, 2020
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!”