Tripathy-Lang reported that 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 Earthquake
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.
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