Kinematics of M6.0 July 2021 Eastern California earthquake as expected within the mapped stress field

I posted a shorter version of this on LinkedIn shortly after the earthquake, but I thought I’d put it somewhere more permanent as well:

Map of the state of stress in North America by Lund Snee and Zoback (2020) showing the location and U.S. Geological Survey focal mechanism for the 8 July 2021 Antelope Valley, California, earthquake. Black lines are measured orientations of the maximum horizontal principal stress (SHmax), and the colored background represents the style of faulting (relative stress magnitudes, represented by the Aϕ parameter). Inset to lower-left is a close-up of the area surrounding the epicenter location. In that area, several black lines indicate a north–south SHmax orientation, and the teal background color represents normal/strike-slip faulting.

Sometimes, an earthquake occurs somewhere with limited historical data, and its focal mechanism reveals something unexpected about the stress field. The M6.0 Antelope Valley event that occurred on the 8th of July this year near Markleeville, CA, in the Eastern Sierra, is not one of those. The normal and strike-slip faulting focal mechanisms of the event and its aftershocks, with fault strikes consistent with the maximum horizontal stress (SHmax) being nearly north–south, are exactly what would be expected for that area.

(Note the jumbled group of SHmax orientations further to the south. This probably doesn’t indicate a “complex” stress field, but instead suggests that some unreliable measurements may have crept into the World Stress Map database there. They should be examined more carefully to determine whether they are erroneous. In our experience mapping the stress field, high-quality data almost always reveals coherent patterns. This is spectacularly visible in the bottom-right inset, which shows the Permian Basin of southeast New Mexico and west Texas.)

The tectonic setting for the event is the western margin of the northern Basin and Range Province, in what’s called the Walker Lane. That area is transitional, accommodating an appreciable portion of transform motion associated with the Pacific–North America plate boundary but also experiencing some extension. The Walker Lane–Eastern California shear zone system has been quite active relative to the San Andreas in recent years—it’s hosted three of California's four M7+ earthquakes in the past three decades.