Seismic Swarm S20211031.1 Near Walker, California: Geological Context and Event Analysis

An earthquake swarm designated S20211031.1 was recorded 6 km east of Walker in Mono County, California. The sequence began at 22:52 on 30 October 2021 and concluded at 02:57 on 2 November 2021, spanning 52 hours and 4 minutes. During this interval, 33 earthquakes were detected, with magnitudes ranging from 0.2 to 2.8 and focal depths between 0 and 7 km.

The largest event reached magnitude 2.8 at a depth of 5 km on 31 October 2021 at 06:35:44 UTC. Subsequent notable shocks included a magnitude 2.2 event at 4 km depth shortly after initiation and a magnitude 2.0 shock at 4 km depth on 31 October at 18:29:02. The majority of events clustered between 2 and 5 km depth, consistent with shallow crustal faulting in the region.

The Walker area lies within the Walker Lane shear zone, a northwest-trending belt of distributed deformation separating the Sierra Nevada block from the Basin and Range province. This zone accommodates approximately 20–25% of the relative motion between the Pacific and North American plates through right-lateral strike-slip and normal faulting. Active structures include segments of the Antelope Valley Fault and related Quaternary faults that exhibit evidence of Holocene displacement.

Seismic swarms in this tectonic setting commonly arise from fluid migration or aseismic slip along fault networks rather than mainshock-aftershock sequences driven by static stress transfer. Historical records since 2000 document 12 prior swarms in the immediate vicinity, occurring in 2000, 2003, 2005, 2007, 2010 (two events), 2013, 2014 (two events), 2017, and 2021 (two events). These episodes underscore recurrent, low-magnitude activity without associated surface rupture.

The 2021 swarm exhibited a typical pattern of rapid onset followed by gradual decay, with 18 events on 31 October alone. Depths remained predominantly above 5 km, indicating brittle failure within the seismogenic upper crust. No events exceeded magnitude 3.0, aligning with the background seismicity rate for the Walker Lane interior.

Regional geology features Cenozoic volcanic rocks overlying Mesozoic basement, with active extension producing north-south trending grabens. GPS measurements indicate northwest-directed dextral shear rates of 8–12 mm per year across the zone. Paleoseismic studies reveal recurrence intervals of several thousand years for larger surface-rupturing events on nearby faults, contrasting with the frequent small-magnitude swarms observed instrumentally.

Continued monitoring by regional networks remains essential for characterizing swarm evolution and assessing any potential for escalation. The S20211031.1 sequence provides additional data on the fine-scale behavior of fault systems within this tectonically active corridor.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey Fault Activity Map
Nevada Seismological Laboratory regional reports
SCEC Community Fault Model documentation