Seismic Swarm S20210404.1 Near Little Lake, California: Characteristics and Regional Context

Seismic swarm S20210404.1 began at 10:35 on 3 April 2021 and concluded at 00:30 on 15 April 2021. The events were centered 19 km east-southeast of Little Lake in Kern County, California. Over 277 hours and 54 minutes, 124 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from –0.2 to 2.3 and a median near 0.7. Focal depths clustered between 1 km and 11 km, consistent with shallow crustal faulting.

The swarm initiated with a cluster of small events on 3 April, including magnitudes of 0.7, 0.8, and 1.7 within the first day. Subsequent days showed sporadic peaks, such as the 1.9 event on 4 April and the largest recorded magnitude of 2.3 on 9 April. Depths remained stable in the upper crust, indicating distributed slip on minor fault segments rather than a single through-going rupture.

Little Lake lies within the Eastern California Shear Zone, a network of northwest-trending right-lateral strike-slip faults that accommodate a portion of Pacific–North American plate motion. The local geology comprises Mesozoic basement rocks overlain by Quaternary alluvium and lacustrine deposits of the China Lake basin. Active faulting occurs along the Little Lake fault and nearby structures linked to the Airport Lake fault zone.

Since 2000, the region has hosted 98 swarms, with notable increases in 2019 (37 swarms) and 2020 (30 swarms). This elevated rate followed the 2019 Ridgecrest earthquake sequence, whose M 7.1 mainshock occurred only 5 km from the S20210404.1 centroid. The 2019 sequence produced extensive surface rupture and triggered widespread aftershock activity that continues to influence local stress fields.

The 2021 swarm exhibits typical swarm behavior: high event rates without a clear mainshock–aftershock decay. Magnitudes remained below 3.0, posing negligible hazard, yet the activity underscores persistent strain accumulation along the shear zone. Depths averaging 6 km align with the brittle–ductile transition in this tectonically active corridor.

Continued monitoring is warranted given the proximity to the Ridgecrest rupture zone. Such swarms provide valuable data on fault interaction and may signal evolving stress conditions in the Eastern California Shear Zone.

References
USGS Earthquake Catalog (events 2000–2021)
California Geological Survey, Fault Activity Map of California
Southern California Earthquake Data Center, Regional Seismicity Reports