Seismic Swarm S20230503.1: Analysis of Activity Near Little Lake, California

A seismic swarm designated S20230503.1 occurred in a tectonically active region of eastern California, centered 19 km east-southeast of Little Lake. The sequence began at 21:46 UTC on 2 May 2023 and concluded at 03:56 UTC on 4 May 2023, spanning 30 hours and 9 minutes. During this period, 32 earthquakes were recorded, with magnitudes ranging from 0.7 to 3.5 and focal depths predominantly between 8 and 11 km.

The swarm initiated with a magnitude 1.3 event at 5 km depth. Activity intensified shortly afterward, including a magnitude 2.6 quake at 8 km depth followed within minutes by events of 1.8 and 1.9. The largest event, magnitude 3.5, occurred at 01:40 UTC on 3 May at 8 km depth. Subsequent activity featured a magnitude 3.0 event at 11:43 UTC on the same day. Depths remained consistent in the mid-crustal range, with one final event of magnitude 1.0 recorded at 3 km depth as the sequence tapered off.

This pattern aligns with typical swarm behavior in the area, where clustered events occur without a dominant mainshock-aftershock sequence. Magnitudes stayed below 4.0, indicating moderate energy release distributed across numerous small ruptures along local fault segments.

The Little Lake region lies within the Eastern California Shear Zone, a zone of distributed right-lateral strike-slip faulting that accommodates a portion of the relative motion between the Pacific and North American plates. Deformation here is influenced by the broader Walker Lane belt, resulting in elevated seismicity rates compared to surrounding stable blocks. The crust in this area consists of Mesozoic basement rocks overlain by Cenozoic volcanic and sedimentary units, with active fault systems trending northwest-southeast.

Since 2000, the region has experienced 105 documented swarms, with notable concentrations in 2019 (32 swarms) and 2020 (26 swarms). These episodes reflect ongoing strain accumulation and release along immature fault networks. The 2019 Ridgecrest earthquake sequence, featuring a magnitude 7.1 mainshock on 6 July located just 8 km from the S20230503.1 center, represents the most significant recent event and likely altered local stress conditions, contributing to subsequent swarm activity.

Seismic monitoring in this zone benefits from dense instrumentation deployed after the Ridgecrest sequence, enabling precise location and depth determination for small-magnitude events. Depths clustering around 8–10 km suggest rupture within the seismogenic zone above the brittle-ductile transition.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey Regional Fault Maps
• SeismoSight internal swarm classification records