Seismic Swarm S20201113.2: Analysis of Activity Near Little Lake, California

Seismic swarm S20201113.2 was recorded in the region 18 km east-southeast of Little Lake, California. The sequence began at 22:38 on 12 November 2020 and concluded at 15:50 on 26 November 2020. Over 329 hours and 12 minutes, a total of 181 earthquakes were registered.

Analysis of the first 100 events reveals predominantly low-magnitude activity clustered at shallow depths. Initial events on 12 November included magnitudes of 0.7 at 6 km and 9 km depth. Subsequent recordings through 13 November showed magnitudes ranging from 0.0 to 1.1, with depths between 3 km and 12 km. Activity on 14 November featured a peak magnitude of 1.7 at 7 km depth, alongside multiple events near 1.0–1.4. By 15–16 November, magnitudes reached 3.5 at 7 km, accompanied by several events at 1.1–1.5 and depths mostly 2–10 km. Later events through 19 November maintained magnitudes below 2.0, with depths averaging 5–10 km and occasional shallower occurrences near 2–3 km. The distribution indicates a concentrated swarm pattern without significant escalation beyond the recorded maximum.

This swarm occurred within the Eastern California Shear Zone, a tectonically active region characterized by distributed right-lateral strike-slip faulting. The area lies near the transition between the San Andreas Fault system and the Walker Lane belt, where Pacific-North American plate motion is accommodated across multiple fault strands. Little Lake sits adjacent to the Coso Volcanic Field, an area influenced by both tectonic extension and geothermal activity that contributes to elevated seismicity rates.

Historical records since 2000 document 101 swarms in the vicinity, with notable increases in frequency during 2019 (30 swarms) and 2020 (23 swarms). Earlier years showed lower counts, such as 7 in 2004 and 2010, and 5 in 2006. The 2019 Ridgecrest earthquake sequence, including a magnitude 7.1 mainshock on 6 July located 12 km from the current swarm center, represents the strongest event in the region since 2000 and likely influenced local stress conditions.

The combination of swarm activity and proximity to major historical ruptures underscores the ongoing seismic hazard in this portion of the Mojave Desert. Continued monitoring remains essential for understanding fault interactions in this complex tectonic setting.

References

• United States Geological Survey Earthquake Catalog (earthquake.usgs.gov)
• California Geological Survey Regional Fault Maps
• Southern California Earthquake Data Center (scedc.caltech.edu)