Seismic Swarm S20190902.1 Near Little Lake, California

Seismic swarm S20190902.1 occurred approximately 20 km east-southeast of Little Lake in Kern County, California, within the Eastern California Shear Zone. The sequence began at 00:38 UTC on 2 September 2019 and concluded at 15:30 UTC on 3 September 2019, spanning 38 hours and 52 minutes. During this period, 76 earthquakes were recorded, with magnitudes ranging from 0.1 to 2.4 and focal depths predominantly between 1 km and 12 km.

The swarm exhibited typical characteristics of clustered microseismicity, with events distributed across shallow crustal levels. Early activity on 2 September included several events near magnitude 1.0–1.5 at depths of 2–10 km, followed by a peak magnitude 2.1 event at 22:37 UTC. Activity continued into 3 September, culminating in a magnitude 2.4 event at 14:12 UTC at 3 km depth. Depths showed no consistent migration pattern, remaining concentrated in the upper 10 km of the crust.

This region lies within the Mojave Desert portion of the Walker Lane belt, where northwest-trending right-lateral strike-slip faults accommodate a significant fraction of Pacific–North American plate motion. The local geology features Quaternary alluvium overlying Mesozoic basement rocks intruded by Cenozoic volcanic units associated with the nearby Coso Volcanic Field. Seismicity here reflects both tectonic loading on regional faults and possible contributions from hydrothermal or magmatic processes at depth.

Historical records indicate recurrent swarm activity in this area. Since 2000, 65 swarms have been documented, with notable yearly totals including 14 in 2019, 7 each in 2004 and 2010, and 6 in 2006. These episodes commonly involve low-magnitude events clustered over hours to days, often without a clear mainshock–aftershock sequence.

The September 2019 swarm followed the July 2019 Ridgecrest earthquake sequence by roughly two months. Static and dynamic stress changes from the magnitude 6.4 and 7.1 mainshocks likely influenced the timing and location of later swarms in the broader region, consistent with observed aftershock patterns extending into the Little Lake area.

Overall, swarm S20190902.1 represents a modest but characteristic expression of background seismicity in an actively deforming tectonic setting. Continued monitoring remains essential for understanding potential links between swarm activity, regional fault systems, and volcanic features.

References

USGS Earthquake Hazards Program catalog (earthquake.usgs.gov)
California Geological Survey, Regional Geologic Maps
USGS Professional Paper on the 2019 Ridgecrest Sequence