Seismic Swarm S20150602.1 Near Little Lake, California

Seismic swarm S20150602.1 was recorded 22 km east of Little Lake in Kern County, California. The sequence began at 08:24 on 1 June 2015 and concluded at 14:26 on 2 June 2015, encompassing 36 earthquakes over 30 hours and 2 minutes. All events occurred at shallow depths between 0 km and 9 km, with the majority clustered between 2 km and 3 km.

The swarm featured low to moderate magnitudes. The largest event reached magnitude 3.5 at 01:31 on 2 June, followed closely by a magnitude 3.4 event later that afternoon. Other notable shocks included magnitudes 2.9, 2.4, 2.3, and 2.2. Smaller events dominated the catalog, with 22 earthquakes below magnitude 1.0. Depths remained consistent throughout, indicating a compact source volume typical of swarm behavior rather than mainshock-aftershock sequences.

This activity aligns with the tectonic setting of the region. Little Lake lies within the Eastern California Shear Zone, where distributed right-lateral strike-slip faulting accommodates Pacific-North America plate motion. The area borders the Coso Volcanic Field, known for geothermal systems and frequent microseismicity driven by both tectonic stress and fluid migration. Shallow focal depths observed in the swarm are consistent with this environment, where brittle failure occurs in the upper crust.

Since 2000, the Little Lake area has experienced 31 documented swarms. Yearly counts show variability: six swarms each in 2004 and 2010, five in 2006, three in 2013, and lower numbers in other years. This pattern underscores episodic swarm activity along local fault networks. The 2015 swarm fits within this historical recurrence.

A significant regional event occurred on 6 July 2019, when the magnitude 7.1 Ridgecrest earthquake struck approximately 9 km from the swarm center. That mainshock and its aftershock sequence highlighted the seismic potential of the same fault system. Post-2019 studies have refined fault models in the area, confirming the role of conjugate faulting and stress transfer in triggering both large events and subsequent swarms.

Seismic swarms in this part of California often reflect transient increases in pore-fluid pressure or aseismic slip on nearby faults. The 2015 sequence, with its rapid onset and decay, exemplifies these characteristics. No damage or felt reports beyond minor shaking were associated with the swarm, consistent with its modest magnitudes.

Ongoing monitoring by regional networks continues to track microseismicity in the Little Lake–Coso region. Such data contribute to improved understanding of strain accumulation and the interplay between tectonic and volcanic processes along the western margin of the Basin and Range province.

References

SeismoSight internal swarm catalog S20150602.1
USGS Earthquake Catalog (events 2000–2019)
Southern California Seismic Network annual reports