Seismic Swarm S20221008.1: Activity East of Julian, California

An earthquake swarm designated S20221008.1 was recorded approximately 23 km east of Julian, California. The sequence began at 22:45 on 7 October 2022 and concluded at 06:59 on 8 October 2022, spanning 8 hours and 14 minutes. During this interval, 28 earthquakes were registered.

The events exhibited magnitudes between 0.3 and 3.4, with focal depths consistently ranging from 9 to 14 km. The largest shock, magnitude 3.4, occurred at the onset at a depth of 11 km. Subsequent activity featured predominantly smaller events clustered in the initial hours, followed by sporadic lower-magnitude tremors through the early morning of 8 October. Depths remained stable, suggesting a localized source volume within the mid-crust.

This swarm aligns with the broader tectonic framework of the Peninsular Ranges in southern California. The region lies within a zone of distributed deformation influenced by the San Andreas transform boundary and subsidiary structures such as the Elsinore Fault Zone. Granitic and metamorphic basement rocks typical of the Peninsular Ranges host these minor faults, which can accommodate episodic seismic clusters.

Historical records indicate recurrent swarm activity in the area since 2000. Eight swarms have been documented, occurring in 2007 (1 swarm), 2009 (3 swarms), 2010 (2 swarms), 2017 (1 swarm), and 2022 (1 swarm). Such sequences reflect the region’s ongoing response to regional strain accumulation and release.

Seismic swarms in this setting often arise from transient processes including pore-fluid pressure changes or aseismic slip on fault segments. The 2022 event’s compact duration and limited magnitude range are consistent with these mechanisms. No damage or felt reports beyond the immediate vicinity were associated with the sequence.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey Regional Fault Maps
• Southern California Earthquake Data Center Historical Records