Earthquake Swarm S20050616.1 Near Yucaipa, California

An earthquake swarm designated S20050616.1 occurred 2 km northeast of Yucaipa, California, from 20:53 on 16 June 2005 to 16:24 on 18 June 2005. The sequence lasted 43 hours and 31 minutes and included 67 events. The largest shock reached magnitude 4.8 at a depth of 10 km, followed by events ranging from magnitude 0.7 to 3.6, with most hypocenters clustered between 12 and 16 km depth.

Swarm activity is characterized by a rapid onset of multiple events without a single dominant mainshock, followed by a gradual decline. In this case, the initial 4.8 event was succeeded within minutes by several magnitude 3+ aftershocks, after which activity transitioned to smaller-magnitude events distributed over the subsequent two days. Depths remained relatively consistent, indicating a compact source volume within the seismogenic crust.

The Yucaipa area lies within the Transverse Ranges of southern California, where northwest-trending right-lateral strike-slip faults of the San Andreas system interact with east-west compressional structures. This tectonic setting produces frequent small-to-moderate earthquakes as accumulated strain is released along secondary faults and fault splays. Historical records document recurrent seismic swarms in the broader Inland Empire region, reflecting ongoing Pacific-North American plate boundary deformation.

Seismic monitoring in southern California has improved markedly since the early 2000s, allowing finer resolution of swarm parameters. The 2005 sequence fits the pattern of short-duration, low-to-moderate magnitude swarms observed along the southern San Andreas and adjacent fault zones. Such events typically do not produce surface rupture but contribute to long-term strain release.

References

USGS Earthquake Hazards Program – Southern California Seismic Network
California Geological Survey – Fault Activity Map of California
Southern California Earthquake Data Center – Regional Seismicity Catalog