Seismic Swarm S20030222.1 Near Big Bear City, California

Swarm S20030222.1 was recorded beginning at 18:52 on 21 February 2003 and concluding at 07:08 on 12 March 2003. The events centered 5 km north of Big Bear City, California, with 615 earthquakes detected over 444 hours and 16 minutes. This activity represents the sole swarm documented in the region since 1 January 2000, following a prior swarm in 2001. The first 100 events provide a clear picture of swarm dynamics. The sequence initiated with a magnitude 0.8 event at a depth of -2 km. Within hours on 22 February, activity intensified, featuring a magnitude 4.9 earthquake at 5 km depth, followed rapidly by magnitudes 4.0, 4.3, and 3.7. Subsequent events ranged primarily between magnitudes 1.0 and 3.4, with depths mostly between 1 km and 5 km, though several registered at shallower or slightly negative values. Notable later shocks in this subset included a magnitude 4.1 at 2 km depth. The pattern shows clustered shallow seismicity with rapid succession of moderate events early on, transitioning to lower-magnitude tremors. Big Bear City lies within the Transverse Ranges of southern California, a tectonically active zone influenced by the San Andreas Fault system and subsidiary structures such as the North Frontal Fault. The region experiences frequent seismicity due to right-lateral strike-slip motion along the San Andreas and associated reverse faulting in the San Bernardino Mountains. Historical earthquakes, including the 1992 Big Bear event of magnitude 6.5, underscore the area's potential for both swarm activity and larger mainshocks. Depths in the observed swarm align with typical crustal seismicity in this compressional regime, where events often occur between 2 km and 10 km. Such swarms contribute to ongoing monitoring of fault interactions in the eastern California shear zone. The 2003 sequence, while producing no reported damage, highlights the value of dense seismic networks in distinguishing swarm behavior from foreshock sequences.

References

USGS Earthquake Hazards Program
California Geological Survey
Southern California Seismic Network reports