Earthquake Swarm S20031113.1: Seismic Activity Near Borrego Springs, California

Earthquake swarm S20031113.1 occurred 13 km NNW of Borrego Springs, California, beginning at 17:06 on 12 November 2003 and concluding at 10:33 on 14 November 2003. Over 41 hours and 27 minutes, the swarm produced 31 earthquakes. The events clustered primarily on 13 November, with the largest reaching magnitude 3.3 at a depth of 12 km.

The sequence featured predominantly low-magnitude shocks. Depths ranged from 8 km to 13 km, consistent with shallow crustal activity in the region. A notable concentration of events took place between 05:17 and 06:38 on 13 November, including the magnitude 3.3 main shock followed by multiple aftershocks between 0.6 and 1.8 in magnitude. Later events tapered off, with the final recorded shock at magnitude 1.6.

This swarm fits within the broader seismic character of the area. Borrego Springs lies within the Anza-Borrego Desert, part of the San Andreas transform boundary system. The local geology is dominated by the San Jacinto Fault Zone, a major right-lateral strike-slip fault system that accommodates a significant portion of Pacific-North American plate motion. The fault zone features multiple strands, including the Coyote Creek Fault, which passes near the swarm epicenters and has a history of producing both large earthquakes and swarm-like sequences.

Seismic swarms in this setting often arise from fluid migration or aseismic slip along fault segments rather than a single mainshock-aftershock cascade. The 2003 activity aligns with this pattern, showing rapid onset and decay without a dominant triggering event beyond the initial magnitude 3.3 shock.

Historical records since 2000 indicate three swarms in the immediate region: one in 2001 and two in 2002. These prior episodes similarly involved small-magnitude events clustered over short time periods, underscoring recurrent swarm behavior along the San Jacinto Fault Zone.

The combination of fault geometry, crustal heterogeneity, and possible hydrothermal influences in the Salton Trough vicinity contributes to the area's elevated swarm frequency. Continued monitoring remains essential given the fault zone's capacity for larger events.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center Swarm Database