Seismic Swarm S20150531.1: Analysis of Activity Near Borrego Springs, California

An earthquake swarm designated S20150531.1 occurred 12 km northeast of Borrego Springs in San Diego County, California. The sequence began at 08:06 UTC on 31 May 2015 and concluded at 17:23 UTC on 2 June 2015, spanning 57 hours and 16 minutes. During this period, 45 earthquakes were recorded.

The swarm took place within the San Jacinto Fault Zone, a major component of the broader plate-boundary system separating the Pacific and North American tectonic plates. This region experiences frequent seismic activity due to right-lateral strike-slip faulting. Depths of the recorded events ranged primarily between 5 km and 13 km, consistent with shallow crustal deformation typical of the area.

The largest event reached magnitude 3.5 at a depth of 12 km on 31 May at 13:02:56 UTC. Subsequent activity included multiple events of magnitude 1.0–2.2, with the majority clustering between 9 km and 11 km depth. Notable later shocks occurred on 1 June at 18:35:51 UTC (magnitude 2.2, 10 km depth) and on 2 June with three closely spaced events around 17:20 UTC, each near magnitude 1.3–1.4.

Seismic swarms in this locale often reflect fluid migration or aseismic slip along fault segments rather than a single mainshock-aftershock sequence. The 2015 swarm followed patterns observed in prior episodes, with rapid onset, numerous small-magnitude events, and gradual decay.

Historical records indicate 20 swarms have occurred in the same area since 1 January 2000. These episodes were distributed across the following years: one each in 2001, 2002, 2003, 2006, 2011, 2014, and 2015; two in 2009; five in 2010; three in 2012; and three in 2013. Such recurrence underscores the persistent seismic character of the San Jacinto Fault Zone.

The geological setting features Quaternary alluvium overlying metamorphic and granitic basement rocks, intersected by northwest-trending faults. Ongoing monitoring by regional networks continues to document microseismicity that informs hazard assessments for nearby communities and infrastructure.

References

USGS Earthquake Catalog (events and locations)
California Geological Survey fault database (regional tectonics)
SeismoSight internal swarm classification S20150531.1 (event parameters and historical counts)