Earthquake Swarm S20030628.1 Near Loma Linda, California

An earthquake swarm designated S20030628.1 occurred 5 km south-southeast of Loma Linda, California, between 23:18 UTC on 27 June 2003 and 22:22 UTC on 28 June 2003. The sequence lasted 23 hours and 4 minutes and comprised 25 events with magnitudes ranging from 1.4 to 2.7. Depths clustered between 9 km and 12 km, consistent with shallow crustal faulting in the region.

The swarm began with a magnitude 1.6 event at 23:18 on 27 June. Subsequent activity included multiple events near magnitude 1.7 during the early hours of 28 June, followed by a magnitude 2.3 shock at 04:05. Peak activity featured a magnitude 2.7 earthquake at 13:54 on 28 June. All events remained below magnitude 3.0, producing no reported damage or felt reports beyond local instrumental detection.

Loma Linda lies within the tectonically active Peninsular Ranges of Southern California, part of the broad transform boundary between the Pacific and North American plates. The area is dominated by the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the relative plate motion. The swarm location aligns with the northern segment of this fault zone, where subsidiary faults and step-overs create localized stress concentrations that favor swarm-type sequences rather than single large ruptures.

Seismicity in this portion of the Inland Empire has been documented since the late nineteenth century. The San Jacinto Fault has generated several moderate to large earthquakes, including the 1918 magnitude 6.7 event near San Jacinto and the 1968 magnitude 6.6 Borrego Mountain earthquake. Ongoing microseismicity reflects continuous strain accumulation and release along the fault system, with swarm episodes occurring periodically as fluid migration or aseismic slip transiently increases pore pressure on fault surfaces.

Analysis of the 2003 swarm indicates a compact source volume with events tightly clustered in both time and space. The absence of a clear mainshock-aftershock pattern is characteristic of swarms, which often reflect distributed slip on a network of small faults rather than rupture of a single plane. Depths near 10 km place the activity within the seismogenic zone where brittle failure dominates.

Current monitoring by regional seismic networks continues to record background seismicity along the San Jacinto Fault, underscoring the persistent seismic hazard in the greater Los Angeles–Inland Empire corridor. The 2003 swarm remains a useful case study for understanding how low-magnitude sequences contribute to long-term strain release without producing damaging ground motion.

References

USGS Earthquake Hazards Program – San Jacinto Fault Zone
Southern California Earthquake Data Center – Regional seismicity catalog
California Geological Survey – Fault activity map of California