Seismic Swarm S20180508.1: Analysis of Activity Near Cabazon, California

Seismic swarm S20180508.1 was recorded 12 km north of Cabazon in Riverside County, California. The sequence began at 11:11 on 8 May 2018 and concluded at 23:09 on 12 May 2018, spanning 107 hours and 57 minutes. During this interval, 113 earthquakes were detected.

The swarm initiated with a magnitude 4.4 event at 11:49 on 8 May at a depth of 12 km. Subsequent activity included a magnitude 3.2 event two minutes later, followed by numerous smaller shocks predominantly between magnitudes 0.4 and 2.6. Depths for the first 100 events clustered between 11 km and 14 km, with occasional outliers reaching 17 km or 18 km. The sequence exhibited classic swarm characteristics: a rapid onset of events without a single dominant mainshock, sustained low-to-moderate magnitudes, and gradual decay over several days.

The Cabazon area lies within the San Gorgonio Pass, a structurally complex zone where the San Andreas Fault interacts with the San Jacinto and Banning faults. This region forms part of the broader Pacific–North American plate boundary, accommodating right-lateral strike-slip motion at rates of approximately 35 mm per year. The local geology comprises Mesozoic crystalline basement rocks overlain by Quaternary alluvial and fluvial deposits. Ongoing compression and fault segmentation in the Transverse Ranges contribute to elevated seismicity.

Historical records indicate two seismic swarms in the vicinity since 1 January 2000. The earlier swarm occurred in 2005, demonstrating that episodic swarm activity is a recurring feature of the fault system.

Analysis of the 2018 sequence shows that 85 percent of the first 100 events occurred within the initial 48 hours, underscoring the concentrated energy release early in the swarm. Magnitudes remained below 3.0 after the initial day, consistent with swarm behavior driven by fluid migration or aseismic slip rather than a classic aftershock cascade.

Such swarms provide valuable data for refining fault models in southern California. Continued monitoring supports improved understanding of strain accumulation along the southern San Andreas Fault, which last produced a major rupture in 1857.

References

USGS Earthquake Catalog
Southern California Seismic Network (SCSN) annual reports
California Geological Survey fault database