Seismic Swarm S20190829.1 Near Mira Loma, California

A seismic swarm designated S20190829.1 occurred 3 km north-northwest of Mira Loma in Riverside County, California. The sequence began at 14:49 UTC on 28 August 2019 and concluded at 06:00 UTC on 2 September 2019, spanning 111 hours and 11 minutes. During this interval, 69 earthquakes were recorded.

The events were predominantly low-magnitude, ranging from 0.5 to 2.0. The majority clustered at shallow depths of 1–3 km, with a smaller number occurring between 4 km and 11 km. Activity peaked on 29 August, featuring multiple events above magnitude 1.5, including a magnitude 2.0 quake at 04:50 UTC. Subsequent days showed declining frequency, with isolated events continuing until the final recorded tremor on 2 September.

This swarm exemplifies typical swarm behavior in Southern California, where numerous small earthquakes occur in close spatial and temporal proximity without a dominant mainshock. Such sequences often reflect fluid migration or aseismic slip along fault segments rather than a single large rupture.

Mira Loma lies within the tectonically active Peninsular Ranges province of Southern California. The region sits near the boundary between the Pacific and North American plates, where right-lateral strike-slip motion along the San Andreas Fault system and subsidiary faults, including segments of the Elsinore and Chino fault zones, accommodates regional strain. Shallow seismicity is common due to the presence of fractured basement rocks and sedimentary basins that amplify ground motion.

Since 2000, ten swarms have been documented in the immediate area, occurring in 2002, 2003, 2005, 2008, 2012, 2013, 2015, 2018, and twice in 2019. These recurrent episodes indicate persistent low-level strain release along local fault structures without escalation to larger events.

No damage or injuries were associated with swarm S20190829.1. Continued monitoring by regional networks remains essential for understanding fault interactions in this densely populated portion of the Inland Empire.

References

USGS Earthquake Catalog
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center Swarm Records