Seismic Swarm S20240907.1 Near Ontario, California

A seismic swarm designated S20240907.1 occurred 5 km southeast of Ontario in San Bernardino County, California. The sequence began at 17:05 on 7 September 2024 and concluded at 01:59 on 9 September 2024, lasting 32 hours and 53 minutes. During this period, 40 earthquakes were recorded.

The events exhibited magnitudes between 0.7 and 3.9, with the largest shock reaching 3.9 at 17:34 on 7 September. Focal depths remained shallow, concentrated between 4 and 6 km. The sequence featured an initial cluster of moderate events, including a 3.4 magnitude earthquake at the onset, followed by the peak magnitude roughly 30 minutes later. Subsequent activity declined gradually, with smaller events persisting into the early hours of 9 September.

This swarm aligns with the historical pattern of episodic seismic clusters in the area. Since 2000, nine prior swarms have been documented in the same locale, occurring in 2002, 2005, 2008, 2012, 2015, 2018, and three times in 2019. Such recurrent activity reflects the underlying tectonic setting rather than isolated anomalies.

The region sits within the southern California fault network at the boundary between the Pacific and North American plates. Right-lateral strike-slip motion dominates, accommodated primarily by the San Andreas Fault system and subsidiary structures such as the Cucamonga and Sierra Madre fault zones. These faults cut through Quaternary alluvial deposits and older crystalline basement rocks of the Transverse Ranges. Shallow crustal seismicity is common because of the thin seismogenic layer, typically extending no deeper than 10–15 km in this sector.

Geological records indicate that the broader Inland Empire has experienced repeated moderate earthquakes throughout the Holocene, driven by strain accumulation along these northwest-trending faults. The September 2024 swarm occurred at depths consistent with the upper seismogenic zone, where brittle failure predominates in granitic and metamorphic rocks.

No damage or injuries were associated with the sequence, consistent with the modest magnitudes involved. Continued monitoring remains essential given the proximity to densely populated urban corridors and critical infrastructure.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• California Geological Survey Fault Activity Map
• Southern California Earthquake Data Center historical records