Recent Earthquake Swarm Near Anza, California
An earthquake swarm designated S20240829.1 occurred 8 km NNW of Anza, California, beginning at 04:54 on 28 August 2024 and concluding at 06:22 on 30 August 2024. Over 49 hours and 28 minutes, the sequence produced 31 events. Depths ranged primarily between 13 km and 16 km, with one shallower event at 2 km. Magnitudes remained modest, peaking at 3.3, while most events registered below 1.0.
The swarm initiated with a magnitude 0.5 event at 13 km depth. Activity intensified midday on 28 August, including the largest shock of magnitude 3.3 at 15 km depth, followed by several smaller aftershocks clustered around 15–16 km. Subsequent events on 29 August concentrated at slightly shallower depths near 14 km, with two magnitude 1.8 shocks. The sequence tapered off by late 30 August.
This pattern aligns with the tectonic setting of the region. Anza lies within the Peninsular Ranges of Southern California, where the San Jacinto Fault Zone accommodates a significant portion of the relative motion between the Pacific and North American plates. The fault zone consists of multiple right-lateral strike-slip segments that have generated both moderate and large historical earthquakes. Crustal deformation here occurs at rates of several millimeters per year, producing frequent small-magnitude seismicity.
Seismic swarms are common in this area due to fluid migration and stress transfer along fault networks. Since 2000, 71 swarms have been recorded near Anza, with notable increases in frequency during 2020 (12 swarms) and 2018 and 2022 (7 swarms each). Earlier decades showed lower annual counts, typically one to four events per year. Depths of swarm events generally fall between 10 km and 16 km, consistent with the brittle-ductile transition zone in the local crust.
The August 2024 swarm did not produce any reported damage or felt shaking beyond light tremors near the epicentral area. Such sequences contribute to ongoing monitoring of strain accumulation along the San Jacinto Fault, which last hosted a major rupture in the early 20th century. Continued observation helps refine models of fault interaction and seismic hazard in the Inland Empire region.
References
USGS Earthquake Hazards Program
California Geological Survey
Southern California Seismic Network catalogs