Seismic Swarm S20190905.1: Analysis of Activity Near Anza, California
Seismic swarm S20190905.1 occurred 7 km southwest of Anza, California, from 21:24 on 4 September 2019 to 00:48 on 9 September 2019. Over 99 hours and 24 minutes, the swarm produced 56 earthquakes. All events registered magnitudes between 0.1 and 1.4, with the majority falling below 1.0. Depths ranged primarily from 1 to 9 km, though a few later events reached 15–16 km.
The temporal distribution showed peak activity on 4–5 September, with 38 events recorded in the first 48 hours. Subsequent days featured declining frequency, tapering to isolated events by 8–9 September. Magnitudes remained consistently low, indicating a classic swarm sequence without a dominant mainshock. Depths clustered in the upper crust, consistent with shallow faulting in the region.
This swarm fits established patterns in the Anza area, where the San Jacinto Fault Zone drives frequent seismic episodes. The fault zone forms part of the San Andreas system and accommodates significant right-lateral strike-slip motion. Its complex segmentation and proximity to the Peninsular Ranges contribute to elevated microseismicity. Historical records document 19 swarms in the locale since 1 January 2000, occurring in 2002, 2005, 2010, 2011, 2014, 2015, 2016, 2017 (four episodes), 2018 (six episodes), and 2019 (two episodes including S20190905.1).
Such swarms typically reflect localized stress adjustments along fault segments rather than large-scale tectonic loading. The shallow focal depths observed align with the brittle-ductile transition zone in this portion of the San Jacinto Fault, where fluid migration or aseismic slip may trigger clustered events. No damage or felt reports accompanied the sequence, underscoring its minor energetic release.
Ongoing monitoring by regional networks continues to track activity along this fault segment, which has produced larger events historically and remains capable of moderate earthquakes. The 2019 swarm reinforces the value of dense seismic catalogs for characterizing background rates and identifying precursory patterns in Southern California fault systems.
References
USGS Earthquake Catalog
California Geological Survey Fault Database
Southern California Seismic Network bulletins