Seismic Swarm S20180420.1: Analysis of Activity near Anza, California

The seismic swarm designated S20180420.1 occurred 14 km south-southeast of Anza, California, within the tectonically active Peninsular Ranges. This sequence began at 22:07 UTC on 19 April 2018 and concluded at 19:50 UTC on 28 June 2018, spanning 1677 hours and 42 minutes. During this period, 1109 earthquakes were recorded, consistent with swarm behavior characterized by clustered, non-mainshock-aftershock patterns. The Anza region lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the Pacific-North American plate boundary strain. This zone features complex fault geometry, including multiple strands and step-overs that promote swarm activity through fluid migration and aseismic slip. Historical seismicity in the area reflects ongoing deformation, with shallow crustal events typical of depths between 2 and 13 km. Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.1 to 2.9, with the majority below 1.0 and only four events reaching or exceeding 2.0. Depths remained shallow, averaging approximately 5 km, indicating brittle failure in the upper crust. The sequence initiated with events near magnitude 0.1–1.1 on 19 April, followed by a gradual increase in rate and occasional peaks, such as the magnitude 2.9 events on 22 and 25 April. Temporal clustering was evident in the initial days, with over 30 events recorded by 21 April, reflecting rapid energy release without a dominant mainshock. Swarm activity has been recurrent in this locale. Since 2000, 35 swarms have been documented, with annual counts showing variability: single events in 2001–2003 and 2009, rising to six in 2017 and three in 2018. This pattern aligns with the fault zone’s capacity for episodic, diffuse seismicity driven by its structural complexity. Overall, S20180420.1 exemplifies typical swarm dynamics in the San Jacinto Fault Zone, providing data for understanding strain accommodation in Southern California.

References

SeismoSight internal swarm classification data (S20180420.1 parameters and event list).
USGS Earthquake Hazards Program regional fault and seismicity summaries.