Seismic Swarm S20190531.1: Analysis of Activity Near Glen Avon, California

Seismic swarm S20190531.1 was recorded in the region 4 km northwest of Glen Avon, California. The sequence began at 15:11 on 30 May 2019 and concluded at 20:14 on 19 June 2019, spanning 485 hours and 2 minutes. During this period, 1003 earthquakes were registered.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.5 to 2.9, with the majority below 2.0. Depths were consistently shallow, primarily between 2 km and 4 km, though isolated events reached 1 km and 6 km. The largest event in this subset measured 2.9 on 31 May 2019 at 13:39:44. Multiple events clustered within short time intervals, such as the series between 13:39 and 13:42 on 31 May, indicating typical swarm behavior with rapid succession rather than a single mainshock-aftershock pattern.

This swarm aligns with the area's documented seismic history. Since 1 January 2000, six prior swarms have occurred in the vicinity, specifically in 2003, 2005, 2008, 2013, 2015, and 2018. These episodes underscore recurring, episodic seismicity without escalation to larger destructive events.

The Glen Avon region lies within the Peninsular Ranges of Southern California, part of the broader Pacific-North American plate boundary. Tectonic deformation here is driven by right-lateral strike-slip motion along the San Andreas Fault system and subsidiary structures, including segments of the Elsinore and San Jacinto fault zones. The local geology features Mesozoic granitic and metamorphic basement rocks overlain by Quaternary alluvial sediments, which amplify ground shaking during seismic events. Historical records indicate that such swarms often result from fluid migration or aseismic slip along fault networks, releasing accumulated strain in low-magnitude bursts.

Updated monitoring by regional networks confirms that Southern California experiences thousands of earthquakes annually, with swarms representing a common mode of strain release in this transform boundary setting. Depths in the 1–6 km range correspond to the brittle upper crust, where frictional locking predominates until episodic failure occurs.

References

• SeismoSight internal swarm classification data for S20190531.1 and historical statistics.
• United States Geological Survey (USGS) Earthquake Hazards Program: regional tectonic framework of the Peninsular Ranges and San Andreas Fault system.