Seismic Swarm S20180527.1: Analysis of Activity Near Anza, California

Seismic swarm S20180527.1 was recorded in the Anza region of Southern California, beginning at 11:57 on 26 May 2018 and concluding at 08:08 on 31 May 2018. Over this 116-hour period, 54 earthquakes were detected at a location 14 km east-southeast of Anza. The events exhibited low magnitudes, with the largest reaching 2.9, and focal depths primarily between 7 km and 18 km. This swarm represents typical background seismicity in a tectonically active zone.

The temporal distribution showed the highest event rate during the initial 24 hours, followed by a gradual decline. Magnitudes remained predominantly below 1.0 after the initial peak, consistent with swarm behavior where energy release occurs through numerous small events rather than a mainshock-aftershock sequence. Depths clustered around 12–16 km, suggesting activity within the brittle upper crust along regional fault structures.

Anza lies within the San Jacinto Fault Zone, a major component of the broader San Andreas Fault system. This right-lateral strike-slip fault accommodates a significant portion of the Pacific-North American plate motion, with slip rates estimated at 10–15 mm per year. The fault zone features multiple strands and step-overs that promote complex rupture patterns and frequent microseismicity. The region’s geology includes Mesozoic granitic basement rocks overlain by Quaternary alluvial sediments, which influence wave propagation and site response during shaking.

Earthquake swarms in this area are often linked to fluid migration or aseismic slip transients that trigger brittle failure on favorably oriented faults. Historical records indicate persistent swarm activity, with 32 documented swarms since 2000. Yearly occurrences include single events in 2002, 2003, 2009, and 2012; two in 2005, 2010, and 2015; three in 2011 and 2014; five in 2016; six in 2017; and four in 2018 up to the time of this swarm. Such clustering underscores the fault zone’s capacity for episodic, distributed seismicity without large-magnitude releases.

Monitoring by regional seismic networks provides high-resolution catalogs that aid in identifying precursory patterns and refining hazard models. Continued observation of swarms like S20180527.1 contributes to understanding stress transfer and long-term fault behavior in Southern California.

References

USGS Earthquake Hazards Program – San Jacinto Fault Zone overview
Southern California Earthquake Data Center – Regional seismicity catalogs
California Geological Survey – Fault activity and slip-rate data