Seismic Insights into the 2015 Anza Earthquake Swarm S20150401.1

The Anza region in Southern California lies within the Peninsular Ranges, where the San Jacinto Fault Zone intersects with subsidiary structures that accommodate significant right-lateral shear between the Pacific and North American plates. This tectonic setting produces frequent microseismicity and episodic swarms driven by fluid migration and aseismic slip along fault segments at depths of 5–15 km.

Swarm S20150401.1 began at 04:59 on 31 March 2015 and concluded at 12:47 on 8 May 2015, centered 14 km southeast of Anza. Over 919 hours and 47 minutes the sequence produced 543 events. The first 100 recorded events, spanning 31 March to 5 April, displayed characteristic swarm behavior: low-magnitude earthquakes clustered tightly in space and time without a dominant mainshock.

Magnitudes ranged from –0.1 to 2.4, with the majority between 0.3 and 1.1. Depths concentrated around 7 km, though initial and later events reached 3 km and 15 km respectively, indicating activation across a modest vertical extent of the fault zone. The largest event, magnitude 2.4, occurred on 5 April at 12:56:55 at 8 km depth. Earlier peaks included a magnitude 2.0 on 31 March and a 1.9 on 4 April, yet no event exceeded magnitude 3.0, consistent with swarm-type sequences rather than typical mainshock-aftershock patterns.

Temporal evolution showed an initial burst of activity within the first 12 hours, followed by steady decay punctuated by small bursts on 1–2 April and a modest resurgence on 5 April. Most events occurred at depths of 6–9 km, suggesting a primary slip horizon near the base of the seismogenic zone in this locale.

Since 1 January 2000 the Anza area has hosted 28 documented swarms. Yearly counts include single swarms in 2001, 2015 and 2014; two each in 2002, 2003, 2005, 2009 and 2012; three in 2013; four each in 2011 and 2014; and five in 2010. This recurrence underscores the region’s persistent susceptibility to swarm activity linked to its intricate fault network and hydrothermal influences.

The 2015 sequence fits established patterns of swarm recurrence in the San Jacinto Fault Zone, where clusters of small events often delineate creeping or fluid-pressurized patches without producing damaging ground motion. Continued monitoring of such swarms aids in refining probabilistic forecasts for larger triggered events along the broader San Andreas system.

References

USGS Earthquake Catalog California Geological Survey Fault Activity Map Southern California Seismic Network Annual Reports