Seismic Swarm S20160720.2 Near Anza, California: Geological Context and Event Analysis

Seismic swarm S20160720.2 occurred in the region 19 km east-southeast of Anza, California, within the Peninsular Ranges province. This area lies at the boundary between the Pacific and North American tectonic plates, where right-lateral strike-slip faulting dominates along the San Jacinto Fault Zone. The swarm initiated at 22:27 on 19 July 2016 and concluded at 04:25 on 26 July 2016, spanning 149 hours and 58 minutes during which 68 earthquakes were recorded.

The events exhibited low to moderate magnitudes, with the largest reaching 2.6. Depths ranged primarily between 7 and 14 km, consistent with shallow crustal seismicity typical of the San Jacinto system. Most events clustered between 8 and 13 km depth, reflecting activity along fault segments that accommodate regional shear strain. The temporal distribution showed peak activity on 20 July, including multiple events within minutes of one another, followed by a gradual decline through 25 July.

Historical records indicate that swarm activity is recurrent in this locale. Since 1 January 2000, 31 swarms have been documented, with annual frequencies varying from one to five events. Notable years include 2010 with five swarms and 2011 and 2013 with four each. This pattern underscores the region's propensity for episodic, clustered seismicity rather than isolated mainshock-aftershock sequences.

Geologically, the Anza area features crystalline basement rocks of the Peninsular Ranges batholith, cut by northwest-trending faults that form part of the broader San Andreas transform boundary. Strain accumulation from Pacific plate motion drives frequent microseismicity, and swarms such as S20160720.2 often occur without a single dominant rupture, instead reflecting distributed slip on fault networks or fluid migration at depth. Depths observed align with the brittle-ductile transition zone in this tectonic setting.

Analysis of the swarm reveals a predominance of events below magnitude 1.0, interspersed with a few reaching 1.5–2.6. This distribution suggests efficient stress release through numerous small ruptures, a hallmark of swarm behavior in mature fault zones. No events exceeded magnitude 3.0, limiting potential for significant surface effects.

The recurrence of swarms since 2000 highlights ongoing tectonic adjustment along the San Jacinto Fault. Such activity contributes to long-term strain relief while posing challenges for seismic hazard assessment due to the diffuse nature of energy release.

References

SeismoSight internal swarm classification database.
USGS Earthquake Catalog for Southern California (2000–2016).
Southern California Seismic Network regional fault maps.