Seismic Swarm S20161026.1: Analysis of Activity Near Anza, California

Anza, California, lies within the tectonically active San Jacinto Fault Zone, a major branch of the San Andreas Fault system in Southern California. This region experiences frequent seismic activity due to right-lateral strike-slip faulting along multiple strands, including the Anza and Coyote Creek segments. The local geology features a complex network of faults cutting through metamorphic and granitic basement rocks overlain by sedimentary deposits, contributing to elevated seismicity rates compared to surrounding areas.

Swarm S20161026.1 was recorded 14 km east-southeast of Anza. The sequence began at 22:41 on 25 October 2016 and concluded at 20:26 on 31 October 2016, spanning 141 hours and 44 minutes. During this interval, 73 earthquakes were detected, with the majority occurring at depths between 5 km and 14 km. Magnitudes remained low, peaking at 2.7 on 29 October, consistent with typical swarm behavior where events cluster in time and space without a dominant mainshock.

The temporal distribution showed initial low-magnitude events on 25 October, followed by increased activity on 26 and 29 October. Depths averaged near 9 km, suggesting nucleation within the seismogenic zone of the fault system. Such swarms often reflect fluid migration or aseismic slip triggering, common in this part of the San Jacinto Fault Zone.

Historical records indicate 32 swarms in the area since 1 January 2000. Annual counts include one in 2001, two each in 2002, 2003, 2005, and 2009, five in 2010, four in 2011, three in 2012, four in 2013, two in 2014, three in 2015, and two in 2016. This pattern underscores persistent swarm-prone conditions driven by the fault zone's structural complexity.

References

• United States Geological Survey Earthquake Catalog (USGS)
• Southern California Seismic Network (SCSN) reports on San Jacinto Fault Zone seismicity