Seismic Swarm S20140418.1: Analysis of Activity Near Anza, California

The seismic swarm designated S20140418.1 was recorded 8 km west of Anza, California. It began at 05:22 on 18 April 2014 and concluded at 12:49 on 26 April 2014, encompassing 191 earthquakes over 199 hours and 26 minutes. Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.0 to 2.5, with the majority below 1.0. Depths clustered tightly around 4–5 km, though occasional events reached 15–16 km. The sequence initiated with a 0.0 magnitude event at 4 km depth, followed by gradual escalation. A notable 2.5 magnitude quake occurred at 20:19 on 18 April at 4 km depth. Subsequent events maintained shallow foci, with isolated deeper occurrences such as a 0.8 magnitude at 16 km on 18 April at 23:09. The pattern indicates sustained microseismicity without a dominant mainshock. The Anza region lies within the San Jacinto Fault Zone, a major component of the broader San Andreas system in Southern California. This strike-slip fault network accommodates significant right-lateral shear between the Pacific and North American plates. Historical records document recurrent swarm activity here, driven by fluid migration and stress transfer along fault segments. Since 2000, 20 swarms have been identified in the area, distributed across years including single events in 2001, 2002, 2003, 2005 (two), 2009, and 2014, with higher frequencies in 2010 (three), 2011 (four), 2012 (three), and 2013 (three). Such swarms reflect the zone's elevated seismicity rates, where small events often cluster without producing damaging ground motion. Depths typically remain crustal, consistent with the observed 3–16 km range in this sequence. Ongoing monitoring underscores the importance of these episodes for understanding fault dynamics in this tectonically active corridor. References
SeismoSight internal swarm classification records.
United States Geological Survey regional fault maps and historical seismicity summaries.