Seismic Swarm S20150211.2 Near Anza, California

Seismic swarm S20150211.2 was recorded 8 km north of Anza in Riverside County, Southern California. The sequence began at 23:20 on 10 February 2015 and concluded at 20:23 on 23 February 2015, spanning 309 hours and 2 minutes. During this period, 168 earthquakes were detected. The Anza region lies within the Peninsular Ranges province and is intersected by the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the Pacific-North American plate boundary deformation. This fault zone has produced numerous moderate earthquakes historically and remains one of the most active seismic sources in Southern California. Depths of the recorded events ranged primarily between 8 km and 17 km, consistent with the brittle seismogenic zone of the San Jacinto Fault. Analysis of the first 100 events reveals predominantly low-magnitude activity. The initial event registered magnitude 0.4 at 3 km depth. Subsequent shocks through 16 February maintained magnitudes below 2.0, with the largest reaching 1.9. Depths clustered around 12–14 km during the early phase, indicating a compact source volume. Event frequency was highest in the first 48 hours, followed by a gradual decline, characteristic of swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence. Historical records maintained by SeismoSight document 17 swarms in the same area since 1 January 2000. These occurred in the following years with the indicated counts: 2001 (1), 2002 (1), 2003 (1), 2005 (2), 2009 (1), 2010 (2), 2011 (3), 2012 (1), 2013 (1), and 2014 (4). This recurring pattern underscores the persistent seismic productivity of the northern San Jacinto Fault near Anza. The February 2015 swarm fits within the established regional pattern of episodic, low-magnitude clustering. Such sequences contribute to long-term strain release along the fault and are routinely monitored by the Southern California Seismic Network. No damage or felt reports above intensity III were associated with this particular swarm.

References

United States Geological Survey Earthquake Catalog California Geological Survey Fault Activity Map Southern California Earthquake Data Center SeismoSight internal swarm classification records