Seismic Swarm S20171110.1 Near Anza, California: Analysis and Regional Context

Seismic swarm S20171110.1 was recorded 23 km east-southeast of Anza, California, beginning at 03:28 on 9 November 2017 and concluding at 23:46 on 11 November 2017. Over 68 hours and 18 minutes, the event sequence included 44 earthquakes. Magnitudes ranged from 0.3 to 3.5, with the largest event occurring at 00:23 on 10 November at a depth of 7 km. Most events clustered between 5 km and 8 km depth, though several reached 12–14 km.

The swarm initiated with a magnitude 3.1 event, followed by a rapid succession of smaller shocks. Activity peaked in the first 24 hours, with additional notable events including a magnitude 2.7 at 04:35 on 9 November and a magnitude 3.5 on 10 November. Later stages showed declining frequency and magnitude, tapering to isolated events below magnitude 1.0 by 11 November.

This sequence occurred within the San Jacinto Fault Zone, a major right-lateral strike-slip system in Southern California that accommodates a significant portion of the Pacific–North American plate boundary motion. The Anza region lies near the intersection of the San Jacinto and San Andreas fault systems, where complex fault geometry promotes clustered seismicity. Depths of 5–14 km align with the seismogenic zone of these faults, where brittle failure predominates in the upper crust.

Since 1 January 2000, 32 swarms have been documented in the area. Annual counts include one in 2001, two each in 2002 and 2005, one in 2009, five in 2010, three each in 2011 and 2012, four each in 2013 and 2016, two each in 2014, 2015, and 2017. This recurrence underscores persistent swarm behavior driven by fluid migration and stress transfer along the fault network.

Such swarms typically lack a dominant mainshock and instead reflect distributed slip on multiple fault strands. The 2017 event fits this pattern, with no single shock exceeding magnitude 4.0 and a gradual decay in activity. Ongoing monitoring by regional networks continues to track strain accumulation in this high-hazard corridor.