Seismic Swarm S20161127.1: Analysis of Activity Near Holtville, California

Seismic swarm S20161127.1 occurred approximately 7 km southwest of Holtville in Imperial County, California. The sequence began at 08:14 on 26 November 2016 and concluded at 08:53 on 28 November 2016, spanning 48 hours and 39 minutes. During this period, 50 earthquakes were recorded.

The events clustered at shallow depths, predominantly between 5 km and 11 km. The largest magnitude reached 3.3 at 08:20 on 26 November, followed by several events exceeding magnitude 2.5, including a 3.0 at 12:40 on 27 November. Activity showed two main peaks: an initial burst on the first day and a stronger cluster during the afternoon and evening of 27 November. Depths remained consistent within the upper crust, with no events deeper than 12 km.

This swarm fits the established pattern of seismic activity in the Imperial Valley. The region lies within the Salton Trough, a pull-apart basin formed by right-lateral strike-slip motion along the Pacific-North American plate boundary. The Imperial Fault and the Brawley Seismic Zone accommodate significant strain through both mainshock-aftershock sequences and swarm-type episodes. These swarms typically involve numerous small- to moderate-magnitude events without a single dominant mainshock.

Since 1 January 2000, twelve swarms have been documented in the same locale. Their distribution by year includes one in 2000, two in 2003, one in 2005, one in 2008, three in 2009, three in 2010, and one in 2011. Such recurrence underscores the persistent tectonic loading in the area and the tendency for strain release through episodic swarm behavior rather than isolated large events.

The geological setting features young sedimentary deposits overlying crystalline basement, with active faulting driven by northwest-directed shear and minor extension. Historical records show that similar swarms have preceded or accompanied larger earthquakes on nearby faults, although most remain confined to low to moderate magnitudes. Monitoring of these sequences provides valuable data on fault interaction and fluid involvement in triggering.

Continued observation of the Imperial Valley remains essential for understanding regional seismic hazard. The 2016 swarm illustrates the characteristic short-duration, high-event-count nature of activity in this tectonically active corridor.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Fault Database
SeismoSight internal swarm classification records