Seismic Swarm S20161231.1: Analysis of Activity Near Brawley, California
Seismic swarm S20161231.1 was recorded in the Imperial Valley region of southern California, centered 2 km west-southwest of Brawley. The sequence began at 11:36 on 31 December 2016 and concluded at 16:22 on 3 January 2017, spanning 76 hours and 46 minutes. During this interval, 317 earthquakes were registered.
The first 100 events provide a clear window into the swarm’s character. These events occurred between 11:36 on 31 December and approximately 23:17 the same day. Magnitudes ranged from 0.9 to 3.8, with the majority falling between 1.0 and 2.5. Depths were shallow, predominantly between 2 km and 11 km, and a single event reached 14 km. The largest shock in this subset measured 3.8 at 23:06:56 and was located at 14 km depth. Several events of magnitude 3.0 or greater clustered between 21:49 and 23:06, indicating a brief intensification phase roughly 10 hours after onset.
The temporal distribution shows an initial low-magnitude phase lasting several hours, followed by a marked increase in both rate and size after 21:00. Events of magnitude 2.0 and above became more frequent during this window, yet the overall energy release remained modest. Depths stayed consistently shallow, consistent with activity within the upper crust.
The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates. The Brawley Seismic Zone, a north-northwest-trending band of faulting, connects the southern San Andreas Fault with the Imperial Fault and accounts for much of the region’s frequent seismicity. Geothermal fields and fluid migration along these structures commonly trigger swarm-like sequences rather than single large mainshock-aftershock patterns.
Historical records document 71 swarms in the area since 1 January 2000. Yearly counts show considerable variability, with peaks in 2010 (12 swarms) and 2013 (13 swarms). The 2016–2017 sequence fits within this established pattern of episodic, low-to-moderate magnitude activity.
Such swarms rarely produce damaging ground motion, yet they underscore the persistent strain accumulation along the plate boundary. Continuous monitoring by regional seismic networks remains essential for distinguishing routine swarm behavior from any potential escalation.
References
- United States Geological Survey Earthquake Catalog
- California Geological Survey Regional Fault Maps
- SeismoSight internal swarm classification records