Seismic Swarm S20191112.1: Analysis of Activity Near El Centro, California

Seismic swarm S20191112.1 was recorded 1 km north of El Centro, California, beginning at 02:07 UTC on 12 November 2019 and concluding at 06:08 UTC on 13 November 2019. Over this 28-hour period, 84 earthquakes were detected, providing valuable data on localized seismic behavior in the Imperial Valley.

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 along the San Andreas Fault system. This region experiences frequent seismicity due to right-lateral strike-slip faulting and associated normal faults. The Brawley Seismic Zone, extending through the valley, is a primary source of swarm activity, where clusters of earthquakes occur without a single dominant mainshock. Depths of events in swarm S20191112.1 ranged primarily from 5 km to 18 km, consistent with shallow crustal deformation typical of the area.

The swarm featured a maximum magnitude of 3.9, recorded shortly after initiation, followed by numerous smaller events. Magnitudes spanned 1.0 to 3.9, with the majority below 2.0. Temporal distribution showed peak activity in the initial hours and a secondary cluster around midday on 12 November, illustrating the episodic nature of swarm sequences. Depths remained relatively stable, indicating consistent rupture within the brittle upper crust.

Historically, the Imperial Valley has hosted repeated earthquake swarms linked to fluid migration and stress transfer along fault networks. Since 2000, 25 swarms have been documented in the vicinity, with notable concentrations in 2010 (8 events) and 2009 (4 events). Earlier occurrences in 2000, 2003, 2005, 2008, 2011, 2012, 2016, and 2017 underscore the persistent seismic character of the Salton Trough. These patterns reflect ongoing plate-boundary deformation rather than isolated anomalies.

Such swarms contribute to understanding regional hazard by highlighting zones of elevated microseismicity. While individual events rarely cause damage, their cumulative effect informs models of fault interaction and potential escalation to larger ruptures along the Imperial Fault or nearby structures.

References

USGS Earthquake Catalog (historical swarm data cross-verified for regional context)
California Geological Survey reports on Imperial Valley tectonics
SeismoSight internal swarm classification records