Seismic Swarm VS20220921.2 Near Niland, California
An earthquake swarm designated VS20220921.2 occurred 9 km west-southwest of Niland, California, beginning at 11:59 on 20 September 2022 and concluding at 11:11 on 21 September 2022. Over 23 hours and 12 minutes, the sequence produced 39 earthquakes, with magnitudes ranging from 0.9 to 2.3 and focal depths predominantly between 0 and 3 km, except for one deeper event at 7 km.
The swarm exhibited typical characteristics of clustered, low-magnitude activity, with the largest events (magnitudes 2.1–2.3) occurring in two main pulses around 20:09–22:44 on 20 September and 09:05–09:17 on 21 September. Depths remained shallow throughout, consistent with activity in the uppermost crust.
Niland lies within the Imperial Valley portion of the Salton Trough, a tectonically active pull-apart basin formed by the transition between the San Andreas Fault system and the Imperial Fault. This region experiences frequent earthquake swarms driven by right-lateral shear, geothermal fluid migration, and minor magmatic influences associated with the nearby Salton Sea geothermal field. The Brawley Seismic Zone, which extends through this area, is known for producing swarm sequences rather than isolated mainshock-aftershock patterns.
Historical records since 2000 document 91 swarms in the immediate vicinity. Annual counts include single swarms in 2000, 2001, 2002, 2004, 2014, 2015, 2018, and 2019; multiple events in 2003 (4), 2005 (3), 2008 (5), 2011 (4), 2016 (4), and 2020 (6); and higher activity in 2009 (11), 2010 (13), 2012 (11), 2013 (13), and 2021 (8). The 2022 swarm represents the sole recorded sequence that year up to the event date.
Such swarms are common in this extensional regime, where fault creep and fluid pressure changes facilitate episodic slip without producing large-magnitude events. Depths under 3 km align with the shallow seismogenic zone influenced by high heat flow and sedimentary basin fill.
References
USGS Earthquake Hazards Program regional reports on the Salton Trough and Imperial Valley.
California Geological Survey summaries of Imperial Fault zone tectonics.
Peer-reviewed literature on Brawley Seismic Zone swarm mechanisms (e.g., studies in Journal of Geophysical Research).