Seismic Swarm S20250422.1: Analysis of Activity Near Calipatria, California
On 22 April 2025, a seismic swarm designated S20250422.1 was recorded 15 km west-northwest of Calipatria in Imperial County, California. The sequence began at 09:05 and concluded at 19:16, encompassing 28 earthquakes over 10 hours and 11 minutes. Magnitudes ranged from 0.3 to 2.8, with the majority of events occurring at shallow depths between 1 and 4 km. The largest shock measured 2.8 at 3 km depth shortly after initiation, followed by numerous smaller events that clustered in the afternoon hours.
This swarm exhibits typical characteristics of Imperial Valley seismicity, where events are predominantly shallow and low-magnitude, reflecting brittle failure within the upper crust. Depths remained consistent until a single deeper event at 10 km late in the sequence, suggesting limited vertical migration of activity.
The Calipatria region lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the Imperial Fault. The area forms part of the Brawley Seismic Zone, where right-lateral strike-slip faulting accommodates Pacific-North American plate motion. Geothermal gradients are elevated due to crustal thinning and magmatic intrusions beneath the Salton Sea, promoting swarm-type seismicity rather than large mainshock-aftershock sequences.
Historical records since 2000 document 102 swarms in the immediate vicinity. Activity peaked in 2010 and 2013 with 13 swarms each, while quieter years such as 2014 and 2015 recorded only one swarm annually. The 2025 event marks the first swarm of the year, continuing the long-term pattern of episodic, clustered microseismicity.
Such swarms are driven by fluid migration along fault networks and aseismic slip transients. The 22 April sequence aligns with this model, showing rapid onset, high event rates, and quick decay without a dominant mainshock. No damage or felt reports of significance were associated with the activity.
Continued monitoring remains essential in this high-strain environment, where swarm recurrence informs probabilistic assessments of larger earthquakes along the southern San Andreas and Imperial faults.
References
United States Geological Survey Earthquake Catalog
California Geological Survey Regional Fault Maps
Scripps Institution of Oceanography Salton Trough Studies