Seismic Swarm VS20250711.1: Analysis of Activity Near Calipatria, California
A seismic swarm designated VS20250711.1 was recorded 12 km west-northwest of Calipatria, California, in the Imperial Valley. The sequence began at 06:06 on 11 July 2025 and concluded at 00:39 on 13 July 2025, spanning 42 hours and 33 minutes. During this period, 132 earthquakes were registered. Analysis of the first 100 events reveals predominantly low-magnitude activity with shallow focal depths. Event parameters indicate magnitudes ranging from 0.6 to 4.3, with the largest shock occurring at 09:55:42 on 11 July at a depth of 11 km. The majority of events clustered between 3 km and 7 km depth, consistent with shallow crustal processes. Temporal distribution showed peak activity in the initial hours, including a 3.3 magnitude event at 08:22:37 and a 3.0 magnitude event at 08:18:37. Subsequent events maintained lower magnitudes, with only isolated instances exceeding 2.0. The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by right-lateral strike-slip motion along the Pacific-North American plate boundary. This region experiences frequent seismicity due to interactions among the San Andreas Fault, Imperial Fault, and Brawley Seismic Zone. Geothermal gradients and fluid migration along faults contribute to swarm-like sequences rather than mainshock-aftershock patterns. Historical records document 102 swarms in the area since 2000. Notable years include 2010 and 2013 with 13 events each, followed by elevated activity in 2009 (11 swarms) and 2012 (11 swarms). The 2025 sequence represents the second swarm of the year, aligning with ongoing tectonic strain accumulation. The swarm's characteristics—high event count, limited maximum magnitude, and shallow depths—align with typical behavior in this fault network. No damage or felt reports beyond minor shaking were associated with the activity.
References
United States Geological Survey Earthquake Catalog California Geological Survey Regional Fault Maps SeismoSight Internal Swarm Classification Database