Seismic Swarm S20230428.1 Near Heber, California: Geological Context and Event Analysis

The region surrounding Heber, California, lies within the Imperial Valley portion of the Salton Trough, a tectonically active extensional basin formed by the interaction of the Pacific and North American plates along the San Andreas fault system. This area is characterized by right-lateral strike-slip faulting, primarily along the Imperial Fault and the Brawley Seismic Zone, which accommodate significant portions of the plate boundary motion. The Salton Trough experiences elevated heat flow and frequent microseismicity due to its position in a transtensional regime, with sedimentary basins overlying crystalline basement rocks that facilitate fluid migration and swarm activity.

Earthquake swarms in this locale are well-documented phenomena, often linked to fluid pressure changes or aseismic slip transients rather than mainshock-aftershock sequences. Historical records since 2000 indicate 28 such swarms in the vicinity, distributed across multiple years with notable clusters in 2010 (8 events) and recurring activity in 2009, 2021, and others. These swarms typically involve hundreds of events over days to weeks, concentrated at shallow to mid-crustal depths of 3–11 km.

Swarm S20230428.1 initiated at 00:40 on 28 April 2023, approximately 4 km south of Heber, and concluded at 04:29 on 2 May 2023, spanning 99 hours and 49 minutes with a total of 186 recorded earthquakes. Analysis of the initial 100 events reveals a magnitude range predominantly between 1.0 and 4.0, with the largest shock reaching M4.0 on 29 April at 12:07 (depth 10 km), immediately followed by an M3.2 event. Depths clustered between 3 km and 11 km, averaging around 8 km, consistent with the brittle-ductile transition zone in the Imperial Valley crust. Early activity featured smaller events (M1.0–M2.6) with variable timing, transitioning to a peak on 29 April that included multiple M2.0+ shocks. Subsequent events showed a gradual decline in frequency and intensity, illustrating the swarm's characteristic diffuse energy release without a dominant mainshock.

This pattern aligns with prior swarms in the Brawley Seismic Zone, underscoring the region's ongoing tectonic adjustment. Monitoring data from such episodes contribute to refined hazard assessments for the Imperial Valley, where proximity to the U.S.-Mexico border amplifies potential impacts on infrastructure and populations.

References

USGS Earthquake Catalog California Geological Survey regional reports SeismoSight internal swarm classification records