Investigating the origin of seismic clustering in the Salton Trough

Claire D. Fagan, Allen L. Husker, & Joann M. Stock

Submitted August 30, 2026, SCEC Contribution #15307, 2026 SCEC Annual Meeting Poster #TBD

The Salton Trough region is one of the most geologically active regions in California. It experiences oblique extension between the Pacific and North American Plates, geothermal and magmatic activity, changes in hydrological loading, and very high seismicity. Due to the region’s geologic complexity, understanding the cause of seismicity in the region is often challenging, especially for seismic swarms, whose origination mechanisms are poorly understood. We used Southern California Earthquake Data Center (SCEDC) catalog data from January 2023 to June 2026 to identify seismic events and separate them into (1) swarms, (2) mainshock-aftershock sequences, (3) mixtures containing attributes of both (1) and (2), and (4) background seismicity. We employed a weighted nearest-neighbors technique to group the events, and separated the clusters into (1), (2), and (3) based on the relative timing of their mean event and comparison of overall event times with the centroid time of moment release. We then spatially correlated between the seismic activity and borehole data from geothermal fields within the Salton Trough, finding that swarm and mixture events occur within 3 km of geothermal fluid injection sites twice as often as mainshock-aftershock sequences and background seismicity. Swarms and mixtures within 3 km of injection sites occurred at shallower depths and lower magnitudes than events farther from injection wells. Many of the seismic clusters greater than 3 km from injection sites were planar. To further investigate these clusters, we mapped a new 3D fault plane catalog in the Salton Trough using all relocated seismicity from the SCEDC catalog and compared the clusters to all regional fault planes. This region is known as the Brawley Shear Zone, and these new fault planes contribute to 60 km of stair-step faulting within the Salton Trough, which may increase fluid migration in the region and increase seismic clustering. Seismic catalogs give a clear image of the new faults and repeated swarms have occurred over the same faults for decades. Overall, seismic swarms and mixtures within 3 km of geothermal wells and fields likely occur due to the increased pore fluid pressure generated by geothermal activity, while clusters away from geothermal wells and fields occur due to friction generated by frequent strike-slip faulting.

Key Words
Swarms, Geothermal Drilling, Fault Mapping

Citation
Fagan, C. D., Husker, A. L., & Stock, J. M. (2026, 08). Investigating the origin of seismic clustering in the Salton Trough. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology