Seismic Swarm S20070526.1 near Mesa, California

A seismic swarm designated S20070526.1 occurred 1 km east-northeast of Mesa, California, between 17:45 on 25 May 2007 and 23:31 on 26 May 2007. The event sequence lasted 29 hours and 46 minutes and produced 25 earthquakes.

The swarm began with low-magnitude activity that rapidly escalated. A magnitude 3.4 event at 7 km depth occurred at 20:04 on 25 May, followed by additional events reaching magnitude 3.2 at 7 km depth later that evening. A second magnitude 3.4 earthquake struck at 6 km depth on 26 May at 08:04. Depths throughout the swarm ranged from 4 km to 8 km, with the majority clustered between 5 km and 7 km. Magnitudes remained predominantly below 2.5 after the two largest shocks, indicating a typical swarm pattern of clustered, moderate-energy releases without a single dominant mainshock.

This sequence represents the second swarm recorded in the area since 1 January 2000. The only prior swarm occurred in 2003 and consisted of a single event cluster.

The Mesa region lies within southern California’s tectonically active zone, where the Pacific Plate slides northwestward relative to the North American Plate along the San Andreas Fault system and associated secondary faults. Ongoing right-lateral strike-slip motion produces frequent small earthquakes and occasional swarms as stress is released across distributed fault networks. Depths of 4–8 km are consistent with the brittle upper crust in this portion of the plate boundary.

Historical seismic records show that southern California experiences multiple swarms annually, often linked to fluid migration or aseismic slip transients that load nearby faults. The 2007 swarm fits this regional pattern of short-lived, spatially confined activity.

References
SeismoSight internal swarm classification S20070526.1
USGS Earthquake Hazards Program – Southern California seismicity overview