Seismic Swarm S20120510.1 Near Mesa, California

An earthquake swarm designated S20120510.1 was recorded 3 km east of Mesa, California, between 10:06 on 9 May 2012 and 08:25 on 11 May 2012. Over the 46-hour period, 27 events were detected, with magnitudes ranging from 0.2 to 2.4 and focal depths primarily between 4 km and 15 km. The sequence exhibited typical swarm characteristics, featuring a rapid onset of activity followed by a gradual decline without a single dominant mainshock.

The largest events, all magnitude 2.4, occurred within a narrow time window on 10 May at depths of 11 km. Most events clustered around 9–11 km depth, consistent with shallow crustal faulting in the region. Smaller events (magnitudes below 1.0) dominated the later stages, indicating a decaying energy release typical of fluid-driven or stress-triggered swarm behavior.

California lies at the boundary between the Pacific and North American tectonic plates, where right-lateral strike-slip motion along the San Andreas Fault system and associated subsidiary faults produces frequent seismic activity. The Mesa area sits within this broader transform boundary, where distributed fault networks accommodate regional strain. Historical records show that the region has experienced episodic swarm activity, with four documented swarms since 2000: one in 2003, two in 2007, and the 2012 sequence analyzed here. Such recurrent swarms reflect localized stress perturbations rather than large-scale tectonic loading.

Seismic swarms differ from typical aftershock sequences by lacking a clear mainshock–aftershock relationship and often persisting for days to weeks. In tectonically active zones like coastal and southern California, swarms may be influenced by fluid migration along fault zones or aseismic slip transients. Depths recorded in this swarm align with the seismogenic zone where brittle failure occurs in the upper crust.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the area, contributing to improved understanding of fault interactions and hazard assessment. The 2012 swarm remains a useful case study for characterizing low-magnitude clustered events in transform-margin settings.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Fault Activity Map
Southern California Seismic Network bulletins