Analysis of Seismic Swarm S20130321.2 in Northern California
Seismic swarm S20130321.2 occurred in Northern California, a region shaped by the ongoing interaction between the Pacific and North American tectonic plates along the San Andreas Fault system and associated subsidiary faults. This tectonic setting produces frequent seismic activity, including swarms of small earthquakes often linked to fluid migration or geothermal processes in areas such as the Geysers geothermal field. The swarm initiated at 23:45 on 20 March 2013 and concluded at 20:14 on 26 March 2013, spanning 140 hours and 29 minutes during which 251 earthquakes were recorded.
Analysis of the first 100 events reveals predominantly low-magnitude activity clustered at shallow depths. Magnitudes ranged from negative values to a peak of 2.6, with the majority between 0.0 and 0.8. Depths concentrated between 2 and 7 km, though several events occurred at or near the surface. Notable sequences include a cluster around 22:40 on 21 March featuring magnitudes up to 2.1, and an intense burst on 22 March between 14:01 and 14:27 that included the swarm's largest event of magnitude 2.6 at 2 km depth, followed by multiple aftershocks of 1.3 to 1.4. Later events maintained similar depth patterns but showed slightly declining frequency by 23 March.
Such swarms are characteristic of Northern California's seismic regime, where historical records since 2000 document 16 comparable episodes. These occurred in 2004 (1 swarm), 2005 (1), 2008 (7), 2010 (1), 2011 (1), 2012 (2), and 2013 (3). The elevated count in 2008 and 2013 underscores episodic clustering, potentially tied to regional stress adjustments or hydrothermal influences prevalent in the area's volcanic and geothermal terrains.
This event aligns with established patterns of microseismicity in Northern California, where most activity remains below magnitude 3.0 and poses minimal surface hazard. Depths under 10 km indicate brittle failure in the upper crust, consistent with plate boundary deformation models. Continued monitoring supports improved forecasting of swarm evolution in tectonically active zones.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional reports
SeismoSight internal swarm classification data