Seismic Swarm VS20080316.1: Analysis of Activity Near Calipatria, California
A notable seismic swarm, designated VS20080316.1, occurred in the Imperial Valley region of southern California. The sequence began at 19:47 on 15 March 2008 and concluded at 18:27 on 21 March 2008, spanning 142 hours and 39 minutes. It was centered 9 km west-northwest of Calipatria and produced 111 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity, with magnitudes ranging from 0.2 to 2.2 and focal depths between 0 and 17 km. The majority clustered between 2 and 8 km depth, consistent with shallow crustal processes in the area.
This swarm unfolded within the tectonically complex Salton Trough, a sediment-filled extensional basin formed by right-lateral shear along the Pacific-North American plate boundary. The trough lies at the southern terminus of the San Andreas Fault system and transitions into the Imperial Fault and Brawley Seismic Zone. These structures accommodate oblique extension and strike-slip motion, resulting in frequent microseismicity and episodic swarms rather than large mainshock-aftershock sequences. The region’s geology features thick Quaternary sediments overlying basement rocks, which influence wave propagation and contribute to the observed shallow event depths.
Historical records indicate recurring swarm activity in this locale. Since 1 January 2000, ten swarms have been documented, distributed across specific years as follows: one each in 2000, 2001, 2002, and 2004; three in 2003; and three in 2005. Such patterns underscore the persistent strain accumulation and release along local fault segments.
The temporal distribution of the 2008 swarm shows an initial burst of events on 15–16 March, followed by a gradual decline with intermittent clusters through 20 March. Magnitudes remained modest, with only a few events reaching or exceeding 2.0, and no significant escalation occurred. Depths stayed consistently shallow, supporting interpretations of fluid migration or aseismic slip triggering brittle failure in the upper crust.
This activity highlights the Imperial Valley’s role as a high-strain-rate corridor within the broader plate-boundary zone. Ongoing monitoring by regional seismic networks continues to track such episodes, providing data for refined hazard assessments in this populated agricultural and geothermal area.
References
- USGS Earthquake Catalog (earthquake.usgs.gov)
- California Geological Survey regional reports
- SeismoSight internal swarm classification records