Seismic Swarm VS20080929.1: Analysis of Activity near Calipatria, California
The seismic swarm designated VS20080929.1 occurred 15 km west-northwest of Calipatria in California's Imperial Valley. It began at 05:17 on 29 September 2008 and concluded at 09:50 on 30 September 2008, encompassing 37 earthquakes over 28 hours and 33 minutes. This event exemplifies the recurrent swarm activity characteristic of the region, driven by transtensional tectonics along the Pacific-North American plate boundary.
The Imperial Valley lies within the Salton Trough, a pull-apart basin formed by the interaction of the San Andreas Fault system and the Brawley Seismic Zone. Shallow crustal extension and geothermal fluid migration facilitate frequent microseismicity at depths typically between 0 and 7 km. The swarm's events aligned with this pattern, with recorded depths ranging from 0 to 7 km and magnitudes predominantly below 2.0.
Sequence initiation featured a magnitude 1.1 event at 05:17 on 29 September, followed by a magnitude 1.7 at 07:34 and a magnitude 2.6 at 07:50. Subsequent activity included multiple events between 0.3 and 1.9 through the morning hours. A notable escalation occurred around 11:37 with a magnitude 2.9 earthquake at 3 km depth, the swarm's largest. Later events tapered, concluding with a magnitude 0.9 at 09:50 on 30 September.
This swarm fits into a broader historical context. Since 1 January 2000, 13 swarms have been documented in the area, distributed across years as follows: one each in 2000, 2001, 2002, and 2004; three each in 2003, 2005, and 2008. Such clustering reflects episodic strain release rather than isolated mainshock-aftershock sequences.
Geological monitoring in the Imperial Valley benefits from dense seismic networks that capture these low-magnitude sequences, aiding understanding of fault interactions and volcanic-adjacent processes near the Salton Sea. No significant damage or felt reports beyond minor shaking were associated with VS20080929.1, consistent with its modest energy release.
References
- SeismoSight internal swarm classification records
- USGS Earthquake Catalog for Imperial Valley events (2000–2008)