Seismic Swarm S20090614.1 Near Calipatria, California

The seismic swarm designated S20090614.1 occurred in the Imperial Valley of southern California, centered 13 km west-northwest of Calipatria. This sequence began at 12:06 on 13 June 2009 and concluded at 17:27 on 14 June 2009, spanning 29 hours and 20 minutes during which 53 earthquakes were recorded. Magnitudes ranged from 0.3 to 3.3, with the largest event reaching 3.3 at a depth of 4 km on 14 June at 05:00:40. Depths throughout the swarm remained shallow, predominantly between 0 and 7 km, consistent with activity in the upper crust of this tectonically active basin.

The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the Pacific and North American plates along the San Andreas Fault system. The region is characterized by right-lateral strike-slip faulting and includes the Brawley Seismic Zone, where clusters of small earthquakes are common. Geothermal activity associated with the nearby Salton Sea further contributes to elevated seismicity through fluid migration that can trigger swarm-like sequences. Historical records indicate that earthquake swarms have been a recurring feature here, reflecting the interplay between fault mechanics and hydrothermal processes.

Analysis of the 2009 swarm reveals two main phases of elevated activity. The initial phase on 13 June featured events clustered around 12:35 and a stronger burst near 19:15–19:28, including magnitudes of 2.7 and 2.3. Activity continued at lower rates overnight before intensifying again on 14 June between 04:00 and 05:19, culminating in the sequence’s largest shock of 3.3. Subsequent events through the afternoon tapered off by late evening. The temporal distribution shows typical swarm behavior, with no single mainshock dominating but rather a diffuse progression of comparable events.

Since 1 January 2000, 19 swarms have been documented in the area. Yearly occurrences include single swarms in 2000, 2001, 2002, and 2004; four in 2003; three in 2005; five in 2008; and three in 2009. These statistics underscore the persistent seismic character of the Imperial Valley, where swarm activity often serves as a background process rather than a precursor to larger events.

The shallow focal depths and moderate magnitudes observed align with the region’s known geology, where brittle failure occurs within the top few kilometers of sediment and crystalline basement. Such patterns support ongoing monitoring by regional seismic networks to better understand fluid-driven seismicity and its relation to the broader San Andreas system.

References

USGS Earthquake Catalog
California Geological Survey, Imperial Valley tectonic summaries
SCEC Community Fault Model documentation