Earthquake Swarm S20090818.1: Analysis of Seismic Activity near Borrego Springs, California

An earthquake swarm designated S20090818.1 occurred approximately 12 km north-northwest of Borrego Springs in San Diego County, California. The sequence began at 11:30 on 17 August 2009 and concluded at 08:38 on 19 August 2009, spanning 45 hours and 8 minutes. During this period, 39 earthquakes were recorded, with magnitudes ranging from 0.2 to 2.6 and focal depths primarily between 2 km and 14 km.

The swarm exhibited a clustered pattern of low-to-moderate events, with the largest shock reaching magnitude 2.6 at 01:48 on 18 August at a depth of 10 km. Subsequent activity included multiple events of magnitude 1.0–1.7 clustered within the first few hours, followed by a gradual decline in both frequency and intensity. Depths remained consistent in the mid-crustal range for most events, though several shallower shocks occurred toward the end of the sequence on 19 August. This temporal distribution reflects typical swarm behavior, where energy release occurs through numerous small ruptures rather than a single dominant mainshock-aftershock sequence.

Borrego Springs lies within the Anza-Borrego Desert region, part of the broader San Andreas Fault system. The area is influenced by the San Jacinto Fault Zone, particularly the Coyote Creek Fault strand, which accommodates a significant portion of the right-lateral strike-slip motion between the Pacific and North American plates. The local geology features a complex network of faults cutting through sedimentary basins and crystalline basement rocks of Mesozoic age, overlain by Quaternary alluvium and desert sediments. This tectonic setting promotes frequent small-magnitude seismicity, including swarms, due to fluid migration and stress transfer along interconnected fault segments.

Historical records indicate that eight swarms have occurred in the region since 1 January 2000. Prior episodes took place in 2001 (one swarm), 2002 (two swarms), 2003 (one swarm), 2005 (two swarms), and 2009 (two swarms, including the present event). Such recurrent swarm activity underscores the ongoing strain accumulation and release characteristic of this portion of the southern California fault network.

The 2009 swarm provides insight into the shallow crustal dynamics near Borrego Springs, where events clustered at depths of 10–14 km suggest rupture within the seismogenic zone. The absence of a clear magnitude progression and the rapid onset of multiple comparable events highlight the swarm's diffuse nature, consistent with triggered slip on secondary faults rather than a primary through-going structure.

References

• SeismoSight internal swarm classification records
• USGS Earthquake Catalog (historical events near Borrego Springs)
• California Geological Survey fault zone maps (San Jacinto Fault Zone)