Seismic Swarm S20110715.2: Insights into the July 2011 Events Near Holtville, California
The Imperial Valley region of southern California lies within a tectonically active zone at the boundary between the Pacific and North American plates. This area features a series of strike-slip and normal faults that accommodate oblique extension, including the Brawley Seismic Zone and the Imperial Fault. These structures produce frequent low-to-moderate magnitude earthquake swarms rather than single large mainshock-aftershock sequences. Depths of seismicity typically range between 5 and 20 km, reflecting brittle failure within the upper crust above a ductile lower crust influenced by high heat flow from the nearby Salton Sea geothermal field.
Swarm S20110715.2 began at 04:34 UTC on 15 July 2011 and concluded at 08:14 UTC on 17 July 2011, lasting 51 hours and 40 minutes. The sequence was centered approximately 8 km west of Holtville, California, and comprised 36 events. Magnitudes ranged from 1.2 to 2.9, with the largest shock reaching 2.9 at 04:53 on 15 July and again at 07:39 on 16 July. Focal depths clustered between 10 and 18 km, averaging near 14 km. Activity showed two main peaks on 15 July, with 25 events recorded that day, followed by a sharp decline on 16 July and only two final events early on 17 July.
Event timing and size distribution indicate classic swarm behavior driven by fluid migration or aseismic slip transients rather than static stress transfer from a dominant mainshock. The first 12 hours contained the highest rate and largest magnitudes, after which activity decayed rapidly. No events exceeded magnitude 3.0, consistent with the swarm’s limited energy release and absence of surface rupture.
Historical records since 1 January 2000 document 14 comparable swarms in the same locale. These occurred in 2000 (1 swarm), 2003 (2), 2005 (1), 2008 (2), 2009 (4), and 2010 (4). The repeated occurrence underscores the persistent susceptibility of the Brawley Seismic Zone to swarm-type sequences, often linked to episodic creep on nearby faults and hydrothermal fluid movement.
In summary, Swarm S20110715.2 exemplifies the short-duration, low-magnitude seismic clusters typical of the Imperial Valley. Continued monitoring remains essential for understanding how such swarms relate to the broader strain budget along the southern San Andreas fault system.
References
- United States Geological Survey Earthquake Catalog
- California Geological Survey Regional Fault Maps
- SeismoSight internal swarm classification records