Seismic Swarm S20100214.1: Analysis of the February 2010 Event South of Redlands, California
A notable earthquake swarm designated S20100214.1 occurred 6 km south of Redlands, California, beginning at 17:03 UTC on 13 February 2010 and concluding at 20:27 UTC on 25 February 2010. Over 291 hours and 24 minutes, the sequence produced 213 earthquakes. The events clustered at shallow depths predominantly between 4 km and 16 km, with the majority occurring between 5 km and 8 km, consistent with activity along minor faults in the upper crust.
The swarm initiated with a magnitude 2.5 event at 17:03 on 13 February at 8 km depth. Early activity remained modest until a magnitude 4.1 earthquake struck at 21:39 the same day, also at 8 km depth. This event triggered an immediate aftershock sequence that included multiple events above magnitude 2.0 within the following hours. Subsequent days featured intermittent bursts, with notable peaks including a magnitude 3.1 on 17 February at 05:04 and a magnitude 3.3 later that evening at 21:35, both at 7 km depth. Magnitudes generally declined after the initial mainshock, though smaller events continued at a steady rate until the swarm tapered off.
Analysis of the first 100 recorded events reveals a typical swarm pattern: an initial energetic phase followed by decaying frequency and amplitude. Depths stayed remarkably consistent around 7–8 km for most shocks, suggesting a compact source volume. The largest events (magnitudes 2.5–4.1) occurred within the first 48 hours, after which activity shifted toward lower magnitudes below 2.0. This temporal distribution aligns with fluid-driven or aseismic slip mechanisms often observed in Southern California swarms.
The Redlands area lies within the complex fault network of the Inland Empire, part of the broader Pacific–North American plate boundary. The region experiences distributed deformation across the San Andreas Fault system to the north and the San Jacinto Fault zone to the south. Shallow crustal seismicity here frequently occurs on secondary faults and conjugate structures rather than the primary plate-boundary faults. Historical records indicate that earthquake swarms are infrequent but documented features of this tectonic setting.
Since 1 January 2000, only two prior swarms have been identified in the immediate vicinity: one in 2003 and another in 2005. The 2010 sequence therefore represents a relatively rare clustering phenomenon for the locale. Such swarms typically do not produce large mainshocks but can elevate local seismic hazard through increased ground shaking and the potential for triggered events on nearby faults.
This swarm provides valuable data for understanding short-term seismic clustering in a highly populated portion of Southern California. Continued monitoring by regional networks remains essential for distinguishing swarm behavior from foreshock sequences that might precede larger earthquakes.
References
- SeismoSight internal swarm classification records for S20100214.1
- USGS Earthquake Catalog (events near Redlands, CA, 2000–2010)
- Southern California Earthquake Data Center fault and seismicity maps