A seismic swarm designated S20081221.1 was recorded 11 km south of Tres Pinos in San Benito County, California. The sequence began at 01:33 on 21 December 2008 and concluded at 11:44 on 24 December 2008, spanning 82 hours and 10 minutes. During this period, 60 earthquakes were detected.
The swarm exhibited typical characteristics of clustered seismicity in the region, with events distributed across shallow depths. The largest event reached magnitude 4.0 at a depth of 6 km on 21 December at 17:35:36 UTC. Other notable events included magnitudes of 3.6, 3.5, 3.0, and 2.9, primarily occurring within the first two days. Depths ranged mainly from 4 km to 7 km, with the majority concentrated between 4 km and 6 km, indicating activity within the upper crustal layers.
Analysis of the provided event list reveals a rapid initial phase on 21 December, followed by a gradual decline in frequency and magnitude over subsequent days. Smaller events (magnitudes below 1.0) dominated the later stages, consistent with aftershock-like decay patterns observed in swarm sequences. No single mainshock dominated the activity; instead, multiple events above magnitude 2.0 occurred throughout the period.
The location lies within the broader San Andreas Fault system, specifically near the junction of the San Andreas and Calaveras faults. This tectonically active zone accommodates right-lateral strike-slip motion between the Pacific and North American plates. The area has a well-documented history of microseismicity and occasional earthquake swarms driven by fault interactions and fluid migration in the crust.
Historical records indicate 11 swarms have occurred in the vicinity since 1 January 2000. These include one swarm in 2000, two in 2001, one each in 2003, 2004, and 2005, three in 2006, and two in 2007. Such recurrent swarm activity underscores the persistent strain accumulation and release along local fault segments.
This swarm provides valuable data for understanding short-term seismic clustering in central California. Continued monitoring by regional networks supports ongoing assessment of fault behavior in this high-hazard area.