Seismic swarm S20011228.1 was recorded 18 km south-southeast of Tres Pinos in San Benito County, California. The sequence began at 21:14 on 28 December 2001 and concluded at 05:58 on 7 January 2002, spanning 224 hours and 44 minutes. During this interval, 171 earthquakes were detected.
The swarm initiated with a magnitude 4.6 event at 6 km depth. Subsequent activity included a rapid succession of smaller shocks, with the first 100 events showing magnitudes predominantly between 0.6 and 3.6. Depths clustered between 3 km and 6 km, although a few events reached 8 km or shallowed to 1–2 km. The largest follow-up shocks reached magnitude 3.6 on 29 December and magnitude 3.1 early on 30 December. Most events remained below magnitude 2.5 after the opening hours, consistent with swarm behavior in which energy release is distributed across numerous modest earthquakes rather than a single mainshock-aftershock sequence.
Tres Pinos lies within the central California Coast Ranges, where the San Andreas Fault system accommodates the majority of relative motion between the Pacific and North American plates. The town is situated southeast of Hollister, an area long recognized for aseismic fault creep along the Calaveras and San Andreas faults. Shallow crustal seismicity is common because the fault zone here transitions from locked to creeping segments, producing frequent microearthquakes and occasional swarms. Regional geology comprises Mesozoic Franciscan Complex rocks overlain by Cenozoic sedimentary units that have been deformed by ongoing transform tectonics.
Instrumental records since 2000 document only two swarms in this immediate vicinity prior to the close of 2001. One occurred in 2000 and another in 2001, indicating that clustered seismic activity of this type remains infrequent even in this tectonically active setting.
The spatial and temporal pattern of S20011228.1 aligns with known characteristics of fault-zone swarms in central California. Shallow focal depths and the absence of a dominant mainshock suggest distributed slip on minor fractures within the broader fault zone rather than rupture of a single large asperity. Such sequences contribute to strain release without generating damaging ground motions, although the initial magnitude 4.6 event was widely felt in nearby communities.
Continued monitoring by regional seismic networks remains essential for distinguishing isolated swarms from potential precursors to larger events along the San Andreas system.
SeismoSight internal swarm catalog
USGS Earthquake Catalog (ANSS Comprehensive Catalog)
USGS Quaternary Fault and Fold Database of the United States