Seismic Swarm S20211231.1 Near Palomar Observatory, California

A seismic swarm designated S20211231.1 was recorded 2 km NNW of Palomar Observatory in San Diego County, California. The sequence began at 11:00 on 30 December 2021 and concluded at 12:15 on 3 January 2022, spanning 97 hours and 14 minutes. During this interval, 63 earthquakes were detected, with magnitudes ranging from 0.2 to 2.9 and focal depths predominantly between 4 and 7 km.

The events clustered tightly in both space and time, exhibiting the characteristic features of a swarm rather than a mainshock-aftershock sequence. Magnitudes remained modest throughout, with only four events exceeding magnitude 2.0. The largest shock, magnitude 2.9, occurred at 09:33 on 31 December at a depth of 4 km. Subsequent activity showed a gradual decline in both rate and maximum magnitude, consistent with swarm behavior driven by fluid migration or aseismic slip along local fault structures.

Palomar Mountain lies within the northern Peninsular Ranges, underlain by Mesozoic granitic rocks of the Peninsular Ranges Batholith. The region experiences distributed deformation linked to the broader Pacific–North America plate boundary. Nearby active faults, including strands of the Elsinore and San Jacinto fault systems, accommodate right-lateral shear and produce both discrete earthquakes and episodic swarms. Depths of 4–7 km align with the brittle–ductile transition zone typical of this tectonic setting.

Historical records maintained by SeismoSight indicate that 25 swarms have occurred in the immediate vicinity since 1 January 2000. Yearly counts show an apparent increase in frequency after 2016: one swarm each in 2005, 2014, and 2016; four in 2017; five in 2018; three in 2019; seven in 2020; and three in 2021. This pattern may reflect improved detection thresholds or changes in local stress conditions, although the precise drivers remain under investigation.

The 2021–2022 swarm fits within the established recurrence pattern for the area. Most events nucleated at depths of 5–6 km, mirroring the depth distribution observed in prior swarms. The absence of any shock larger than magnitude 3.0 and the rapid decay of activity after the peak on 31 December are typical for this source zone.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the Palomar area. Such swarms provide valuable data on fault-zone properties and may serve as indicators of evolving crustal stress. No damage or felt reports were associated with S20211231.1, underscoring the low hazard posed by these small-magnitude sequences.

References

USGS Earthquake Hazards Program – regional fault maps and seismicity catalogs
California Geological Survey – Peninsular Ranges geologic framework
SeismoSight internal swarm classification database