Earthquake Swarm S20211223.1 Near Palomar Observatory

An earthquake swarm designated S20211223.1 occurred 3 km NNW of Palomar Observatory in San Diego County, California. The sequence began at 15:29 on 22 December 2021 and concluded at 17:16 on 23 December 2021, spanning 25 hours and 47 minutes. During this period, 46 earthquakes were recorded, with magnitudes ranging from 0.3 to 3.1 and focal depths primarily between 4 and 14 km.

The swarm exhibited typical characteristics of clustered seismic activity, with the majority of events occurring in two main pulses on 23 December. Early morning activity included multiple events near magnitude 2.0 at depths of 4–6 km, followed by a later peak that featured the largest event of magnitude 3.1 at 12:44. Depths remained shallow throughout, consistent with activity in the upper crust of the Peninsular Ranges.

Geologically, the Palomar region lies within the northern Peninsular Ranges, underlain by Mesozoic granitic rocks of the Peninsular Ranges Batholith. This terrain forms part of the broader Pacific–North American plate boundary, where right-lateral strike-slip faulting accommodates relative plate motion. The nearest major structure is the Elsinore Fault Zone, which trends northwest–southeast and has produced moderate earthquakes historically. The area experiences distributed microseismicity due to its position between the San Andreas and San Jacinto fault systems.

Seismic swarms have been documented in this locale since at least 2005. Records since 1 January 2000 show 24 distinct swarms, with increasing frequency in recent years: one each in 2005, 2014, and 2016; four in 2017; five in 2018; three in 2019; seven in 2020; and two in 2021. These episodes typically involve low-magnitude events at similar shallow depths and are interpreted as fluid-driven or aseismic-slip triggered sequences rather than foreshock–mainshock patterns.

The S20211223.1 swarm fits this regional pattern, with no reported damage or felt reports exceeding minor intensities. Such activity underscores the ongoing tectonic adjustment within the Peninsular Ranges and contributes to refined hazard assessments for nearby infrastructure, including the observatory itself.

References

USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
Southern California Earthquake Data Center Swarm Database