Seismic Swarm S20220109.1 Recorded Near Palomar Observatory, California

An earthquake swarm designated S20220109.1 occurred approximately 3 km NNW of Palomar Observatory in San Diego County, California. The sequence began at 21:12 UTC on 8 January 2022 and concluded at 15:57 UTC on 9 January 2022, spanning 18 hours and 45 minutes. During this period, 37 earthquakes were registered, with magnitudes ranging from 0.2 to 3.2 and focal depths between 4 and 7 km.

The swarm exhibited a typical clustered pattern, with the largest event—a magnitude 3.2 earthquake—occurring early in the sequence at 23:46 UTC on 8 January at a depth of 4 km. Subsequent activity included several events above magnitude 2.0, such as a 2.1 at 23:51 UTC and a 2.2 at 03:56 UTC on 9 January, both at 5 km depth. The majority of events remained below magnitude 1.5, consistent with swarm behavior where no single mainshock dominates.

Palomar Mountain lies within the Peninsular Ranges province of southern California, a region shaped by ongoing tectonic interaction between the Pacific and North American plates. The area features granitic and metamorphic bedrock typical of the Peninsular Ranges batholith, emplaced during the Mesozoic era. Seismic activity here is driven by northwest-trending strike-slip faults that accommodate right-lateral shear along the plate boundary. Historical records indicate recurrent low-to-moderate magnitude seismicity, often manifesting as swarms rather than isolated large events.

Since 2000, 26 swarms have been documented in the immediate vicinity. Activity increased notably after 2017, with yearly counts of 4 swarms in 2017, 5 in 2018, 3 in 2019, 7 in 2020, and 4 in 2021. This recent uptick aligns with broader patterns of episodic swarm activity observed along secondary fault structures in the region.

The S20220109.1 swarm provides a clear example of such episodic clustering. Events were tightly grouped both temporally and spatially, with most occurring within a narrow depth band of 5–7 km. No damage or felt reports beyond instrumental detection were associated with the sequence, underscoring the low-hazard character of these small-magnitude swarms despite their frequency.

Continued monitoring of swarm occurrences remains essential for refining seismic hazard assessments in the Peninsular Ranges. The geological framework of active faulting and the observed increase in swarm frequency since the mid-2010s highlight the dynamic nature of crustal deformation in this sector of the Pacific–North American plate boundary.

References

SeismoSight internal swarm catalog (S20220109.1 parameters and historical statistics since 2000).
United States Geological Survey Earthquake Catalog (regional tectonic setting and Peninsular Ranges geology).