Seismic Swarm S20211220.1: Analysis of Activity Near Petrolia, California

Seismic swarm S20211220.1 was recorded beginning at 20:10 on 20 December 2021 and concluding at 23:37 on 24 December 2021. The events occurred 7 km north of Petrolia in Humboldt County, California, with a total of 113 earthquakes detected over 99 hours and 26 minutes.

Analysis of the initial events reveals a mainshock of magnitude 6.2 at 27 km depth at 20:10:31 on 20 December, followed by a rapid sequence of aftershocks predominantly in the magnitude 1.5–3.2 range. Depths clustered between 18 km and 31 km, with the majority occurring around 24–27 km. Notable subsequent events included magnitudes of 3.8 at 03:26:39 on 21 December and 3.7 at 04:34:19 on the same day, both at depths near 26–27 km. Activity showed a gradual decline after the first 24 hours, transitioning to lower-frequency events below magnitude 3.0.

The Petrolia region lies within the Mendocino Triple Junction, the onshore junction of the Pacific, North American, and Gorda plates. This tectonically complex area features the southern termination of the Cascadia Subduction Zone and the Mendocino Fracture Zone, producing frequent seismicity through a combination of thrust, strike-slip, and normal faulting. Crustal deformation rates remain high due to ongoing plate interactions, with historical records indicating recurrent moderate-to-large earthquakes.

Regional seismic history includes two prior swarms since 2000, occurring in 2002 and 2015. The 1992 Petrolia earthquake of magnitude 7.2 demonstrated the potential for significant ground shaking and associated hazards in the immediate vicinity. Current monitoring by the USGS and regional networks confirms ongoing microseismicity consistent with the area's tectonic setting.

References

USGS Earthquake Catalog California Geological Survey Regional Reports Mendocino Triple Junction Tectonic Studies (USGS Professional Papers)