Analysis of Seismic Swarm S20100110.2 Off Ferndale, California

The seismic swarm designated S20100110.2 occurred 46 km west of Ferndale, California, in the offshore region of Humboldt County. It began at 00:27 on 10 January 2010 and concluded at 02:53 on 12 January 2010, spanning 50 hours and 25 minutes. During this period, 30 earthquakes were recorded, ranging in magnitude from 2.3 to 6.5 and at depths primarily between 11 km and 28 km.

The sequence initiated with a magnitude 6.5 event at 00:27:39 on 10 January at 28 km depth. Subsequent activity included multiple events above magnitude 3.0, such as a 4.4 quake at 02:21:39 and a 3.9 at 03:07:01, both on the same day. Later notable shocks comprised a magnitude 3.9 at 11:48:31 and a magnitude 4.0 at 06:44:37 on 11 January. The final recorded event was a magnitude 3.1 at 02:53:36 on 12 January at a shallower depth of 14 km. Depths showed a general trend of slight shallowing toward the swarm's end, with most events clustered between 20 km and 26 km initially.

This swarm reflects typical clustered seismicity in a tectonically complex zone. The Ferndale area lies near the Mendocino Triple Junction, where the Pacific, North American, and Gorda plates interact. The Gorda Plate's internal deformation and the Mendocino Fracture Zone contribute to frequent moderate earthquakes. Historical records document similar swarms and mainshocks, including the 1992 magnitude 7.2 Cape Mendocino event and earlier activity linked to the 1906 San Francisco earthquake's northern extent.

Ongoing monitoring by regional networks continues to track strain accumulation along these boundaries. Updated assessments confirm persistent seismic hazard in northern California due to the Cascadia subduction zone's influence and strike-slip faulting. Such swarms provide data for refining models of plate boundary behavior without indicating immediate larger rupture potential.

References

United States Geological Survey Earthquake Catalog California Geological Survey Regional Reports SeismoSight internal swarm classification S20100110.2