Location:

Period:

4 May 2002 04:58:52 - 6 May 2002 18:34:18 (2 days 13 hours 35 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

5 swarms found nearby.

2015

S20150129.1(14.4km)

28 Jan

20 hours

25 earthquakes

2021

S20211220.1(12.7km)

20 Dec

4 days 3 hours

113 earthquakes

2022

S20221220.1(25.4km)

20 Dec

6 days 3 hours

165 earthquakes

2024

S20241205.1(11.0km)

5 Dec

8 days 15 hours

194 earthquakes

S20241205.2(1.2km)

5 Dec

1 day 21 hours

108 earthquakes

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Seismic Swarm S20020504.1: Analysis of Earthquake Activity Near Ferndale, California

Seismic swarms represent clusters of earthquakes occurring in a localized area over a short period without a clear mainshock-aftershock sequence. Swarm S20020504.1, recorded 41 km southwest of Ferndale, California, exemplifies this phenomenon. The event sequence began at 04:58 on 4 May 2002 and concluded at 18:34 on 6 May 2002, spanning 61 hours and 35 minutes with a total of 44 earthquakes.

Magnitudes ranged from 1.1 to 4.4, with the peak event of magnitude 4.4 occurring on 4 May at 13:56:32 UTC at a depth of 6 km. Depths were predominantly shallow, concentrated between 3 km and 8 km, consistent with activity in the upper crust. The sequence initiated with events of magnitude 3.0–3.1, escalated through a series of magnitude 3.5–3.9 shocks within the first 12 hours, and gradually diminished, featuring mostly magnitude 2.0–3.2 events on subsequent days.

This distribution indicates a rapid onset followed by sustained low-to-moderate energy release, typical of fluid-driven or stress-triggered swarms rather than tectonic mainshock sequences. Shallow focal depths suggest involvement of near-surface fault structures accommodating distributed strain.

The Ferndale region lies within the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates converge. This tectonic setting produces complex faulting along the Mendocino Fracture Zone and the southern Cascadia Subduction Zone. Historical seismicity includes the 1992 Cape Mendocino earthquake (magnitude 7.2), which generated significant ground shaking and demonstrated the area's capacity for larger events. Ongoing monitoring by regional networks confirms persistent microseismicity linked to plate boundary deformation.

Updated geological assessments emphasize the role of the Gorda Plate deformation zone in generating frequent small-magnitude swarms. These events help delineate active fault segments and inform probabilistic seismic hazard models for northern California coastal communities.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Regional Fault Maps
Pacific Northwest Seismic Network Historical Data Reports