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Location:
28 km SSW of Scotia, California
Period:
28 Jan 2015 21:11:14 - 29 Jan 2015 17:27:07 (20 hours 15 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
25
5 swarms found nearby.
2002
S20020504.1(14.4km)
4 May
2 days 13 hours
44 earthquakes
2021
20 Dec
4 days 3 hours
113 earthquakes
2022
S20221220.1(28.5km)
20 Dec
6 days 3 hours
165 earthquakes
2024
S20241205.1(23.7km)
5 Dec
8 days 15 hours
194 earthquakes
S20241205.2(15.4km)
5 Dec
1 day 21 hours
108 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20150129.1 Near Scotia, California: Geological Context and Event Analysis

The region 28 km SSW of Scotia, California, lies within Humboldt County in the northern Coast Ranges, adjacent to the Mendocino Triple Junction. This tectonic setting marks the convergence of the Pacific, North American, and Juan de Fuca plates, producing complex fault interactions along the Mendocino Fracture Zone and the southern terminus of the Cascadia Subduction Zone. The area experiences elevated seismicity due to right-lateral strike-slip motion on the San Andreas Fault system transitioning offshore, combined with subduction-related stresses. Crustal depths in this zone typically range from 10 to 30 km, consistent with the observed event depths.

Historical records indicate recurrent seismic swarms in this portion of the Mendocino Triple Junction, often linked to fluid migration or stress triggering along minor faults. Since 2000, only one prior swarm has been documented in the immediate vicinity, occurring in 2002. These episodic clusters differ from mainshock-aftershock sequences by lacking a dominant primary rupture and instead featuring numerous events of comparable magnitude over short durations.

Swarm S20150129.1 began at 21:11 on 28 January 2015 and concluded at 17:27 on 29 January 2015, spanning 20 hours and 15 minutes. A total of 25 earthquakes were recorded, with magnitudes between 0.4 and 3.1 and focal depths from 10 to 31 km. The sequence initiated with the largest event (magnitude 3.1 at 21 km depth), followed rapidly by events of 2.5 and 2.9. Subsequent activity included a mix of magnitudes mostly below 2.5, with occasional peaks at 2.8 and 2.7. Depths remained concentrated around 20 km for the majority of events, though shallower (10–11 km) and deeper (28–31 km) outliers occurred midway through the swarm.

Temporal analysis reveals an initial high-rate phase in the first hour, with nine events, followed by a gradual decline. Later events on 29 January maintained similar depths but showed slightly lower average magnitudes. This pattern suggests a short-lived triggering mechanism, possibly aseismic slip or pore-pressure changes within the fractured crust near the triple junction, rather than progressive mainshock rupture.

Such swarms contribute to understanding regional strain accumulation without producing significant surface rupture or damage, given the modest magnitudes involved. Continued monitoring in this tectonically active corridor remains essential for refining hazard models associated with the Mendocino Triple Junction.

References

USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
NOAA National Centers for Environmental Information Seismicity Database