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Note:This page contains AI-generated content for informational and entertainment purposes only. It may contain inaccuracies. Raw event data is from USGS and EMSC. All statistics, lists, and derived information are generated by this site. Full disclaimerFound an error?
Location:
6 km NNE of Cayucos, California
Period:
7 Jan 2004 17:27:41 - 18 Jan 2004 20:07:03 (11 days 2 hours 39 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
217
15 swarms found nearby.
2003
22 Dec
37 days 3 hours
2456 earthquakes
22 Dec
15 days 17 hours
643 earthquakes
S20031223.1(29.4km)
22 Dec
1 day 0 hours
127 earthquakes
23 Dec
2 days 19 hours
170 earthquakes
2004
31 Jan
9 days 8 hours
199 earthquakes
4 Feb
2 days 23 hours
57 earthquakes
S20040301.1(29.2km)
1 Mar
24 days 6 hours
484 earthquakes
3 Mar
8 days 3 hours
172 earthquakes
25 Apr
19 days 16 hours
216 earthquakes
27 May
2 days 11 hours
49 earthquakes
13 Jul
86 days 1 hours
888 earthquakes
S20040727.1(26.1km)
26 Jul
10 days 1 hours
140 earthquakes
2005
26 Apr
3 days 20 hours
44 earthquakes
2009
S20090620.2(19.0km)
20 Jun
4 days 17 hours
84 earthquakes
2025
18 Nov
2 days 18 hours
43 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20040108.1 Near Cayucos, California

A notable earthquake swarm designated S20040108.1 occurred 6 km north-northeast of Cayucos in San Luis Obispo County, California. The sequence began at 17:27 on 7 January 2004 and concluded at 20:07 on 18 January 2004, spanning 266 hours and 39 minutes. During this period, 217 earthquakes were recorded, providing valuable data on local seismic patterns.

Analysis of the first 100 events reveals predominantly shallow focal depths between 0 and 8 km, with the majority clustered between 2 and 6 km. Magnitudes ranged from 0.8 to 3.5, indicating low-to-moderate energy release typical of swarm activity rather than a mainshock-aftershock sequence. The largest event, magnitude 3.5, occurred on 10 January at a depth of 2 km. Smaller events (magnitudes 1.0–2.0) dominated the catalog, comprising over 70 percent of the initial activity. Temporal distribution showed peak rates on 7–8 January and again around 10 January, with events occurring at irregular intervals throughout the swarm duration.

The Cayucos region lies within the central California coastal zone, part of the broader Pacific-North American plate boundary. This area experiences distributed deformation influenced by the San Andreas Fault system to the east and offshore structures such as the Hosgri Fault Zone. The local geology features a mix of Franciscan Complex basement rocks, sedimentary units of the Monterey and Paso Robles Formations, and Quaternary marine terraces. Active faulting in the vicinity includes northwest-trending strike-slip and reverse faults that accommodate regional transpression.

Seismic swarms have been documented in this portion of the coast since at least the early 2000s. Records indicate four such swarms occurred after 1 January 2000, with the initial episode taking place in 2003. These sequences often reflect fluid migration or aseismic slip along minor faults rather than large-scale tectonic loading. The 2004 swarm's shallow depths align with known seismogenic zones in the region, where brittle failure occurs within the upper crust.

Ongoing monitoring by regional networks continues to refine understanding of these episodic events and their relationship to larger structures. Such data contribute to improved hazard assessment for coastal communities between Morro Bay and Cambria.

References

USGS Earthquake Catalog California Geological Survey Regional Fault Maps Southern California Seismic Network Reports