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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:
12 km N of Cayucos, California
Period:
22 Dec 2003 19:29:39 - 7 Jan 2004 12:52:45 (15 days 17 hours 23 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
643
22 swarms found nearby.
2003
22 Dec
37 days 3 hours
2456 earthquakes
S20031223.1(24.4km)
22 Dec
1 day 0 hours
127 earthquakes
23 Dec
2 days 19 hours
170 earthquakes
S20040101.1(28.1km)
31 Dec
2 days 2 hours
67 earthquakes
2004
7 Jan
11 days 2 hours
217 earthquakes
31 Jan
9 days 8 hours
199 earthquakes
S20040202.1(27.0km)
1 Feb
1 day 12 hours
34 earthquakes
4 Feb
2 days 23 hours
57 earthquakes
S20040301.1(24.1km)
1 Mar
24 days 6 hours
484 earthquakes
3 Mar
8 days 3 hours
172 earthquakes
S20040317.1(28.6km)
16 Mar
7 days 18 hours
281 earthquakes
S20040405.1(26.9km)
4 Apr
18 days 8 hours
226 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(21.2km)
26 Jul
10 days 1 hours
140 earthquakes
S20041014.1(25.1km)
13 Oct
8 days 7 hours
97 earthquakes
2005
26 Apr
3 days 20 hours
44 earthquakes
S20051002.1(26.9km)
1 Oct
10 days 17 hours
151 earthquakes
2006
S20060407.1(27.5km)
6 Apr
11 days 10 hours
151 earthquakes
2009
S20090620.2(14.2km)
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?

The 2003 Cayucos Earthquake Swarm: Geological Context and Event Analysis

The Cayucos region in central California lies within a tectonically active zone shaped by the interaction between the Pacific and North American plates. This coastal area, approximately 12 km north of Cayucos, experiences shallow crustal seismicity influenced by the broader San Andreas Fault system and associated offshore structures such as the Hosgri Fault. The local geology features sedimentary basins and faulted Franciscan Complex rocks, which have recorded episodic seismic activity throughout the Holocene.

SeismoSight internal classification identifies Swarm S20031222.2 as the sole swarm recorded in the region since 1 January 2000. The sequence began at 19:29 on 22 December 2003 and concluded at 12:52 on 7 January 2004, spanning 377 hours and 23 minutes. A total of 643 earthquakes were registered during this period.

Analysis of the first 100 events reveals predominantly shallow focal depths between 0 and 13 km, consistent with the area's crustal structure. Magnitudes ranged from 0.8 to 4.2, with several events exceeding magnitude 3.0 in the initial hours. The sequence opened with a magnitude 3.8 shock at 4 km depth, followed rapidly by a magnitude 4.0 event at 3 km. Subsequent activity included a magnitude 4.2 event at 2 km depth within the first two hours, alongside numerous events of magnitude 2.0–3.2 clustered at depths of 3–8 km. Later events in the initial 100 showed a gradual decline in both frequency and peak magnitude while maintaining shallow depths.

This swarm reflects typical swarm behavior in central coastal California, where fluid migration or aseismic slip along minor faults can trigger dense clusters of small-to-moderate events without a clear mainshock-aftershock pattern. The concentration of activity near 4–8 km depth aligns with the brittle-ductile transition zone in this part of the plate boundary.

References

SeismoSight internal swarm classification S20031222.2
USGS Earthquake Catalog (regional tectonic framework)
California Geological Survey coastal fault maps