Location:

Period:

31 Jan 2004 13:28:36 - 9 Feb 2004 22:15:21 (9 days 8 hours 46 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

199

22 swarms found nearby.

2003

S20031222.1(5.9km)

22 Dec

37 days 3 hours

2456 earthquakes

S20031222.2(5.8km)

22 Dec

15 days 17 hours

643 earthquakes

S20031223.1(22.5km)

22 Dec

1 day 0 hours

127 earthquakes

S20031224.1(4.8km)

23 Dec

2 days 19 hours

170 earthquakes

S20040101.1(26.3km)

31 Dec

2 days 2 hours

67 earthquakes

2004

S20040108.1(8.7km)

7 Jan

11 days 2 hours

217 earthquakes

S20040202.1(24.9km)

1 Feb

1 day 12 hours

34 earthquakes

S20040205.1(3.4km)

4 Feb

2 days 23 hours

57 earthquakes

S20040301.1(22.6km)

1 Mar

24 days 6 hours

484 earthquakes

S20040303.1(2.3km)

3 Mar

8 days 3 hours

172 earthquakes

S20040317.1(26.4km)

16 Mar

7 days 18 hours

281 earthquakes

S20040405.1(25.5km)

4 Apr

18 days 8 hours

226 earthquakes

S20040425.1(4.7km)

25 Apr

19 days 16 hours

216 earthquakes

S20040528.1(4.8km)

27 May

2 days 11 hours

49 earthquakes

S20040714.1(5.5km)

13 Jul

86 days 1 hours

888 earthquakes

S20040727.1(19.2km)

26 Jul

10 days 1 hours

140 earthquakes

S20041014.1(22.8km)

13 Oct

8 days 7 hours

97 earthquakes

2005

S20050427.1(13.7km)

26 Apr

3 days 20 hours

44 earthquakes

S20051002.1(25.5km)

1 Oct

10 days 17 hours

151 earthquakes

2006

S20060407.1(25.0km)

6 Apr

11 days 10 hours

151 earthquakes

2009

S20090620.2(11.6km)

20 Jun

4 days 17 hours

84 earthquakes

2025

S20251119.2(5.8km)

18 Nov

2 days 18 hours

43 earthquakes

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Seismic Swarm S20040201.1: Analysis Near Oak Shores, California

Seismic swarm S20040201.1 was recorded in the Central Coast region of California, centered 11 km west-southwest of Oak Shores. The sequence began at 13:28 on 31 January 2004 and concluded at 22:15 on 9 February 2004, spanning 224 hours and 46 minutes. A total of 199 earthquakes were registered during this period. The Oak Shores area lies within the tectonically active Coast Ranges province, where the Pacific and North American plates interact along the San Andreas Fault system. This setting produces frequent shallow crustal seismicity driven by right-lateral strike-slip motion and subsidiary faulting. Depths in the swarm ranged mainly from 0 to 9 km, consistent with the brittle upper crust typical of the Salinian block and adjacent Franciscan Complex terranes. The region has a well-documented history of both mainshock-aftershock sequences and swarm-like activity linked to fluid migration and fault creep. Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes spanned 0.7 to 2.8, with the largest events (2.7–2.8) occurring on 1 February. Depths clustered between 3 and 7 km for most shocks, though occasional events reached 0 km or 9 km. Timing showed an initial burst on 31 January followed by sustained elevated rates through 3 February, with many events separated by minutes to hours. This pattern indicates episodic energy release rather than a single triggering mainshock. Since 1 January 2000, six swarms have occurred in the immediate area. Prior activity included five swarms in 2003 and one in 2004, underscoring recurring swarm behavior in this segment of the fault network. The modest magnitudes and shallow depths observed align with the region’s established seismic character, where swarms often reflect distributed slip on secondary structures without producing significant surface rupture.

References

SeismoSight internal swarm classification database
USGS Earthquake Catalog and regional fault maps
California Geological Survey tectonic summaries