Location:

Period:

25 Apr 2004 00:33:45 - 14 May 2004 16:37:14 (19 days 16 hours 3 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

216

21 swarms found nearby.

2003

S20031222.1(1.6km)

22 Dec

37 days 3 hours

2456 earthquakes

S20031222.2(3.3km)

22 Dec

15 days 17 hours

643 earthquakes

S20031223.1(26.1km)

22 Dec

1 day 0 hours

127 earthquakes

S20031224.1(2.1km)

23 Dec

2 days 19 hours

170 earthquakes

S20040101.1(29.9km)

31 Dec

2 days 2 hours

67 earthquakes

2004

S20040108.1(4.0km)

7 Jan

11 days 2 hours

217 earthquakes

S20040201.1(4.7km)

31 Jan

9 days 8 hours

199 earthquakes

S20040202.1(28.6km)

1 Feb

1 day 12 hours

34 earthquakes

S20040205.1(2.8km)

4 Feb

2 days 23 hours

57 earthquakes

S20040301.1(26.0km)

1 Mar

24 days 6 hours

484 earthquakes

S20040303.1(2.6km)

3 Mar

8 days 3 hours

172 earthquakes

S20040405.1(28.8km)

4 Apr

18 days 8 hours

226 earthquakes

S20040528.1(2.0km)

27 May

2 days 11 hours

49 earthquakes

S20040714.1(4.6km)

13 Jul

86 days 1 hours

888 earthquakes

S20040727.1(22.8km)

26 Jul

10 days 1 hours

140 earthquakes

S20041014.1(26.6km)

13 Oct

8 days 7 hours

97 earthquakes

2005

S20050427.1(9.0km)

26 Apr

3 days 20 hours

44 earthquakes

S20051002.1(28.8km)

1 Oct

10 days 17 hours

151 earthquakes

2006

S20060407.1(28.9km)

6 Apr

11 days 10 hours

151 earthquakes

2009

S20090620.2(15.4km)

20 Jun

4 days 17 hours

84 earthquakes

2025

S20251119.2(3.1km)

18 Nov

2 days 18 hours

43 earthquakes

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Analysis of Earthquake Swarm S20040425.1 Near Lake Nacimiento, California

Earthquake swarm S20040425.1 occurred 14 km south of Lake Nacimiento in San Luis Obispo County, California. The sequence began at 00:33 on 25 April 2004 and concluded at 16:37 on 14 May 2004, spanning 472 hours and registering 216 events. This activity aligns with the broader tectonic setting of the central Coast Ranges, where the region experiences distributed deformation linked to the San Andreas Fault system and subsidiary structures such as the Nacimiento Fault zone. The underlying geology consists primarily of Mesozoic Franciscan Complex rocks, including mélange and coherent terranes that record subduction-related accretion, overlain by Cenozoic sedimentary units. Ongoing right-lateral shear and minor reverse faulting contribute to the observed seismicity in this area. Examination of the first 100 recorded events reveals predominantly low-magnitude earthquakes. Magnitudes ranged from 0.5 to 2.7, with the majority falling between 1.0 and 1.9. Depths were shallow, concentrated between 0 and 9 km, consistent with brittle failure in the upper crust. The initial event on 25 April at 00:33 reached magnitude 1.6 at 1 km depth. Subsequent activity showed clusters of events on 25–26 April, including a magnitude 2.7 shock at 6 km depth. Later events through 2 May maintained similar characteristics, with no clear migration pattern evident in the early sequence. These observations indicate a typical swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock cascade. Historical records indicate that twelve swarms have occurred in the vicinity since 1 January 2000. Five swarms took place in 2003 and seven in 2004, underscoring recurrent seismic unrest in this sector of the Coast Ranges. Such episodic activity reflects the region's moderate strain accumulation and the influence of local fault networks accommodating Pacific–North American plate motion.

References

USGS Earthquake Catalog California Geological Survey Regional Geologic Maps Southern California Earthquake Data Center Swarm Database