Location:

10 km SSW of Lake Nacimiento, California

Period:

20 Jun 2009 08:45:48 - 25 Jun 2009 02:02:18 (4 days 17 hours 16 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

22 swarms found nearby.

2003

S20031222.1(16.0km)

22 Dec

37 days 3 hours

2456 earthquakes

S20031222.2(14.2km)

22 Dec

15 days 17 hours

643 earthquakes

S20031223.1(10.9km)

22 Dec

1 day 0 hours

127 earthquakes

S20031224.1(14.2km)

23 Dec

2 days 19 hours

170 earthquakes

S20040101.1(14.8km)

31 Dec

2 days 2 hours

67 earthquakes

2004

S20040108.1(19.0km)

7 Jan

11 days 2 hours

217 earthquakes

S20040201.1(11.6km)

31 Jan

9 days 8 hours

199 earthquakes

S20040202.1(13.3km)

1 Feb

1 day 12 hours

34 earthquakes

S20040205.1(12.7km)

4 Feb

2 days 23 hours

57 earthquakes

S20040301.1(11.2km)

1 Mar

24 days 6 hours

484 earthquakes

S20040303.1(13.7km)

3 Mar

8 days 3 hours

172 earthquakes

S20040317.1(14.8km)

16 Mar

7 days 18 hours

281 earthquakes

S20040405.1(14.1km)

4 Apr

18 days 8 hours

226 earthquakes

S20040425.1(15.4km)

25 Apr

19 days 16 hours

216 earthquakes

S20040528.1(16.2km)

27 May

2 days 11 hours

49 earthquakes

S20040714.1(17.0km)

13 Jul

86 days 1 hours

888 earthquakes

S20040727.1(7.6km)

26 Jul

10 days 1 hours

140 earthquakes

S20041014.1(11.2km)

13 Oct

8 days 7 hours

97 earthquakes

2005

S20050427.1(24.1km)

26 Apr

3 days 20 hours

44 earthquakes

S20051002.1(14.1km)

1 Oct

10 days 17 hours

151 earthquakes

2006

S20060407.1(13.5km)

6 Apr

11 days 10 hours

151 earthquakes

2025

S20251119.2(17.3km)

18 Nov

2 days 18 hours

43 earthquakes

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Seismic Swarm S20090620.2: Earthquake Activity near Lake Nacimiento, California

The seismic swarm designated S20090620.2 was recorded in the region 10 km south-southwest of Lake Nacimiento, California. Activity commenced at 08:45 on 20 June 2009 and concluded at 02:02 on 25 June 2009. Over this 113-hour, 16-minute interval, a total of 84 earthquakes were registered.

The sequence displayed classic swarm characteristics, with events distributed across a narrow time window and no single dominant mainshock. Magnitudes ranged from 0.2 to 4.3, with the largest event (M4.3) occurring at 12:32:48 on 20 June at a depth of 5 km. A secondary peak reached M3.2 at 17:55:52 the same day, also at 5 km depth. The majority of hypocenters clustered between 5 km and 6 km, indicating a shallow crustal source consistent with local fault structures. Depths extended from 1 km to 9 km overall, though events deeper than 6 km remained infrequent.

This swarm fits within the established seismic history of the area. Since 1 January 2000, 21 swarms have been documented in the vicinity. Earlier episodes occurred in 2003 (5 events), 2004 (13 events), 2005 (2 events), and 2006 (1 event). Such recurrent swarms reflect ongoing tectonic strain release along secondary faults associated with the broader Pacific–North American plate boundary.

The Lake Nacimiento region lies within the Salinian Block of coastal California, where northwest-trending faults accommodate right-lateral shear. Shallow seismicity at depths of 3–9 km aligns with mapped structures in the area, including segments of the Rinconada Fault system. Historical records indicate that swarm-type sequences are common in this setting, often involving fluid migration or aseismic slip that triggers distributed small-magnitude events rather than large, isolated ruptures.

Analysis of the 2009 swarm shows rapid onset followed by a gradual decay in rate, with most energy released in the first 48 hours. Aftershock productivity remained low relative to the main events, reinforcing the swarm classification. Depths remained stable throughout, suggesting a consistent source volume.

References

SeismoSight internal swarm classification S20090620.2
USGS Earthquake Catalog (regional tectonic framework)