Location:

Period:

7 Apr 2009 20:12:55 - 10 Apr 2009 02:05:46 (2 days 5 hours 52 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

4 swarms found nearby.

2005

S20050216.1(21.2km)

16 Feb

12 hours

32 earthquakes

2018

S20181130.2(14.8km)

30 Nov

132 days 1 hours

9026 earthquakes

S20181201.1(28.5km)

30 Nov

4 days 17 hours

83 earthquakes

2020

S20201107.1(3.5km)

7 Nov

6 days 8 hours

150 earthquakes

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Seismic Swarm S20090408.1 Near Knik, Alaska: Characteristics and Regional Context

An earthquake swarm designated S20090408.1 was recorded 4 km north of Knik, Alaska, beginning at 20:12 on 7 April 2009 and concluding at 02:05 on 10 April 2009. Over 53 hours and 52 minutes, the sequence comprised 45 events. Magnitudes ranged from 0.5 to 4.6, with the largest shock occurring at the onset. Focal depths varied between 13 km and 45 km, indicating activity within the upper to mid-crustal levels.

The swarm exhibited a typical pattern of clustered seismicity without a single dominant mainshock-aftershock sequence. Initial events included the magnitude-4.6 shock at 33 km depth, followed rapidly by smaller events clustered around 37–45 km. Subsequent activity migrated to shallower depths, with several events recorded between 13 km and 27 km after the first 12 hours. Later phases on 8–10 April featured repeated events near 19–40 km, maintaining low-to-moderate magnitudes.

Knik lies within the Cook Inlet basin of south-central Alaska, a region shaped by ongoing convergence between the Pacific and North American plates. Subduction along the Aleutian megathrust drives regional deformation, producing both shallow crustal earthquakes and deeper intraslab events. The local crust is influenced by the Castle Mountain fault system and associated structures that accommodate lateral motion and shortening. Historical records document recurrent moderate seismicity in this sector, consistent with the broader tectonic regime of the Yakutat collision zone.

Since 1 January 2000, only one prior swarm has been identified in the immediate area, occurring in 2005. The 2009 sequence therefore represents a relatively infrequent clustered episode amid background seismicity. Depths recorded during the swarm align with known seismogenic zones in the overriding plate and upper portion of the subducting slab beneath Cook Inlet.

Analysis of the event distribution shows no clear migration direction, suggesting a diffuse triggering mechanism possibly linked to fluid migration or static stress changes along pre-existing fractures. The absence of surface rupture and the modest maximum magnitude are characteristic of swarm behavior rather than a classic foreshock-mainshock sequence.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Alaska Earthquake Center, University of Alaska Fairbanks
SeismoSight internal swarm classification records