Location:

Period:

30 Nov 2018 18:24:48 - 5 Dec 2018 11:31:29 (4 days 17 hours 6 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.1; 1 km SE of Point MacKenzie, Alaska; (30 Nov 2018) (10km from the swarm center)

5 swarms found nearby.

2002

S20020206.1(18.3km)

6 Feb

4 days 15 hours

76 earthquakes

2005

S20050216.1(16.9km)

16 Feb

12 hours

32 earthquakes

2009

S20090408.1(28.5km)

7 Apr

2 days 5 hours

45 earthquakes

2010

S20100920.1(17.8km)

20 Sep

2 days 17 hours

89 earthquakes

2018

S20181130.2(16.5km)

30 Nov

132 days 1 hours

9026 earthquakes

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Seismic Swarm S20181201.1 Near Point MacKenzie, Alaska

A notable seismic swarm occurred near Point MacKenzie, Alaska, beginning at 18:24 on 30 November 2018 and concluding at 11:31 on 5 December 2018. Over 113 hours and 6 minutes, 83 earthquakes were recorded, centered approximately 3 km south-southeast of Point MacKenzie. Magnitudes ranged from 1.0 to 4.5, with depths predominantly between 28 km and 60 km, indicating activity within the deeper crustal layers influenced by regional tectonics.

The sequence featured several events of moderate strength, including a magnitude 4.5 earthquake at 04:52 on 2 December at 51 km depth and a magnitude 3.8 event at 00:46 on 5 December at 36 km depth. Earlier peaks included magnitude 3.7 and 3.4 shocks on 1 and 3 December, respectively. Shallower events, such as one at 8 km depth on 3 December, were less common but contributed to the overall distribution. The temporal clustering showed heightened activity in the first 48 hours, followed by a gradual decline.

This swarm unfolded in a tectonically active zone where the Pacific Plate subducts beneath the North American Plate along the Aleutian megathrust. The Cook Inlet region, encompassing Point MacKenzie, experiences frequent seismicity due to this convergent boundary, with additional contributions from crustal faults and volcanic processes in the nearby arc. Depths observed in the swarm align with typical Wadati-Benioff zone seismicity in south-central Alaska.

Historically, five swarms have been documented in the area since 2000, occurring in 2002, 2005, 2009, 2010, and 2018. These episodes reflect episodic release of stress in a setting prone to both swarms and larger mainshock-aftershock sequences. On the same day the swarm initiated, a magnitude 7.1 earthquake struck 1 km southeast of Point MacKenzie, approximately 10 km from the swarm centroid, underscoring the interconnected nature of seismic events in this high-hazard environment.

Such swarms provide valuable data for understanding stress transfer and fault interactions in subduction settings. Monitoring continues to refine models of regional hazard, given Alaska's position on the Pacific Ring of Fire and its record of significant historical earthquakes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center historical records