Location:

Period:

8 Aug 2020 12:41:41 - 12 Aug 2020 06:51:49 (3 days 18 hours 10 minutes)

Volcanoes in 100km radius:

Dutton(89km), Emmons Lake Volcanic Center(92km), Pavlof(93km), Pavlof Sister(96km)

Earthquakes:

7 swarms found nearby.

2007

PS20071002.1(43.8km)

2 Oct

2 hours

5 earthquakes

2009

S20090502.1(28.1km)

2 May

1 day 8 hours

54 earthquakes

2011

S20110717.1(2.8km)

16 Jul

2 days 17 hours

34 earthquakes

2020

S20200728.1(5.7km)

27 Jul

5 days 4 hours

111 earthquakes

S20200805.1(20.3km)

4 Aug

1 day 16 hours

45 earthquakes

PS20201019.2(106.1km)

19 Oct

1 day 4 hours

12 earthquakes

2023

S20230716.3(8.4km)

16 Jul

6 days 3 hours

65 earthquakes

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Seismic Swarm S20200809.1 Near King Cove, Alaska: Geological Context and Event Analysis

Seismic swarm S20200809.1 occurred approximately 81 km east-southeast of King Cove on the Alaska Peninsula. The sequence began at 12:41 UTC on 8 August 2020 and concluded at 06:51 UTC on 12 August 2020, spanning 90 hours and 10 minutes. During this period, 43 earthquakes were recorded, with magnitudes ranging from 1.2 to 5.5 and focal depths between 12 km and 45 km.

The swarm initiated with a magnitude 1.5 event at 21 km depth, followed shortly by the largest event—a magnitude 5.5 earthquake at 38 km depth. Subsequent activity included multiple events above magnitude 3.0, such as a magnitude 3.9 at 45 km depth and a magnitude 4.2 at 44 km depth. The majority of events clustered between 20 km and 35 km depth, consistent with intermediate-depth seismicity in the region. Activity declined steadily after 10 August, with only smaller-magnitude events recorded on the final day.

The Alaska Peninsula lies within the tectonically active Aleutian subduction zone, where the Pacific plate converges with and subducts beneath the North American plate at rates of approximately 6–7 cm per year. This setting produces frequent earthquakes across a wide range of depths, from shallow crustal events to deeper intraslab activity. King Cove and surrounding areas sit near the eastern end of the Shumagin Islands segment, a portion of the megathrust known for variable coupling and episodic slow-slip events.

Historical records document numerous significant earthquakes along this margin, including the 1938 magnitude 8.2 event and the 1946 magnitude 8.6 Unimak Island earthquake that generated a trans-Pacific tsunami. The 1912 Novarupta-Katmai eruption, while primarily volcanic, occurred within the same broad tectonic framework and was accompanied by intense seismicity. Modern instrumental monitoring has revealed that earthquake swarms are recurrent but relatively infrequent in this sector compared with the central Aleutians.

Since 2000, five swarms have been identified in the immediate vicinity, occurring in 2007, 2009, 2011, and twice in 2020. The 2020 sequence described here represents the second swarm of that year. These episodes typically feature moderate-magnitude events without a clear mainshock-aftershock pattern, suggesting fluid migration or stress triggering along pre-existing faults within the subducting slab.

Such swarms contribute to ongoing hazard assessment for nearby communities, including King Cove and the adjacent marine environment. Although no damage was reported from swarm S20200809.1, the magnitude 5.5 event underscores the potential for felt shaking and the need for continued real-time monitoring by regional seismic networks.

References

USGS Earthquake Catalog
Alaska Earthquake Center, University of Alaska Fairbanks
Alaska Division of Geological & Geophysical Surveys seismic reports