Location:

Period:

14 Nov 2020 02:29:00 - 19 Nov 2020 13:53:21 (5 days 11 hours 24 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.6; 2020 Sand Point, Alaska Earthquake; (19 Oct 2020) (4km from the swarm center)

9 swarms found nearby.

2020

PS20200722.1(70.8km)

22 Jul

15 hours

6 earthquakes

PS20201019.2(32.0km)

19 Oct

1 day 4 hours

12 earthquakes

S20201020.1(13.8km)

19 Oct

22 days 19 hours

647 earthquakes

S20201020.2(12.2km)

19 Oct

5 days 0 hours

74 earthquakes

S20201021.2(14.4km)

20 Oct

2 days 3 hours

40 earthquakes

S20201210.1(21.0km)

9 Dec

1 day 13 hours

36 earthquakes

2025

S20250716.3(17.9km)

16 Jul

30 days 21 hours

1640 earthquakes

S20250717.3(20.0km)

16 Jul

7 days 2 hours

211 earthquakes

S20250728.1(10.9km)

27 Jul

2 days 4 hours

44 earthquakes

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Seismic Swarm S20201115.1 Near Sand Point, Alaska: Geological Context and Event Analysis

Seismic swarm S20201115.1 occurred approximately 109 km south-southeast of Sand Point, Alaska, within the tectonically active Aleutian subduction zone. The swarm initiated at 02:29 on 14 November 2020 and concluded at 13:53 on 19 November 2020, spanning 131 hours and 24 minutes. During this interval, 97 earthquakes were recorded, with magnitudes ranging from 1.8 to 4.3 and focal depths between 4 km and 35 km.

The swarm exhibited a typical pattern of clustered, low-to-moderate magnitude events without a single dominant mainshock. Notable activity included a magnitude 4.3 event at 21:14 on 15 November at 35 km depth and a magnitude 3.8 event at 12:45 on 19 November at 33 km depth. Multiple events exceeded magnitude 3.0, including several at depths around 20 km, consistent with activity along the plate interface or within the overriding crust.

This swarm followed the magnitude 7.6 Sand Point earthquake of 19 October 2020, whose epicenter lay only 4 km from the swarm centroid. The preceding mainshock likely altered local stress conditions, contributing to the subsequent swarm through afterslip or triggered seismicity. Since 1 January 2000, five swarms have been documented in the region, with this event representing the first recorded swarm in that timeframe.

The Sand Point area lies along the Alaska Peninsula segment of the Aleutian arc, where the Pacific plate subducts northwestward beneath the North American plate at rates of approximately 6–7 cm per year. This convergent margin produces frequent earthquakes across a range of depths, from shallow crustal events to deeper intraslab activity. The Shumagin Islands sector, immediately adjacent to the swarm location, is characterized by variable coupling along the megathrust, influencing both great earthquakes and swarm-type sequences.

Seismic swarms in subduction zones often reflect fluid migration, slow slip events, or stress diffusion following larger ruptures. Depths recorded during swarm S20201115.1 align with the expected range for the seismogenic zone in this portion of the arc. Continued monitoring remains essential given the proximity to populated coastal communities and the potential for larger triggered events.

References
USGS Earthquake Catalog (events 2000–2020)
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification S20201115.1