Location:

Period:

22 Jul 2020 06:32:56 - 4 Aug 2020 23:57:22 (13 days 17 hours 24 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

332

7 swarms found nearby.

2007

PS20071002.1(87.8km)

2 Oct

2 hours

5 earthquakes

2011

S20110505.2(11.9km)

5 May

3 days 7 hours

50 earthquakes

2020

PS20200722.1(94.4km)

22 Jul

15 hours

6 earthquakes

S20200722.2(21.1km)

22 Jul

5 days 1 hours

126 earthquakes

PS20201019.2(70.8km)

19 Oct

1 day 4 hours

12 earthquakes

2025

S20250716.2(17.7km)

16 Jul

1 day 7 hours

103 earthquakes

S20250717.1(25.9km)

16 Jul

15 hours

59 earthquakes

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Earthquake Swarm S20200722.1: Seismic Activity Near Sand Point, Alaska

The Shumagin Islands region southeast of Sand Point, Alaska, lies within the Aleutian subduction zone where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes extending to depths exceeding 50 km. The area has a well-documented history of large megathrust events, such as the 1938 magnitude 8.2 earthquake and the 1946 magnitude 8.6 Unimak event, underscoring its potential for significant seismic hazards.

Swarm S20200722.1 was recorded beginning at 06:32 on 22 July 2020 and concluded at 23:57 on 4 August 2020. Over 329 hours and 24 minutes, 332 earthquakes were detected at a location 68 km south-southeast of Sand Point. Analysis of the first 100 events reveals a magnitude range from 1.1 to 4.1, with the largest shock occurring at 07:42 on 22 July at 52 km depth. Most events clustered between 15–30 km depth, though shallower activity as low as 1 km and isolated deeper events were also observed. The sequence exhibited a typical swarm pattern with no single dominant mainshock, instead featuring multiple events of comparable size distributed over the initial 48 hours before activity gradually declined.

Historical records indicate that earthquake swarms in this sector remain uncommon. Since 1 January 2000, only three such swarms have occurred in the immediate region: one in 2007, one in 2011, and the 2020 sequence. This low frequency suggests that swarm activity may represent episodic fluid migration or localized stress adjustments within the subduction interface rather than routine aftershock sequences.

The 2020 swarm’s spatial distribution aligns with the broader seismotectonic framework of the Shumagin gap, a segment of the subduction zone previously identified for its relative seismic quiescence between great earthquakes. Depths and magnitudes recorded during the swarm are consistent with both interplate and intraplate processes active in this portion of the Aleutian arc.

References

USGS Earthquake Catalog Alaska Earthquake Center SeismoSight internal swarm classification S20200722.1