Location:

Period:

22 Jul 2020 06:39:17 - 1 Aug 2020 01:55:54 (9 days 19 hours 16 minutes)

Volcanoes in 100km radius:

Stepovak Bay Group(88km), Kupreanof(94km)

Earthquakes:

135

2 swarms found nearby.

2020

PS20200722.1(38.1km)

22 Jul

15 hours

6 earthquakes

PS20201019.2(97.0km)

19 Oct

1 day 4 hours

12 earthquakes

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Seismic Swarm S20200723.1: Analysis of Activity Near Sand Point, Alaska

Seismic swarm S20200723.1 occurred in the Shumagin Islands region of Alaska, centered approximately 66 km east-southeast of Sand Point. The sequence began at 06:39 UTC on 22 July 2020 and concluded at 01:55 UTC on 1 August 2020, spanning 235 hours and 16 minutes. During this period, 135 earthquakes were recorded.

The first 100 events exhibited magnitudes between 1.4 and 5.0, with focal depths predominantly ranging from 13 km to 45 km. Notable events included a magnitude 5.0 earthquake at 11:08 UTC on 23 July at 44 km depth and a magnitude 4.5 event at 03:12 UTC on 24 July at 44 km depth. Shallower activity, such as events at 2–3 km depth, appeared sporadically but remained infrequent. The temporal distribution showed peak rates within the initial 48 hours, followed by a gradual decline, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

This swarm took place along the Alaska Peninsula, where the Pacific Plate subducts beneath the North American Plate at the Aleutian Trench. The Shumagin segment forms part of this convergent margin, characterized by oblique subduction and variable coupling along the interface. Historical seismicity in the area includes both megathrust events and intermediate-depth earthquakes within the Wadati-Benioff zone. Depths observed during the swarm align with typical activity in the overriding crust and upper portion of the subducting slab.

The broader tectonic setting features frequent moderate-to-large earthquakes due to ongoing plate convergence at rates of approximately 6–7 cm per year. Nearby volcanic centers, including those on the Alaska Peninsula, reflect the influence of slab dehydration and magma generation at depth. No significant surface deformation or volcanic unrest was associated with this particular sequence.

Since 1 January 2000, only one swarm has been identified in the region according to SeismoSight classification, occurring in 2020. This highlights the relative rarity of clustered seismic sequences compared with isolated events in the Shumagin Islands.

References
USGS Earthquake Catalog
Alaska Earthquake Center annual reports
SeismoSight internal swarm classification database