Location:

Period:

26 Oct 2020 03:34:41 - 5 Nov 2020 21:35:39 (10 days 18 hours)

Volcanoes in 100km radius:

None

Earthquakes:

233

7 swarms found nearby.

2007

PS20071002.1(103.7km)

2 Oct

2 hours

5 earthquakes

2020

PS20200722.1(103.9km)

22 Jul

15 hours

6 earthquakes

PS20201019.2(14.3km)

19 Oct

1 day 4 hours

12 earthquakes

S20201020.1(22.1km)

19 Oct

22 days 19 hours

647 earthquakes

S20201019.1(25.5km)

19 Oct

6 days 4 hours

164 earthquakes

S20201210.1(13.6km)

9 Dec

1 day 13 hours

36 earthquakes

2025

S20250717.3(22.0km)

16 Jul

7 days 2 hours

211 earthquakes

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Seismic Swarm S20201027.1: Analysis of Activity Near Sand Point, Alaska

A seismic swarm designated S20201027.1 was recorded southeast of Sand Point, Alaska, beginning at 03:34 UTC on 26 October 2020 and concluding at 21:35 UTC on 5 November 2020. Over 258 hours, the swarm produced 233 earthquakes. The events were centered approximately 108 km south-southeast of Sand Point on the Alaska Peninsula.

Analysis of the first 100 events reveals a typical swarm pattern dominated by low-to-moderate magnitudes. The largest event reached magnitude 4.6 at a depth of 28 km early in the sequence. Subsequent events clustered between magnitudes 1.5 and 3.9, with depths ranging from 4 km to 31 km. Most activity occurred between 15 km and 25 km depth, consistent with crustal faulting above the subducting slab. The initial 24 hours accounted for the highest rate of events above magnitude 3.0, after which activity transitioned to smaller events with occasional spikes, such as a magnitude 3.8 at 29 km depth on 27 October.

The swarm occurred 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 generates frequent earthquakes along the megathrust interface and overlying crustal faults. Sand Point lies near the Shumagin Islands segment, a region known for both deep intraslab events and shallower swarm activity linked to fluid migration or stress transfer.

Since 2000, five swarms have been identified in the broader area, with one occurring in 2007 and four recorded in 2020. This distribution underscores episodic clustering rather than steady background seismicity. Such swarms rarely produce damaging ground motion but provide valuable data on fault behavior in the transition zone between locked and creeping segments of the subduction interface.

Ongoing monitoring by regional seismic networks continues to track aftershock decay and any potential for larger triggered events. The 2020 activity aligns with historical patterns of swarm occurrence in this portion of the Alaska Peninsula, where volcanic and tectonic processes interact.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Alaska Earthquake Center, University of Alaska Fairbanks
SeismoSight internal swarm classification records