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Location:
89 km SSW of Sand Point, Alaska
Period:
27 Jul 2020 12:31:14 - 1 Aug 2020 17:03:12 (5 days 4 hours 31 minutes)
Volcanoes in 100km radius:
Dutton(85km), Emmons Lake Volcanic Center(87km), Pavlof(88km), Pavlof Sister(90km)
Earthquakes:
111
6 swarms found nearby.
2007
PS20071002.1(47.9km)
2 Oct
2 hours
5 earthquakes
2011
16 Jul
2 days 17 hours
34 earthquakes
2020
S20200805.1(25.8km)
4 Aug
1 day 16 hours
45 earthquakes
8 Aug
3 days 18 hours
43 earthquakes
PS20201019.2(109.9km)
19 Oct
1 day 4 hours
12 earthquakes
2023
S20230716.3(13.3km)
16 Jul
6 days 3 hours
65 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20200728.1: Analysis of Activity Near Sand Point, Alaska

Seismic swarm S20200728.1 occurred 89 km SSW of Sand Point, Alaska, within the tectonically active Aleutian subduction zone. This region marks the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting megathrust fault system generates frequent seismicity, including both large interplate earthquakes and smaller swarms associated with stress adjustments along the plate interface and overlying crust.

The swarm initiated at 12:31 UTC on 27 July 2020 and concluded at 17:03 UTC on 1 August 2020, spanning 124 hours and 31 minutes. During this interval, 111 earthquakes were recorded. Analysis of the first 100 events reveals a magnitude range from 0.9 to 6.1, with the majority below 2.0. Depths clustered between 6 km and 47 km, consistent with activity near the subduction interface and within the overriding plate.

Notable events included a magnitude 6.1 earthquake at 35 km depth on 28 July at 08:03 UTC, followed by a magnitude 4.0 at 45 km depth on 29 July and a magnitude 4.8 at 38 km depth later that day. Shallower events (under 15 km) occurred alongside deeper ones, indicating distributed faulting across multiple structural levels. The temporal pattern showed an initial burst on 27–28 July, a secondary peak around 29 July, and a gradual decline through 31 July.

This swarm represents the third documented episode in the area since 2000, following single swarms in 2007 and 2011. Such recurrent activity highlights episodic stress release along the Shumagin segment of the Aleutian arc, where locked and creeping patches coexist. Depths and magnitudes align with typical subduction-related sequences, where fluid migration or slow slip may trigger clustered events without producing a dominant mainshock-aftershock sequence.

Geological context underscores the area's elevated hazard. Historical great earthquakes, such as the 1938 M8.2 event and the 1946 M8.6 tsunami-generating quake, originated nearby. Modern instrumentation from networks including the Alaska Earthquake Center confirms ongoing microseismicity at rates exceeding 1,000 events annually within 200 km of Sand Point. Swarm S20200728.1 fits this background while illustrating short-term clustering that can precede or accompany larger ruptures.

Monitoring of such swarms aids in refining probabilistic forecasts for the region. Continued observation through 2024 shows no immediate escalation to great-earthquake thresholds, yet the persistent subduction dynamics warrant sustained vigilance.

References

USGS Earthquake Catalog
Alaska Earthquake Center Annual Reports
SeismoSight Internal Swarm Database