Location:

Period:

4 Aug 2020 10:53:20 - 6 Aug 2020 03:14:43 (1 day 16 hours 21 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

10 swarms found nearby.

2007

PS20071002.1(28.5km)

2 Oct

2 hours

5 earthquakes

S20071002.1(22.6km)

2 Oct

12 days 10 hours

292 earthquakes

S20071026.1(27.8km)

25 Oct

2 days 13 hours

46 earthquakes

2009

S20090502.1(21.3km)

2 May

1 day 8 hours

54 earthquakes

2011

S20110717.1(20.9km)

16 Jul

2 days 17 hours

34 earthquakes

2020

S20200728.1(25.8km)

27 Jul

5 days 4 hours

111 earthquakes

S20200809.1(20.3km)

8 Aug

3 days 18 hours

43 earthquakes

PS20201019.2(98.6km)

19 Oct

1 day 4 hours

12 earthquakes

2023

S20230716.2(28.8km)

16 Jul

1 day 11 hours

30 earthquakes

S20230716.3(16.8km)

16 Jul

6 days 3 hours

65 earthquakes

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Seismic Swarm S20200805.1: Analysis of Activity Near Sand Point, Alaska

A seismic swarm designated S20200805.1 was recorded 80 km south-southwest of Sand Point, Alaska, beginning at 10:53 UTC on 4 August 2020 and concluding at 03:14 UTC on 6 August 2020. Over this 40-hour and 21-minute interval, 45 earthquakes were detected. The events occurred within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates exceeding 6 cm per year, generating frequent seismic and volcanic activity along the arc.

The swarm exhibited a range of magnitudes from 1.3 to 5.2, with two events exceeding magnitude 5.0. The largest shock, magnitude 5.2, occurred at 12:47 UTC on 4 August at a depth of 35 km. A second notable event of magnitude 5.1 followed on 5 August at 16:17 UTC at a shallower depth of 6 km. Depths throughout the sequence varied between 6 km and 40 km, indicating activity across both the shallow megathrust interface and deeper portions of the subducting slab. Smaller events clustered at intermediate depths around 20–25 km, consistent with brittle failure in the overriding plate and upper slab.

Temporal distribution showed peak activity on 4 August, with 21 events recorded within the first 12 hours. Activity continued at a reduced rate on 5 August before tapering off early on 6 August. The sequence lacked a single dominant mainshock-aftershock pattern typical of isolated tectonic events, instead displaying the clustered, non-stationary characteristics of a swarm. Such swarms often reflect fluid migration, aseismic slip transients, or localized stress perturbations within the subduction interface.

This region lies within the Shumagin seismic gap, a segment of the Aleutian megathrust that has not experienced a great earthquake in the instrumental era. Historical records indicate six prior swarms in the vicinity since 2000, occurring in 2007 (three swarms), 2009, 2011, and 2020. These episodes underscore recurring episodic slip behavior along the plate boundary, which may modulate the accumulation of strain toward future large-magnitude ruptures.

The geological setting features complex faulting, including splay faults and forearc structures that accommodate oblique convergence. Earthquake depths extending to 40 km align with the expected geometry of the Wadati-Benioff zone beneath the Alaska Peninsula. Continued monitoring of swarm recurrence provides valuable constraints on the seismic cycle and potential precursory signals in this high-hazard corridor.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (aeic.alaska.edu)
SeismoSight internal swarm classification records