Location:

Period:

19 Oct 2020 21:27:04 - 26 Oct 2020 01:43:43 (6 days 4 hours 16 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

164

4 swarms found nearby.

2020

PS20200722.1(96.6km)

22 Jul

15 hours

6 earthquakes

PS20201019.2(11.8km)

19 Oct

1 day 4 hours

12 earthquakes

S20201027.1(25.5km)

26 Oct

10 days 18 hours

233 earthquakes

S20201210.1(25.4km)

9 Dec

1 day 13 hours

36 earthquakes

AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20201019.1: Analysis of Activity Near Sand Point, Alaska

Seismic swarm S20201019.1 was recorded beginning at 21:27 on 19 October 2020 and concluding at 01:43 on 26 October 2020. The events occurred 131 km south-southeast of Sand Point, Alaska, with a total of 164 earthquakes registered over 148 hours and 16 minutes. This sequence represents the first swarm in the region since systematic monitoring began on 1 January 2000.

The initial 100 events displayed magnitudes primarily between 2.5 and 4.9, with the strongest reaching 4.9 at 00:00 on 20 October 2020. Depths ranged from 13 km to 33 km, clustering most frequently between 17 km and 29 km. Early activity featured several events above magnitude 4.0 within the first hours, including a 4.8 at 22:20 on 19 October and another 4.8 at 00:40 on 20 October. Subsequent events showed a gradual decline in peak magnitudes while maintaining a steady rate of smaller tremors through the afternoon of 20 October. Depths remained consistent in the mid-crustal range, consistent with tectonic release along the subduction interface.

Sand Point lies on Popof Island within the Shumagin Islands group of the Aleutian arc. The area sits above the convergent boundary where the Pacific plate subducts northwestward beneath the North American plate at rates of approximately 6–7 cm per year. This subduction drives the majority of regional seismicity, producing both shallow crustal events and deeper Wadati-Benioff zone activity. The swarm location aligns with the expected depth range for interface and intraslab earthquakes in this segment of the arc.

The Aleutian subduction zone has produced great earthquakes historically, including the 1938 M8.2 event whose rupture zone extends near the Shumagin Islands. Modern instrumentation has documented persistent background seismicity, with clusters often occurring along the plate interface or within the downgoing slab. The 2020 swarm fits within this pattern of episodic activity rather than precursory behavior preceding a larger mainshock.

No surface rupture or significant ground deformation was associated with the sequence. All recorded events remained below magnitudes capable of widespread damage, though felt reports were likely within the Shumagin Islands and on the Alaska Peninsula. The swarm concluded without transition to a classic aftershock sequence, reinforcing its classification as a discrete cluster.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification records