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Location:
76 km S of Sand Point, Alaska
Period:
16 Jul 2025 20:49:24 - 17 Jul 2025 12:23:56 (15 hours 34 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
59
M 7.0+:
M 7.3; 2025 Sand Point, Alaska Earthquake; (16 Jul 2025) (5km from the swarm center)
10 swarms found nearby.
2007
PS20071002.1(75.4km)
2 Oct
2 hours
5 earthquakes
2020
PS20200722.1(106.7km)
22 Jul
15 hours
6 earthquakes
S20200722.1(25.9km)
22 Jul
13 days 17 hours
332 earthquakes
PS20201019.2(54.3km)
19 Oct
1 day 4 hours
12 earthquakes
2025
S20250716.3(26.6km)
16 Jul
30 days 21 hours
1640 earthquakes
S20250716.4(13.9km)
16 Jul
14 days 21 hours
502 earthquakes
S20250717.2(17.0km)
16 Jul
3 days 10 hours
107 earthquakes
S20250716.2(13.0km)
16 Jul
1 day 7 hours
103 earthquakes
S20250717.3(28.2km)
16 Jul
7 days 2 hours
211 earthquakes
23 Jul
3 days 6 hours
60 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20250717.1: Analysis of Activity South of Sand Point, Alaska

Seismic swarm S20250717.1 occurred 76 km south of Sand Point, Alaska, in the Shumagin Islands region of the Aleutian arc. The sequence began at 20:49 UTC on 16 July 2025 and concluded at 12:23 UTC on 17 July 2025, spanning 15 hours and 34 minutes. During this interval, 59 earthquakes were recorded, with magnitudes ranging from 1.5 to 4.2 and focal depths between 1 km and 35 km.

The swarm initiated with several events above magnitude 3.0 within the first hour, including a magnitude 4.0 earthquake at 21:10 and a magnitude 4.2 event at 21:39. Subsequent activity featured a mix of moderate and smaller events, with the largest after the initial phase being magnitude 3.1 at 11:41 on 17 July. Depths clustered primarily between 10 km and 25 km, consistent with crustal seismicity above the subducting slab.

This swarm followed closely after the magnitude 7.3 Sand Point earthquake on 16 July 2025, centered only 5 km from the swarm's core location. The mainshock likely altered local stress conditions, promoting the subsequent clustered seismicity through afterslip or fluid migration along pre-existing faults.

The Shumagin Islands lie within the Aleutian subduction zone, where the Pacific plate converges with the North American plate at approximately 6–7 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes and occasional swarms driven by slab dehydration or stress transfer. Historical records since 2000 show eight prior swarms in the broader area: one in 2007, three in 2020, and four earlier in 2025. These episodes typically last hours to days and rarely exceed magnitude 5.0, distinguishing them from mainshock-aftershock sequences.

Swarm S20250717.1 exhibited classic characteristics of subduction-related swarm activity, with rapid onset, variable depths, and no single dominant event after the first few hours. Such patterns aid in monitoring potential precursory signals or post-seismic relaxation following great earthquakes in the region.

References
USGS Earthquake Hazards Program – Aleutian Arc Tectonic Summary
Alaska Earthquake Center – Regional Seismicity Reports
SeismoSight internal swarm catalog S20250717.1