Location:

Period:

16 Jul 2025 20:42:15 - 31 Jul 2025 18:04:52 (14 days 21 hours 22 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

502

M 7.0+:

M 7.3; 2025 Sand Point, Alaska Earthquake; (16 Jul 2025) (8km from the swarm center)

8 swarms found nearby.

2007

PS20071002.1(62.0km)

2 Oct

2 hours

5 earthquakes

2020

S20200722.2(28.9km)

22 Jul

5 days 1 hours

126 earthquakes

PS20201019.2(66.0km)

19 Oct

1 day 4 hours

12 earthquakes

2023

S20230716.2(26.7km)

16 Jul

1 day 11 hours

30 earthquakes

2025

S20250717.2(12.8km)

16 Jul

3 days 10 hours

107 earthquakes

S20250716.2(24.9km)

16 Jul

1 day 7 hours

103 earthquakes

S20250717.1(13.9km)

16 Jul

15 hours

59 earthquakes

S20250724.1(14.7km)

23 Jul

3 days 6 hours

60 earthquakes

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Seismic Swarm S20250716.4: Analysis of Activity Near Sand Point, Alaska

A seismic swarm designated S20250716.4 was recorded 106 km south-southeast of Sand Point, Alaska, beginning at 20:42 UTC on 16 July 2025 and concluding at 18:04 UTC on 31 July 2025. Over 357 hours and 22 minutes, the sequence produced 502 earthquakes. Analysis of the first 100 events reveals a rapid onset with an initial magnitude 5.4 earthquake at 43 km depth, followed by a dense cluster of smaller events. Magnitudes ranged from 1.5 to 5.4, with multiple events exceeding magnitude 4.0, including additional shocks of 4.4, 5.2, and further 4.2 events. Depths predominantly clustered between 10 km and 40 km, with frequent occurrences near 35 km, consistent with activity along the subduction interface.

The temporal pattern shows the highest rate of events in the first 12 hours, featuring repeated magnitude 3+ shocks interspersed with lower-magnitude aftershocks. Notable sequences include paired 4.4 events within minutes on 16 July and a magnitude 5.2 event early on 17 July at 35 km depth. Later events in the initial 100 maintained a steady but declining frequency, with many registering between 2.0 and 3.5. This distribution indicates a classic swarm progression driven by fluid migration or stress transfer rather than a single mainshock-aftershock decay.

The Sand Point region lies 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 generates the majority of Alaska’s seismicity through thrust faulting at the plate interface and within the overlying crust. Historical records since 2000 document only four prior swarms in the immediate area: one in 2007, two in 2020, and one in 2023. These episodes typically involved hundreds of events over days to weeks but rarely exceeded magnitude 5.0.

On 16 July 2025, a magnitude 7.3 earthquake struck approximately 8 km from the swarm centroid, providing a likely trigger through static and dynamic stress changes. The subsequent swarm occupied a similar depth range to the mainshock aftershock zone, suggesting reactivation of nearby fault segments. Such sequences are common in subduction environments and contribute to long-term strain release along the megathrust.

Seismic monitoring in the Aleutian arc relies on regional networks that detect events down to magnitude 1.5, enabling detailed characterization of swarm evolution. Depths in the 20–40 km range align with the expected location of the plate interface beneath the outer shelf. Continued observation of this swarm will refine models of stress interaction between great earthquakes and smaller clustered activity in the region.

References

SeismoSight internal swarm catalog S20250716.4
Alaska Earthquake Center regional seismicity database
USGS Earthquake Hazards Program tectonic summaries for the Aleutian arc