Location:

Period:

16 Jul 2023 06:48:21 - 17 Jul 2023 17:52:32 (1 day 11 hours 4 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.2; 2023 Sand Point, Alaska Earthquake; (16 Jul 2023) (6km from the swarm center)

5 swarms found nearby.

2007

PS20071002.1(44.1km)

2 Oct

2 hours

5 earthquakes

2020

S20200805.1(28.8km)

4 Aug

1 day 16 hours

45 earthquakes

PS20201019.2(74.0km)

19 Oct

1 day 4 hours

12 earthquakes

2025

S20250716.4(26.7km)

16 Jul

14 days 21 hours

502 earthquakes

S20250717.2(20.1km)

16 Jul

3 days 10 hours

107 earthquakes

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Seismic Swarm S20230716.2 on the Alaska Peninsula: Geological Context and Event Analysis

The Alaska Peninsula lies within one of the most seismically active regions on Earth, situated along the convergent margin where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This subduction drives the formation of the Aleutian volcanic arc and generates frequent earthquakes, including megathrust events and intermediate-depth seismicity within the Wadati-Benioff zone. The peninsula's geology features a complex assemblage of accreted terranes, active volcanoes, and fault systems that accommodate both compressional and strike-slip deformation.

Seismic swarms in this setting often reflect fluid migration, stress transfer, or aftershock sequences following a mainshock. Swarm S20230716.2 began at 06:48 UTC on 16 July 2023 and concluded at 17:52 UTC on 17 July 2023, lasting 35 hours and 4 minutes. During this period, 30 earthquakes were recorded, with the sequence centered near Sand Point. The swarm initiated with a magnitude 7.2 event at 25 km depth, located only 6 km from the swarm centroid. This mainshock, known as the 2023 Sand Point earthquake, represents the largest event in the sequence and aligns with the region's history of producing M7+ earthquakes along the subduction interface.

Subsequent activity included multiple events clustered between magnitudes 2.2 and 4.5, with depths ranging from 5 km to 42 km. Notable aftershocks comprised a magnitude 4.5 at 42 km depth shortly after the mainshock, followed by a series of magnitude 3.0–4.2 events at depths of 7–40 km. Shallower activity (under 15 km) occurred intermittently, potentially indicating crustal adjustment or secondary faulting. The temporal distribution showed the highest rate of events in the first 12 hours, tapering thereafter, consistent with typical post-mainshock decay modified by swarm-like clustering.

Historical records since 2000 indicate only three prior swarms in the Alaska Peninsula region: one in 2007 and two in 2020. This low frequency underscores the episodic nature of swarm activity amid the dominant background of isolated subduction-related events. The 2023 sequence stands out due to its association with a magnitude 7.2 mainshock, highlighting the potential for swarms to accompany significant ruptures in this tectonic environment.

Overall, Swarm S20230716.2 provides insight into the dynamic stress regime of the Alaska Peninsula subduction zone, where mainshock-aftershock sequences can exhibit swarm characteristics driven by the interplay of plate convergence and local structural complexities.

References

USGS Earthquake Catalog (updated through 2023)
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification data