Location:

Period:

6 Dec 2025 21:03:20 - 8 Dec 2025 15:19:41 (1 day 18 hours 16 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

6 swarms found nearby.

2014

S20140717.1(23.7km)

16 Jul

11 days 0 hours

524 earthquakes

2025

S20251206.1(16.8km)

6 Dec

21 days 22 hours

2086 earthquakes

S20251212.1(15.0km)

11 Dec

3 days 7 hours

62 earthquakes

2026

S20260101.1(8.1km)

1 Jan

5 days 17 hours

167 earthquakes

S20260115.3(19.2km)

15 Jan

1 day 9 hours

34 earthquakes

S20260125.2(13.6km)

24 Jan

6 days 20 hours

137 earthquakes

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

Seismic Swarm S20251207.1 Near Yakutat, Alaska

A seismic swarm designated S20251207.1 occurred approximately 117 km north of Yakutat, Alaska, from 21:03 on 6 December 2025 to 15:19 on 8 December 2025. Over 42 hours and 16 minutes, the sequence produced 52 earthquakes. The events clustered tightly in time and space, with magnitudes ranging from 1.9 to 4.6 and focal depths predominantly between 0 and 12 km. The largest event, magnitude 4.6, occurred at 16:56 on 7 December at a depth of 5 km. Several magnitude 3.5 events were also recorded, interspersed with numerous smaller shocks that maintained elevated activity throughout the swarm period.

The swarm region lies within the tectonically complex transition zone where the Yakutat terrane collides with the North American plate. This area experiences ongoing convergence along the Fairweather Fault system and the eastern edge of the Aleutian subduction zone. Shallow crustal seismicity is common due to the interaction of these plates, producing both strike-slip and thrust mechanisms at depths typically less than 15 km. The December 2025 swarm fits this pattern, with nearly all hypocenters confined to the upper 12 km of crust.

Yakutat and its surrounding region have long been recognized for elevated seismic hazard. The 1899 Yakutat Bay earthquakes, with magnitudes estimated above 8, remain among the largest recorded in the area and produced significant coastal deformation. Modern instrumental monitoring since the early 1970s has documented frequent moderate events, reflecting the persistent strain accumulation from plate motion rates of several centimeters per year. Swarm-type sequences, however, remain infrequent. Historical records since 2000 show only two prior swarms in the immediate vicinity: a single-event swarm in 2014 and the current 2025 sequence.

Analysis of the 52 events reveals a rapid onset followed by sustained activity over roughly two days. Early events on 6 December included a magnitude 4.2 shock at 5 km depth. Activity intensified on 7 December, with multiple magnitude 3+ events occurring within short intervals, including a magnitude 3.7 at 12:36 and a magnitude 4.6 later that afternoon. Depths remained shallow throughout, with occasional outliers reaching 12 km. By 8 December, rates declined sharply, terminating with a magnitude 2.6 event at 15:19. The temporal distribution indicates a classic swarm pattern driven by fluid migration or localized stress triggering rather than a single mainshock-aftershock sequence.

Such swarms provide valuable data for understanding fault interactions in the Yakutat collision zone. Continued monitoring by regional seismic networks will help assess whether this sequence signals broader changes in strain release or remains an isolated episode. Residents in coastal southeast Alaska are reminded that even moderate events in this setting can produce felt shaking and warrant preparedness for larger regional earthquakes.

References

USGS Earthquake Catalog
Alaska Earthquake Center
SeismoSight internal swarm classification S20251207.1