Location:

Period:

23 Jan 2018 09:31:40 - 24 Jan 2018 21:58:54 (1 day 12 hours 27 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.9; 261 km SE of Chiniak, Alaska; (23 Jan 2018) (29km from the swarm center)

11 swarms found nearby.

2009

PS20090516.1(173.5km)

15 May

23 hours

5 earthquakes

2018

S20180123.2(48.5km)

23 Jan

10 days 6 hours

191 earthquakes

S20180123.1(13.4km)

23 Jan

9 days 3 hours

140 earthquakes

S20180124.5(44.5km)

23 Jan

10 days 3 hours

109 earthquakes

S20180124.4(51.3km)

23 Jan

2 days 6 hours

50 earthquakes

S20180124.1(29.7km)

23 Jan

1 day 13 hours

59 earthquakes

S20180124.6(7.1km)

23 Jan

2 days 8 hours

55 earthquakes

S20180123.3(34.6km)

23 Jan

5 days 2 hours

78 earthquakes

S20180124.3(76.1km)

23 Jan

3 days 20 hours

85 earthquakes

S20180124.2(74.1km)

23 Jan

2 days 1 hours

36 earthquakes

S20180208.1(66.6km)

7 Feb

1 day 15 hours

32 earthquakes

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

Seismic Swarm PS20180123.1 in the Gulf of Alaska

The Gulf of Alaska lies within a tectonically active margin where the Pacific Plate converges with the North American Plate along the Aleutian subduction zone. This setting produces frequent seismic activity, including megathrust events and strike-slip faulting in the overriding plate and adjacent oceanic crust. The region has a well-documented history of large earthquakes, most notably the magnitude 9.2 Great Alaska Earthquake of 1964, which ruptured a substantial portion of the subduction interface and generated widespread tsunamis.

SeismoSight internal classification identifies Swarm PS20180123.1 as a distinct sequence that began at 09:31 on 23 January 2018 and concluded at 21:58 on 24 January 2018. Within this 36-hour, 27-minute window, nine earthquakes were recorded. The sequence initiated with a magnitude 7.9 event at 14 km depth, followed by eight additional shocks whose magnitudes ranged from 3.1 to 5.5 and depths between 10 km and 32 km. The largest aftershock reached magnitude 5.5.

Analysis of the temporal distribution shows the mainshock was succeeded within minutes by two magnitude 5.0 events, after which activity continued at moderate levels through the first day. A secondary cluster occurred near midnight on 23–24 January, featuring two magnitude 5.5 events separated by roughly 43 minutes. The final recorded event, magnitude 5.0, marked the official end of the swarm. Depths remained predominantly shallow, consistent with crustal faulting rather than deep subduction processes.

Historical records maintained by SeismoSight indicate that only one swarm has been identified in the Gulf of Alaska since 1 January 2000. The preceding swarm occurred in 2009. The 2018 sequence is therefore notable as a relatively rare clustered episode within an otherwise steady background of isolated events.

The magnitude 7.9 mainshock of 23 January 2018 was located 261 km southeast of Chiniak, Alaska, approximately 29 km from the swarm centroid. This event remains the strongest earthquake recorded in the immediate vicinity since 2000. Its occurrence within an oceanic crustal setting underscores the capacity for energetic strike-slip rupture even outside the primary subduction megathrust.

Ongoing monitoring by regional seismic networks continues to refine understanding of stress transfer and aftershock productivity in this portion of the plate boundary. The 2018 swarm provides a useful case study for examining how moderate-magnitude sequences can develop rapidly following a large crustal event in the Gulf of Alaska.

References

USGS Earthquake Catalog
Alaska Earthquake Center Annual Reports
SeismoSight Internal Database