Location:

Period:

13 Oct 2009 05:37:23 - 13 Oct 2009 20:34:29 (14 hours 57 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

16 swarms found nearby.

2007

S20070110.2(109.4km)

10 Jan

3 days 5 hours

108 earthquakes

S20070715.1(68.7km)

15 Jul

7 days 12 hours

182 earthquakes

PS20071227.1(94.0km)

26 Dec

17 hours

7 earthquakes

S20071227.1(92.1km)

26 Dec

4 days 1 hours

89 earthquakes

2009

S20091013.1(16.7km)

13 Oct

6 days 12 hours

187 earthquakes

S20091013.2(39.0km)

13 Oct

2 days 8 hours

41 earthquakes

S20091015.1(35.9km)

15 Oct

4 days 11 hours

82 earthquakes

2012

S20120912.1(92.2km)

11 Sep

11 hours

25 earthquakes

2015

PS20150727.1(167.0km)

27 Jul

3 hours

8 earthquakes

2016

S20160328.1(111.2km)

27 Mar

1 day 19 hours

33 earthquakes

2020

S20201201.2(84.1km)

1 Dec

3 days 9 hours

65 earthquakes

2022

PS20220111.1(71.7km)

11 Jan

3 hours

8 earthquakes

S20220111.1(68.7km)

11 Jan

4 days 15 hours

135 earthquakes

S20220111.2(42.5km)

11 Jan

2 days 1 hours

53 earthquakes

S20220122.1(68.3km)

22 Jan

3 days 22 hours

78 earthquakes

2024

S20241006.1(61.7km)

5 Oct

1 day 23 hours

28 earthquakes

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Seismic Swarm PS20091013.1: Analysis of Activity Near Nikolski, Alaska

On October 13, 2009, seismic swarm PS20091013.1 was recorded 138 km east-southeast of Nikolski, Alaska. The sequence initiated at 05:37 and concluded at 20:34, registering five earthquakes across a span of 14 hours and 57 minutes. This event cluster provides a focused example of short-duration seismic activity in a tectonically active subduction setting.

The sequence began with a magnitude 6.5 earthquake at 05:37:23 UTC, located at a depth of 24 km. Subsequent events included a magnitude 5.3 quake at 07:41:12 UTC (39 km depth), followed closely by a magnitude 5.0 event at 07:46:25 UTC (38 km depth). Later activity featured a magnitude 6.4 earthquake at 20:21:53 UTC (14 km depth) and a final magnitude 2.2 event at 20:34:29 UTC (13 km depth). These occurrences illustrate a pattern of initial larger shocks succeeded by moderate aftershocks that decreased in both magnitude and focal depth over the swarm duration.

Nikolski lies on Umnak Island within the Aleutian archipelago, a region shaped by ongoing convergence between the Pacific and North American plates. The Aleutian subduction zone generates the majority of Alaska’s seismicity, with the Pacific plate descending at rates of approximately 6–7 cm per year. This tectonic framework produces both shallow crustal events and deeper Wadati-Benioff zone earthquakes, contributing to the archipelago’s volcanic arcs and frequent seismic swarms. Historical records document major earthquakes in the Aleutians, including the 1957 magnitude 8.6 event and the 1964 magnitude 9.2 Great Alaska Earthquake, underscoring the zone’s capacity for high-magnitude releases.

Since January 1, 2000, four seismic swarms have been documented in the broader Nikolski area, with the earliest occurring in 2007. Swarm PS20091013.1 represents one of these episodes, highlighting episodic clustering rather than isolated mainshock-aftershock sequences. Such patterns often reflect fluid migration or stress redistribution along the subduction interface, though detailed source mechanisms require further instrumental analysis.

The 2009 swarm’s shallowing focal depths—from 39 km early in the sequence to 13 km by its end—suggest localized adjustments within the overriding plate or upper portions of the subducting slab. Combined with the two events exceeding magnitude 6, the sequence underscores the potential for moderate-to-large events within brief temporal windows in this segment of the Aleutian chain.

References

USGS Earthquake Catalog
Alaska Earthquake Center historical data
SeismoSight internal swarm classification records