Location:

Period:

6 Feb 2002 17:18:45 - 11 Feb 2002 08:31:11 (4 days 15 hours 12 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

4 swarms found nearby.

2005

S20050216.1(9.6km)

16 Feb

12 hours

32 earthquakes

2010

S20100920.1(28.5km)

20 Sep

2 days 17 hours

89 earthquakes

2018

S20181130.2(27.1km)

30 Nov

132 days 1 hours

9026 earthquakes

S20181201.1(18.3km)

30 Nov

4 days 17 hours

83 earthquakes

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Earthquake Swarm S20020206.1: Seismic Activity Southeast of Anchorage, Alaska

Earthquake swarm S20020206.1 occurred 8 km southeast of Anchorage, Alaska, from 17:18 on 6 February 2002 to 08:31 on 11 February 2002. Over 111 hours and 12 minutes, the sequence included 76 events. The swarm began with two moderate shocks of magnitude 5.0 and 5.1 at depths of 36 km and 37 km, followed by numerous smaller events that declined in frequency and size.

The Anchorage region lies within the tectonically active Cook Inlet basin, part of the Aleutian subduction zone where the Pacific Plate converges with the North American Plate at approximately 6 cm per year. This convergence produces a well-defined Wadati-Benioff zone of intermediate-depth seismicity. Most events in the swarm occurred at depths of 30–38 km, consistent with slab-related seismicity beneath the overriding plate. A smaller number of shallower events appeared later in the sequence, including isolated shocks near 4 km and 20 km depth.

South-central Alaska experiences frequent seismic swarms due to both plate-boundary thrusting and crustal faulting. Historical records document major events such as the 1964 Mw 9.2 Great Alaska Earthquake, whose rupture extended through the Anchorage area and produced widespread damage. Smaller swarms have been recorded near the Castle Mountain Fault and along the Border Ranges Fault system, often reflecting stress transfer within the accretionary complex and forearc basin.

The 2002 swarm exhibited classic characteristics of a tectonic sequence: an initial pair of larger events followed by a decaying aftershock-like distribution. Magnitudes ranged from 0.3 to 5.1, with the majority below 2.0. Depths remained concentrated in the 30–40 km range for most of the 111-hour period, indicating activity within the subducting slab rather than shallow crustal faults.

No surface rupture or significant damage was associated with this swarm. The sequence concluded without escalation into a larger mainshock, consistent with many subduction-related swarms that release accumulated strain without producing destructive ground motion at the surface.

References

USGS Earthquake Catalog Alaska Earthquake Center annual reports Alaska Division of Geological & Geophysical Surveys tectonic summaries