Alaska earthquakes

M5.5 earthquake 185 km WSW of Adak, Alaska

A magnitude 5.5 earthquake struck 185 km WSW of Adak, Alaska on Thu, Sep 23, 2021, 10:02 PM HADT (07:02 UTC), 18 km (11 mi) below the surface.

A larger quake followed: M6.1 187 km WSW of Adak, Alaska. This one was a foreshock.

Map

Where this earthquake was located, with the others nearby. Select one for its page.

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The earthquake

Magnitude: 5.5 (mww), moderate
Local time: Thu, Sep 23, 2021, 10:02 PM HADT
UTC: 2021-09-24 07:02:29 UTC
Depth: 18.0 km (11.2 mi), shallow
Epicenter: 51.134° N, 179.022° W
Region: Alaska
Strongest shaking: V, moderate shaking
PAGER alert: Green
Tsunami flag: No
Status: Reviewed by a seismologist

Shaking

The USGS ShakeMap: shaking estimated from seismic stations and the quake’s size, on the Modified Mercalli intensity scale.

Strongest estimated: V, moderate shaking
Peak ground acceleration: 10.9% of gravity
Peak ground velocity: 7.8 cm/s
Map: Automatic

USGS ShakeMap: estimated shaking intensity around the epicenter — Estimated shaking intensity. Map: USGS ShakeMap.

Did you feel it?

No one has reported feeling this quake to the USGS.

Estimated impact

The USGS PAGER system estimates deaths and economic losses from the shaking and the population and buildings exposed to it.

Alert level: Green: little or no damage or loss of life expected

Deaths: estimated chance of each range
Range	Chance
Under 1	69%
1 to 10	29%
10 to 100	2%
100 to 1,000	Under 1%
1,000 to 10,000	Under 1%
10,000 to 100,000	Under 1%
100,000 to 10,000,000	Under 1%

Economic losses: estimated chance of each range
Range, million USD	Chance
Under 1	65%
1 to 10	30%
10 to 100	4%
100 to 1,000	Under 1%
1,000 to 10,000	Under 1%
10,000 to 100,000	Under 1%
100,000 to 10,000,000	Under 1%

Fault motion and ground failure

How the rock slipped, from the moment tensor, and the USGS estimate of landslides and liquefaction.

Faulting: Reverse (thrust)
Moment magnitude: 5.5 Mww
Nodal plane 1: Strike 267°, dip 27°, rake 112°
Nodal plane 2: Strike 62°, dip 65°, rake 79°

How well it is located

Reviewed by a seismologist.

Location uncertainty: About 3.7 km horizontally, 1.8 km in depth
Magnitude uncertainty: ±0.05
Depth from: From modeling of broad-band P waveforms
Station coverage gap: 56°
Stations used: 318

Nearby

Within 100 km, magnitude 2.5 and up: 54 in the 60 days after this one and 20 in the 30 days before it.

After

Before

Earthquakes today Alaska earthquakes Live radar here

Sources

From the USGS Earthquake Hazards Program and its partner networks, event us7000fe0g (also filed as usauto7000fe0g, ak021c9oo6tx, iscgem621109283). Read Fri, Sep 25, 2026, 10:21 PM UTC.