Alaska earthquakes

M5.5 earthquake 132 km WSW of Adak, Alaska

A magnitude 5.5 earthquake struck 132 km WSW of Adak, Alaska on Sun, Apr 21, 1996, 6:50 AM HADT (15:50 UTC), 28 km (17 mi) below the surface.

A larger quake followed: M7.9 77 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 (Mwc, centroid moment), moderate
Magnitude from: USGS National Earthquake Information Center
Local time: Sun, Apr 21, 1996, 6:50 AM HADT
UTC: 1996-04-21 15:50:04 UTC
Depth: 27.5 km (17.1 mi), shallow
Epicenter: 51.437° N, 178.420° W
Region: Alaska
Tsunami flag: No
Status: Reviewed by a seismologist

Magnitude estimates

Different scales measure different things and differ by a few tenths. The USGS marks one as preferred, and it is the one this page uses.

Mag.	Scale	Computed by	Notes
5.5	Mwc, centroid moment	USGS National Earthquake Information Center	Preferred · Reviewed
5.49	Mw, moment	ISC-GEM global catalog	Final · ±0.1
5.5	Mwc, centroid moment	Global CMT project	Preliminary
5.5	Mw, moment	Alaska Earthquake Center	Preliminary

What happened

The USGS’s account of the effects of this earthquake.

Felt (III) on Adak.

From the USGS event page for usp0007gep. U.S. Geological Survey text and figures are in the public domain.

Did you feel it?

No one has reported feeling this quake to the USGS.

Fault motion and ground failure

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

Faulting: Reverse (thrust)
Nodal plane 1: Strike 249°, dip 21°, rake 106°
Nodal plane 2: Strike 51°, dip 70°, rake 84°

How well it is located

Reviewed by a seismologist.

Depth from: CONSTRAINED_BY_DEPTH_PHASES

Nearby

Within 100 km, magnitude 2.5 and up: 128 in the 60 days after this one and 1 in the 30 days before it.

After

The first 60 of 128.

Before

3.9161 km WSW of Adak, AlaskaThu, Apr 4, 12:45 PM HAST · 11139 days ago · 33 km deep · 30 km away

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Sources

From the USGS Earthquake Hazards Program and its partner networks, event usp0007gep (also filed as gcmtb042196b, gcmt19960421155004, ak99655hf3h2, iscgem960083). Read Sun, Oct 4, 2026, 7:58 AM UTC.