M5.6 Earthquake 258 km NW of Attu Station, Alaska — December 29, 2021
2021-12-29 03:57:32 UTC (2021-12-29) · approx. 2:57 PM UTC+11 local
On December 29, 2021 at 03:57 UTC, a magnitude 5.6 shallow crustal earthquake struck 258 km NW of Attu Station, Alaska, at a depth of 10.0 km and coordinates 54.6220°, 170.6514°. The USGS PAGER system issued a green alert level for this event, indicating no significant casualties or damage expected.
Physical scale: An earthquake of magnitude 5.6 releases seismic energy equivalent to roughly 4 kilotons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 4.4 km — a useful intuition for the size of the slip patch on the fault.
The epicenter is located in Alaska, a region characterized by Pacific Plate subduction beneath the North American Plate, producing frequent megathrust and crustal earthquakes along the Aleutian arc. The nearest mapped fault system is the Aleutian megathrust. View all earthquakes in Alaska.
Learn more: Magnitude scale · Aftershock sequences
ShakeMap — predicted shaking intensity
Modeled ground-motion intensity contoured on the Modified Mercalli scale (MMI). Computed by the USGS from the moment-tensor solution and regional ground-motion prediction equations.
Source: U.S. Geological Survey ShakeMap. View interactive ShakeMap on USGS
Population exposure (USGS PAGER)
Estimated population that experienced each level of shaking. Based on the USGS PAGER rapid impact assessment, which combines ShakeMap output with global population grids.
| Shaking intensity (MMI) | Description | Population exposed |
|---|---|---|
| MMI 3 | weak | 4 |
Earthquake Details
170.6514°E
Technical Information
Common Questions
How strong was the magnitude 5.6 earthquake near Attu Station, Alaska?
The magnitude 5.6 earthquake that struck 258 km NW of Attu Station, Alaska on December 29, 2021 at 03:57 UTC had a depth of 10.0 km. For context, this was a moderate earthquake that could be widely felt.
Where did the earthquake occur?
The earthquake epicenter was located at 54.6220°, 170.6514°, which is 258 km NW of Attu Station, Alaska. View all earthquakes in Alaska.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 5.6 earthquakes can continue for days to weeks and gradually diminish over time.
Has Alaska had earthquakes this big before?
The largest recorded earthquake in Alaska was the M9.2 Great Alaska earthquake of March 27, 1964. Today's magnitude 5.6 event is significantly smaller than that historical record.
Authoritative source: USGS event page (event ID: us6000gg09). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, December 29, 2021). M5.6 Earthquake 258 km NW of Attu Station, Alaska — December 29, 2021. Retrieved December 22, 2025 from https://www.earthquaketracker.org/earthquakes/event/us6000gg09/