M5.9 Earthquake 58 km E of Port Alsworth, Alaska — December 21, 2021
2021-12-21 22:42:15 UTC (2021-12-21) · approx. 12:42 PM UTC-10 local
Felt by 1,888 people across a wide area. Maximum shaking intensity MMI 4.5 (moderate).
On December 21, 2021 at 22:42 UTC, a magnitude 5.9 intermediate depth earthquake struck 58 km E of Port Alsworth, Alaska, at a depth of 151.2 km and coordinates 60.1237°, -153.2742°. The earthquake was reported felt by 1,888 peopleacross a wide area, with a maximum shaking intensity of Modified Mercalli Intensity (MMI) 4.5 (moderate). The USGS PAGER system issued a green alert level for this event, indicating no significant casualties or damage expected. The nearest populated place is Diamond Ridge (population 1,156).
Physical scale: An earthquake of magnitude 5.9 releases seismic energy equivalent to roughly 11 kilotons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 7.1 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
Nearest Populated Places
What the Shaking Felt Like
At MMI 4.5 (moderate), people in the most strongly affected areas would have experienced: Mild shaking, similar to a passing truck. Hanging objects swing. Glasses and dishes may rattle. Not commonly felt outdoors.
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
Did You Feel It? — community-reported shaking
Aggregated felt-report intensity from 1,888 citizen responses to the USGS Did You Feel It? system. Each colored zone represents the average MMI from reports in that area.
Source: U.S. Geological Survey Did You Feel It?. View on USGS · submit your own report
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 | 145,871 |
| MMI 4 | light | 218,952 |
| MMI 5 | moderate | 371 |
Earthquake Details
-153.2742°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 5.9 earthquake near Port Alsworth, Alaska?
The magnitude 5.9 earthquake that struck 58 km E of Port Alsworth, Alaska on December 21, 2021 at 22:42 UTC had a depth of 151.2 km. It was felt by 1,888 people with maximum shaking intensity of MMI 4.5 (moderate). For context, this was a moderate earthquake that could be widely felt.
Where did the earthquake occur?
The earthquake epicenter was located at 60.1237°, -153.2742°, which is 58 km E of Port Alsworth, Alaska. The nearest populated place is Diamond Ridge (population 1,156). View all earthquakes in Alaska.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 5.9 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 5.9 event is significantly smaller than that historical record.
Authoritative source: USGS event page (event ID: ak021gbh4rso). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, December 21, 2021). M5.9 Earthquake 58 km E of Port Alsworth, Alaska — December 21, 2021. Retrieved December 22, 2025 from https://www.earthquaketracker.org/earthquakes/event/ak021gbh4rso/