M5.6 Earthquake 181 km SE of Chignik, Alaska — September 16, 2021
2021-09-16 17:51:55 UTC (2021-09-16) · approx. 7:51 AM UTC-10 local
Felt by 2 people across nearby locations.
On September 16, 2021 at 17:51 UTC, a magnitude 5.6 shallow crustal earthquake struck 181 km SE of Chignik, Alaska, at a depth of 36.8 km and coordinates 55.0710°, -156.4993°. The earthquake was reported felt by 2 peopleacross nearby locations, with a maximum shaking intensity of Modified Mercalli Intensity (MMI) 3.3 (light). The USGS PAGER system issued a green alert level for this event, indicating no significant casualties or damage expected. The nearest populated place is Sand Point (population 976).
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
Nearest Populated Places
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 2 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 | 21 |
Earthquake Details
-156.4993°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 5.6 earthquake near Chignik, Alaska?
The magnitude 5.6 earthquake that struck 181 km SE of Chignik, Alaska on September 16, 2021 at 17:51 UTC had a depth of 36.8 km. It was felt by 2 people with maximum shaking intensity of MMI 3.3 (light). For context, this was a moderate earthquake that could be widely felt.
Where did the earthquake occur?
The earthquake epicenter was located at 55.0710°, -156.4993°, which is 181 km SE of Chignik, Alaska. The nearest populated place is Sand Point (population 976). 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: ak021bwmmasc). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, September 16, 2021). M5.6 Earthquake 181 km SE of Chignik, Alaska — September 16, 2021. Retrieved December 22, 2025 from https://www.earthquaketracker.org/earthquakes/event/ak021bwmmasc/