M4.7 Earthquake 128 km SSE of Perryville, Alaska — July 29, 2021
2021-07-29 21:00:04 UTC (2021-07-29) · approx. 10:00 AM UTC-11 local
On July 29, 2021 at 21:00 UTC, a magnitude 4.7 shallow crustal earthquake struck 128 km SSE of Perryville, Alaska, at a depth of 2.5 km and coordinates 54.9511°, -158.0184°. 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 4.7 releases seismic energy equivalent to roughly 169 metric tons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 1.1 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
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 |
|---|
Earthquake Details
-158.0184°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 4.7 earthquake near Perryville, Alaska?
The magnitude 4.7 earthquake that struck 128 km SSE of Perryville, Alaska on July 29, 2021 at 21:00 UTC had a depth of 2.5 km. For context, this was a light earthquake commonly felt near the epicenter.
Where did the earthquake occur?
The earthquake epicenter was located at 54.9511°, -158.0184°, which is 128 km SSE of Perryville, 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 4.7 earthquakes can continue for several days 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 4.7 event is significantly smaller than that historical record.
Authoritative source: USGS event page (event ID: ak0219nnczma). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, July 29, 2021). M4.7 Earthquake 128 km SSE of Perryville, Alaska — July 29, 2021. Retrieved October 3, 2021 from https://www.earthquaketracker.org/earthquakes/event/ak0219nnczma/