M4.0 Earthquake near Andreanof Islands, Aleutian Islands, Alaska — July 28, 2021
2021-07-28 17:31:13 UTC (2021-07-28) · approx. 5:31 AM UTC-12 local
On July 28, 2021 at 17:31 UTC, a magnitude 4.0 shallow crustal earthquake struck near Andreanof Islands, Aleutian Islands, Alaska, at a depth of 38.1 km and coordinates 51.2734°, -177.9335°.
Physical scale: An earthquake of magnitude 4.0 releases seismic energy equivalent to roughly 15 metric tons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 347 m — a useful intuition for the size of the slip patch on the fault.
This earthquake is part of an ongoing aftershock sequence following the magnitude 5.3 mainshock that occurred 14 days ago 151 km WSW of Adak, Alaska. Since the mainshock, the USGS has recorded 29 aftershocks in this area. Aftershock activity is expected to continue for days to weeks as stresses on surrounding faults redistribute.
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
Earthquake Details
-177.9335°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 4.0 earthquake near Andreanof Islands, Aleutian Islands, Alaska?
The magnitude 4.0 earthquake that struck near Andreanof Islands, Aleutian Islands, Alaska on July 28, 2021 at 17:31 UTC had a depth of 38.1 km. For context, this was a light earthquake commonly felt near the epicenter.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 4.0 earthquakes can continue for several days and gradually diminish over time.
Is this the same earthquake as the M5.3 one earlier?
No. This magnitude 4.0 earthquake is an aftershock of the larger magnitude 5.3 mainshock that occurred earlier 151 km WSW of Adak, Alaska. Aftershocks are smaller earthquakes triggered by stress changes from the mainshock.
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Understanding This Data
Magnitude 4.0 earthquakes are often felt but rarely cause damage.
Authoritative source: USGS event page (event ID: us6000ezwr). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, July 28, 2021). M4.0 Earthquake near Andreanof Islands, Aleutian Islands, Alaska — July 28, 2021. Retrieved October 3, 2021 from https://www.earthquaketracker.org/earthquakes/event/us6000ezwr/