M3.9 Earthquake 220 km WSW of Adak, Alaska — July 31, 2021
2021-07-31 21:52:20 UTC (2021-07-31) · approx. 9:52 AM UTC-12 local
On July 31, 2021 at 21:52 UTC, a magnitude 3.9 shallow crustal earthquake struck 220 km WSW of Adak, Alaska, at a depth of 35.0 km and coordinates 50.9969°, -179.4709°.
Physical scale: An earthquake of magnitude 3.9 releases seismic energy equivalent to roughly 11 metric tons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 296 m — a useful intuition for the size of the slip patch on the fault.
Earthquake Details
-179.4709°E
Technical Information
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Common Questions
How strong was the magnitude 3.9 earthquake near Adak, Alaska?
The magnitude 3.9 earthquake that struck 220 km WSW of Adak, Alaska on July 31, 2021 at 21:52 UTC had a depth of 35.0 km. For context, this was a minor earthquake typically detected only by instruments.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 3.9 earthquakes can continue for several days and gradually diminish over time.
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Understanding This Data
Magnitude 3.9 earthquakes are typically only recorded by instruments.
Authoritative source: USGS event page (event ID: us6000f12y). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, July 31, 2021). M3.9 Earthquake 220 km WSW of Adak, Alaska — July 31, 2021. Retrieved October 14, 2021 from https://www.earthquaketracker.org/earthquakes/event/us6000f12y/