M3.9 Earthquake 82 km SW of Atka, Alaska — July 18, 2021
2021-07-18 20:30:13 UTC (2021-07-18) · approx. 8:30 AM UTC-12 local
On July 18, 2021 at 20:30 UTC, a magnitude 3.9 shallow crustal earthquake struck 82 km SW of Atka, Alaska, at a depth of 48.8 km and coordinates 51.7748°, -175.1859°.
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
-175.1859°E
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Common Questions
How strong was the magnitude 3.9 earthquake near Atka, Alaska?
The magnitude 3.9 earthquake that struck 82 km SW of Atka, Alaska on July 18, 2021 at 20:30 UTC had a depth of 48.8 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: us7000en94). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, July 18, 2021). M3.9 Earthquake 82 km SW of Atka, Alaska — July 18, 2021. Retrieved November 4, 2021 from https://www.earthquaketracker.org/earthquakes/event/us7000en94/