M3.9 Earthquake 266 km ESE of Attu Station, Alaska — October 13, 2021
2021-10-13 20:56:27 UTC (2021-10-13) · approx. 8:56 AM UTC+12 local
On October 13, 2021 at 20:56 UTC, a magnitude 3.9 shallow crustal earthquake struck 266 km ESE of Attu Station, Alaska, at a depth of 35.0 km and coordinates 51.6559°, 176.5754°.
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.
This earthquake is part of an ongoing aftershock sequence following the magnitude 5.0 mainshock that occurred 16 days ago 236 km ESE of Attu Station, Alaska. Since the mainshock, the USGS has recorded 36 aftershocks in this area. Aftershock activity is expected to continue for days to weeks as stresses on surrounding faults redistribute.
Earthquake Details
176.5754°E
Technical Information
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Common Questions
How strong was the magnitude 3.9 earthquake near Attu Station, Alaska?
The magnitude 3.9 earthquake that struck 266 km ESE of Attu Station, Alaska on October 13, 2021 at 20:56 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.
Is this the same earthquake as the M5.0 one earlier?
No. This magnitude 3.9 earthquake is an aftershock of the larger magnitude 5.0 mainshock that occurred earlier 236 km ESE of Attu Station, Alaska. Aftershocks are smaller earthquakes triggered by stress changes from the mainshock.
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Understanding This Data
Magnitude 3.9 earthquakes are typically only recorded by instruments.
Authoritative source: USGS event page (event ID: us6000fucm). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, October 13, 2021). M3.9 Earthquake 266 km ESE of Attu Station, Alaska — October 13, 2021. Retrieved December 18, 2021 from https://www.earthquaketracker.org/earthquakes/event/us6000fucm/