QuakeIndex

Earthquake · reviewed by a seismologist

M 3.9ml

140 km S of Unalaska, Alaska

Yes. A magnitude 3.9 earthquake was recorded 140 km S of Unalaska, Alaska, at 10:57 am AKDT on 9 October 2016 (18:57 UTC), 24 km deep.

· 24 km deep · released about 11 tons of TNT

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Distance rings around the epicentre, with the nearest towns at their true direction and later quakes as coloured dots. Not a shaking map — see the USGS ShakeMap for that.
When
Magnitude
M 3.9 (ml)
Depth
24 km below the surface
Felt
no reports yet
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 1199 days ago

This was a small earthquake. Shaking like this is often felt indoors nearby and very rarely causes damage.

At this magnitude and depth, people at rest would likely notice it out to about 86 km (53 mi) from the epicentre. Estimated from magnitude, depth and distance with the intensity equation USGS Did You Feel It? uses; felt reports, where there are any, are the measurement.

How big was it?

This earthquake released about 11 tons of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 4.9 would have released about 32 times as much.

  1. Northridge, California 1994 M 6.7 16,000× the energy of this one
  2. Mineral, Virginia 2011 M 5.8 710× the energy of this one
  3. Magna, Utah 2020 M 5.7 500× the energy of this one
  4. This earthquake M 3.9 about 11 tons of TNT
  5. Around the size people start to feel M 3.0 this one released 22× more

Energy from the Gutenberg–Richter relation (log10E = 1.5M + 4.8), rounded to two figures; bar length is magnitude, which is already a logarithm of energy. Magnitudes of past earthquakes from the USGS catalogue.

For this area

The largest earthquake within 50 km since 2016 was M 5.4 on 2016-07-26 — about 180 times the energy of this one.

Before 2016, the USGS record lists 38 earthquakes of M5 or more within 50 km; the five largest:

  1. 6.5 183 km S of Unalaska, Alaska · 44 km SSE of this epicentre
  2. 6.5 127 km E of Nikolski, Alaska · 36 km WNW of this epicentre
  3. 6.4 118 km E of Nikolski, Alaska · 44 km WNW of this epicentre
  4. 6.4 123 km ESE of Nikolski, Alaska · 41 km W of this epicentre
  5. 6.3 136 km ESE of Nikolski, Alaska · 26 km W of this epicentre

Since 2016: the QuakeIndex catalogue (US, M1.5+), built from the USGS FDSN event service. Before 2016: USGS ComCat, M5+ from 1900. Distances are between epicentres. This is the record, not a forecast.

Where

Epicentre 52.6139, -166.5159. Map © OpenStreetMap contributors.

Aftershocks so far

59 earthquakes have been recorded within 50 km since this one.

  1. 3.2 169 km ESE of Nikolski, Alaska
  2. 2.0 108 km S of Unalaska, Alaska
  3. 1.9 Fox Islands, Aleutian Islands, Alaska
  4. 2.5 145 km ESE of Nikolski, Alaska
  5. 2.3 155 km ESE of Nikolski, Alaska
  6. 1.8 131 km S of Unalaska, Alaska
  7. 2.5 Fox Islands, Aleutian Islands, Alaska
  8. 2.7 161 km ESE of Nikolski, Alaska
  9. 1.9 148 km S of Unalaska, Alaska
  10. 2.1 141 km S of Unalaska, Alaska
  11. 2.2 154 km ESE of Nikolski, Alaska
  12. 2.0 147 km ESE of Nikolski, Alaska
  13. 2.1 166 km ESE of Nikolski, Alaska
  14. 2.4 163 km ESE of Nikolski, Alaska
  15. 2.5 165 km ESE of Nikolski, Alaska

Counted from the QuakeIndex database, which ingests the USGS feed every 30 seconds · as of . QuakeIndex does not forecast aftershocks. When USGS issues an operational aftershock forecast it appears on the USGS event page.

On USGS

QuakeIndex does not issue warnings. For earthquake early warning see ShakeAlert.