225 km SSE of Unalaska, Alaska

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 27 km (17 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 680 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.9 would have released about 32 times as much.

1. Mineral, Virginia 2011 M 5.8 22,000× the energy of this one
2. Magna, Utah 2020 M 5.7 16,000× the energy of this one
3. Around the size people start to feel M 3.0 1.4× the energy of this one
4. This earthquake M 2.9 about 680 lb of TNT

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 4.1 on 2018-12-23 — about 63 times the energy of this one.

Before 2016, the USGS record lists 2 earthquakes of M5 or more within 50 km:

1. 5.5 198 km SSE of Unalaska, Alaska · 36 km WNW of this epicentre Jun 5, 1981
2. 5.4 219 km SSE of Unalaska, Alaska · 11 km W of this epicentre Jun 18, 1973

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 52.1762, -164.7021. Map © OpenStreetMap contributors.

Aftershocks so far

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

1. 4.0 203 km SE of Unalaska, Alaska 19:45 UTC
2. 2.6 194 km SSE of Unalaska, Alaska 05:35 UTC
3. 2.6 176 km SE of Unalaska, Alaska 03:34 UTC
4. 2.7 182 km SSE of Unalaska, Alaska 19:43 UTC
5. 2.9 207 km SSE of Unalaska, Alaska 13:34 UTC
6. 2.4 200 km SSE of Unalaska, Alaska 08:34 UTC
7. 2.2 208 km SSE of Unalaska, Alaska 16:48 UTC
8. 2.2 230 km SSE of Unalaska, Alaska 06:52 UTC
9. 3.0 255 km SSE of Unalaska, Alaska 03:53 UTC
10. 2.2 197 km SSE of Unalaska, Alaska 13:42 UTC
11. 2.6 206 km SSE of Akutan, Alaska 12:03 UTC
12. 3.2 230 km SSE of Akutan, Alaska 08:53 UTC
13. 2.2 196 km SSE of Unalaska, Alaska 09:49 UTC
14. 2.3 192 km SSE of Unalaska, Alaska 05:30 UTC
15. 2.7 207 km SSE of Unalaska, Alaska 18:43 UTC

Counted from the QuakeIndex database, which ingests the USGS feed every 30 seconds · as of 18:55 UTC. QuakeIndex does not forecast aftershocks. When USGS issues an operational aftershock forecast it appears on the USGS event page.

On USGS

• The official USGS event page — the source of record for every value above
• Did you feel it? Report it to USGS — reports go to USGS, not to us
• ShakeMap for this event, when USGS has produced one

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