191 km S of Adak, 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 25 km (16 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 5.4 on 2019-09-09 — about 5,600 times the energy of this one.

Before 2016, the USGS record lists 12 earthquakes of M5 or more within 50 km; the five largest:

1. 6.4 146 km S of Adak, Alaska · 46 km N of this epicentre Mar 10, 1957
2. 6.2 204 km S of Adak, Alaska · 13 km SSE of this epicentre Dec 9, 1918
3. 6.1 198 km S of Adak, Alaska · 8 km SW of this epicentre May 22, 1957
4. 6.1 189 km S of Adak, Alaska · 14 km WNW of this epicentre Jul 3, 1960
5. 6.0 194 km SSW of Adak, Alaska · 44 km W of this epicentre Nov 11, 1993

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 50.1546, -176.8283. Map © OpenStreetMap contributors.

Aftershocks so far

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

1. 3.1 222 km S of Adak, Alaska 20:58 UTC
2. 4.2 167 km S of Adak, Alaska 12:30 UTC
3. 2.6 200 km S of Adak, Alaska 09:24 UTC
4. 3.2 176 km S of Adak, Alaska 16:45 UTC
5. 2.2 156 km S of Adak, Alaska 00:31 UTC
6. 2.8 153 km S of Adak, Alaska 20:22 UTC
7. 2.7 156 km S of Adak, Alaska 02:26 UTC
8. 2.8 149 km S of Adak, Alaska 23:41 UTC
9. 2.7 143 km S of Adak, Alaska 03:25 UTC

Counted from the QuakeIndex database, which ingests the USGS feed every 30 seconds · as of 23:36 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.