154 km SSE of Chignik, 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 42 km (26 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 1,900 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 4.2 would have released about 32 times as much.

1. Northridge, California 1994 M 6.7 180,000× the energy of this one
2. Mineral, Virginia 2011 M 5.8 7,900× the energy of this one
3. Magna, Utah 2020 M 5.7 5,600× the energy of this one
4. This earthquake M 3.2 about 1,900 lb of TNT
5. Around the size people start to feel M 3.0 this one released 2× 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 5.9 on 2021-07-29 — about 11,000 times the energy of this one.

Before 2016, the USGS record lists 17 earthquakes of M5 or more within 50 km; the five largest:

1. 7.0 164 km SE of Chignik, Alaska · 46 km ENE of this epicentre Sep 4, 1989
2. 6.7 121 km SE of Chignik, Alaska · 37 km N of this epicentre Feb 13, 1979
3. 6.2 143 km SE of Perryville, Alaska · 50 km WSW of this epicentre Mar 25, 1927
4. 6.2 189 km SE of Chignik, Alaska · 39 km ESE of this epicentre Mar 18, 1945
5. 6.0 168 km SE of Chignik, Alaska · 40 km ENE of this epicentre Nov 16, 1938

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 55.1271, -157.0717. Map © OpenStreetMap contributors.

Aftershocks so far

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

1. 3.6 154 km SE of Perryville, Alaska 12:04 UTC
2. 2.3 120 km SSE of Chignik, Alaska 11:24 UTC
3. 2.3 122 km SE of Chignik, Alaska 10:54 UTC
4. 1.9 139 km SE of Chignik, Alaska 02:51 UTC
5. 2.7 192 km SE of Perryville, Alaska 23:25 UTC
6. 2.2 148 km SE of Perryville, Alaska 21:55 UTC
7. 2.6 161 km SE of Chignik, Alaska 21:45 UTC
8. 2.2 141 km SE of Perryville, Alaska 20:22 UTC
9. 3.4 135 km SE of Perryville, Alaska 17:28 UTC
10. 2.7 157 km SE of Chignik, Alaska 06:46 UTC
11. 3.0 144 km SE of Perryville, Alaska 06:02 UTC
12. 2.2 150 km SE of Perryville, Alaska 05:15 UTC
13. 2.4 126 km SE of Chignik, Alaska 05:09 UTC
14. 2.5 south of Alaska 22:37 UTC
15. 3.3 156 km SSE of Chignik, Alaska 22:00 UTC

Counted from the QuakeIndex database, which ingests the USGS feed every 30 seconds · as of 15:36 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.