QuakeIndex

Earthquake · reviewed by a seismologist

M 4.0mb

233 km SSE of Perryville, Alaska

Yes. A magnitude 4.0 earthquake was recorded 233 km SSE of Perryville, Alaska at 07:56 UTC on 2021-07-30, 24 km deep.

· 24 km deep · released about 15 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 4.0 (mb)
Depth
24 km below the surface
Felt
no reports yet
Measured shaking
intensity I (ShakeMap)
Nearest mapped fault
Alaska-Aleutian subduction zone, 8 km from its surface trace — near is not the same as on
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 1857 days ago

This was a light earthquake. It is usually felt clearly nearby and can rattle windows and dishes, but damage is uncommon.

At this magnitude and depth, people at rest would likely notice it out to about 94 km (58 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 15 tons of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 5.0 would have released about 32 times as much.

  1. Northridge, California 1994 M 6.7 11,000× the energy of this one
  2. Mineral, Virginia 2011 M 5.8 500× the energy of this one
  3. Magna, Utah 2020 M 5.7 350× the energy of this one
  4. This earthquake M 4.0 about 15 tons of TNT
  5. Around the size people start to feel M 3.0 this one released 32× 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 (31 mi) since our catalogue begins in 2016. No other earthquake of M4 or more has been recorded this close in that time.

Before 2016, the USGS record lists one earthquake of M5 or more within 50 km:

  1. 5.8 249 km SSE of Perryville, Alaska · 17 km SE 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 54.0435, -157.4792. Map © OpenStreetMap contributors.

Aftershocks so far

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

  1. 3.5 273 km SSE of Perryville, Alaska
  2. 2.7 258 km SSE of Perryville, Alaska
  3. 3.2 259 km SSE of Perryville, Alaska
  4. 2.8 249 km SSE of Perryville, Alaska
  5. 2.8 254 km SSE of Perryville, Alaska
  6. 3.3 253 km SSE of Perryville, Alaska
  7. 3.3 263 km SSE of Perryville, Alaska
  8. 3.5 210 km SE of Perryville, Alaska
  9. 3.1 198 km SSE of Perryville, Alaska
  10. 2.4 187 km SSE of Perryville, Alaska
  11. 2.7 238 km SSE of Perryville, Alaska
  12. 2.7 195 km SSE of Perryville, Alaska
  13. 2.8 226 km SE of Perryville, Alaska
  14. 3.1 229 km SE of Perryville, Alaska
  15. 2.3 191 km SSE of Perryville, 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.