QuakeIndex

Earthquake · reviewed by a seismologist

M 5.5mww

140 km SE of Chignik, Alaska

Yes. A magnitude 5.5 earthquake was recorded 140 km SE of Chignik, Alaska at 03:52 UTC on 2021-07-30, 43 km deep. 1 person has reported feeling it to USGS.

· 43 km deep · released about 2.7 kilotons 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 5.5 (mww)
Depth
43 km below the surface
Felt
1 report · strongest reported intensity I
Measured shaking
intensity III (ShakeMap)
USGS impact alert
green
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 326 days ago

This was a moderate earthquake. Shaking at this size can damage poorly built structures close to the epicentre.

At this magnitude and depth, people at rest would likely notice it out to about 258 km (160 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 2.7 kilotons of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 6.5 would have released about 32 times as much.

  1. Northridge, California 1994 M 6.7 63× the energy of this one
  2. Mineral, Virginia 2011 M 5.8 2.8× the energy of this one
  3. Magna, Utah 2020 M 5.7 2× the energy of this one
  4. This earthquake M 5.5 about 2.7 kilotons of TNT
  5. Around the size people start to feel M 3.0 this one released 5,600× 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 4 times the energy of this one.

Before 2016, the USGS record lists 15 earthquakes of M5 or more within 50 km; the five largest:

  1. 7.0 164 km SE of Chignik, Alaska · 34 km E of this epicentre
  2. 6.9 105 km ESE of Perryville, Alaska · 49 km W of this epicentre
  3. 6.7 121 km SE of Chignik, Alaska · 18 km NW of this epicentre
  4. 6.6 128 km SE of Chignik, Alaska · 36 km NNE of this epicentre
  5. 6.0 168 km SE of Chignik, Alaska · 32 km ESE 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 55.3344, -156.9608. Map © OpenStreetMap contributors.

Aftershocks so far

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

  1. 3.4 173 km SE of Chignik, Alaska
  2. 4.0 136 km SSE of Chignik, Alaska
  3. 2.9 155 km SE of Chignik, Alaska
  4. 3.3 149 km SE of Chignik, Alaska
  5. 1.6 143 km SSE of Chignik, Alaska
  6. 2.9 126 km SSE of Chignik, Alaska
  7. 2.3 143 km SE of Chignik, Alaska
  8. 3.6 161 km SSE of Chignik, Alaska
  9. 2.4 147 km SSE of Chignik, Alaska
  10. 2.2 145 km SE of Perryville, Alaska
  11. 3.0 140 km SE of Chignik, Alaska
  12. 2.5 139 km SE of Perryville, Alaska
  13. 2.7 141 km SSE of Chignik, Alaska
  14. 3.9 137 km SE of Chignik, Alaska
  15. 3.8 139 km SE 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.