Reviewed by USGSAlaska Earthquake5y ago
M8.2 earthquake 2021 Chignik, Alaska Earthquake
Thursday, July 29, 2021, 06:15:49 UTC · 55.364°N, 157.888°W
A magnitude 8.2 earthquake struck 2021 Chignik, Alaska Earthquake on Thursday, July 29, 2021, 06:15:49 UTC, at a depth of 35.0 km (shallow). USGS received 512 “Did You Feel It?” reports, with a maximum reported intensity of IX (violent). The USGS PAGER alert is yellow: some casualties and damage are possible. It is the strongest earthquake within 300 km since an M8.2 on November 10, 1938.
Tsunami flagUSGS sets this flag for large events in oceanic regions so tsunami warning centers review them. It does not by itself mean a tsunami occurred. Check tsunami.gov and local authorities for official messages.
- Magnitude
- 8.2
- Great · mww
- Depth
- 35 km
- Shallow
- Felt reports
- 512
- Max intensity IX
- Est. shaking
- VII
- Very strong (ShakeMap)
- PAGER alert
- Yellow
- Impact estimate
- Tsunami
- Flag
- Check tsunami.gov
Outlined circle: this earthquakeDashed circle: 400 km aftershock search areaColoured lines: estimated shaking intensity (USGS ShakeMap)
In context
How big was it, and why here?
Energy released
30 megatons of TNT
That is about 1,998 times the Hiroshima bomb. It released about 63,096 times the energy of a magnitude 5.0.
Tectonic setting
126 km
from the nearest mapped plate boundary, between the North American and Pacific plates. Earthquakes this far from a boundary happen on faults inside a plate, often less frequently but sometimes strongly.
Depth: shallow
35 km
Shallow earthquakes (under 70 km) produce the strongest surface shaking for their size, so they are the ones most likely to cause damage near the epicenter.
Did You Feel It?
Where it was felt
Reports sent by people to USGS, grouped by place (most reports first). Intensity uses the Modified Mercalli scale (I–X): what people felt, not the magnitude.
| Place | Intensity | Reports | Distance |
|---|---|---|---|
| 99603<br>0.110 | III · Weak | 61 | 615 km |
| 99611<br>0.137 | III · Weak | 40 | 743 km |
| 99615<br>0.153 | IV · Light | 33 | 372 km |
| 99669<br>0.159 | III · Weak | 31 | 696 km |
| 99556<br>0.203 | III · Weak | 19 | 620 km |
| 99517<br>0.208 | III · Weak | 18 | 800 km |
| 99577<br>0.208 | II · Weak | 18 | 825 km |
| 99628<br>0.224 | III · Weak | 15 | 400 km |
| 99516<br>0.224 | II · Weak | 15 | 797 km |
| 99507<br>0.230 | II · Weak | 14 | 802 km |
| 99654<br>0.236 | II · Weak | 13 | 832 km |
| 99501<br>0.248 | III · Weak | 11 | 804 km |
USGS PAGER estimate
Who was exposed to shaking
Modelled from the ShakeMap and population data. Alert level yellow: some casualties and damage are possible.
- VII · Very strong shaking
- 1,700 people
- V · Moderate shaking
- 1,500 people
- IV · Light shaking
- 25,000 people
- III · Weak shaking
- 154,000 people
- II · Weak shaking
- 328,000 people
Estimated shaking in nearby towns
- Sand PointPopulation 976VII · Very strong
- KodiakPopulation 6,100IV · Light
- Kodiak StationPopulation 1,300IV · Light
- DillinghamPopulation 2,300IV · Light
- AkutanPopulation 1,000IV · Light
- HomerPopulation 5,000III · Weak
- AnchoragePopulation 292,000III · Weak
- Knik-FairviewPopulation 15,000II · Weak
After the main event
Aftershocks within 400 km
USGS lists 659 M2.5+ earthquakes within 400 km in the following 30 days; the largest was M6.9 on August 14, 2021. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.
- 2.6130 km SSE of Sand Point, Alaska5y ago · 147 km SW of this epicenter
- 3.377 km ESE of Chignik, Alaska5y ago · 86 km NE of this epicenter
- 3.4173 km SE of Sand Point, Alaska5y ago · 111 km S of this epicenter
- 2.679 km ESE of Sand Point, Alaska5y ago · 102 km W of this epicenter
- 2.7138 km SE of Perryville, Alaska5y ago · 40 km SE of this epicenter
- 2.5136 km SE of Chignik, Alaska5y ago · 55 km E of this epicenter
- 3.5123 km SSE of Perryville, Alaska5y ago · 65 km SW of this epicenter
- 2.692 km SSW of Sand Point, Alaska5y ago · 224 km W of this epicenter
Regional history
Past M5+ earthquakes within 300 km
USGS lists 322 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:
- 8.21938 Semidi Islands, Alaska EarthquakeNovember 10, 1938 · 28 km SW of this epicenter
- 7.82020 Perryville, Alaska EarthquakeJuly 22, 2020 · 55 km SW of this epicenter
- 7.62020 Sand Point, Alaska EarthquakeOctober 19, 2020 · 140 km SW of this epicenter
- 7.454 km SE of Sand Point, AlaskaMay 31, 1917 · 133 km W of this epicenter
- 7.190 km SE of King Cove, AlaskaMay 14, 1948 · 226 km W of this epicenter
- 7.0113 km ESE of Chignik, AlaskaMay 4, 1923 · 88 km NE of this epicenter
- 7.071 km SE of King Cove, AlaskaMay 30, 1991 · 253 km W of this epicenter
- 7.0164 km SE of Chignik, AlaskaSeptember 4, 1989 · 92 km E of this epicenter
History · NOAA
Deadliest earthquakes in this area
NOAA’s significant-earthquake database lists 11 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700. The most severe:
- 2020M7.8
Alaska
22 Jul 2020 · 55 km away · intensity VII
Limited damage (under $1M)tsunami
- 1938M8.2
Alaska
10 Nov 1938 · 33 km away · intensity VI
tsunami
- 2021M8.2
Alaska Peninsula
29 Jul 2021 · 5 km away · intensity VIII
tsunami
- 1788M8.0
Alaska Peninsula
6 Aug 1788 · 202 km away
tsunami
- 1906M7.6
Aleutian Islands
23 Dec 1906 · 297 km away
- 2020M7.6
Alaska
19 Oct 2020 · 140 km away · intensity IX
tsunami
NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.