Reviewed by USGSAlaska5y ago
M6.9 earthquake 112 km E of Chignik, Alaska
Monday, October 11, 2021, 09:10:25 UTC · 56.295°N, 156.581°W
A magnitude 6.9 earthquake struck 112 km E of Chignik, Alaska on Monday, October 11, 2021, 09:10:25 UTC, at a depth of 51.6 km (shallow). USGS received 320 “Did You Feel It?” reports, with a maximum reported intensity of VIII (severe). The USGS PAGER alert is green: low likelihood of casualties and damage. It is the strongest earthquake within 300 km since an M6.9 on August 14, 2021.
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
- 6.9
- Strong · mww
- Depth
- 52 km
- Shallow
- Felt reports
- 320
- Max intensity VIII
- Est. shaking
- VI
- Strong (ShakeMap)
- PAGER alert
- Green
- Impact estimate
- Tsunami
- Flag
- Check tsunami.gov
Outlined circle: this earthquakeDashed circle: 100 km aftershock search areaColoured lines: estimated shaking intensity (USGS ShakeMap)
In context
How big was it, and why here?
Energy released
338 kilotons of TNT
That is about 22 times the Hiroshima bomb. It released about 708 times the energy of a magnitude 5.0.
Tectonic setting
183 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
52 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>0.119 | III · Weak | 52 | 488 km |
| 99615<br>0.134 | III · Weak | 42 | 252 km |
| 99577<br>0.208 | II · Weak | 18 | 697 km |
| 99507<br>0.236 | III · Weak | 13 | 675 km |
| 99516<br>0.248 | II · Weak | 11 | 670 km |
| 99664<br>0.255 | III · Weak | 10 | 629 km |
| 99502<br>0.255 | II · Weak | 10 | 667 km |
| 99567<br>0.255 | II · Weak | 10 | 717 km |
| 99628<br>0.277 | IV · Light | 7 | 333 km |
| 99504<br>0.277 | III · Weak | 7 | 681 km |
| 99503<br>0.285 | III · Weak | 6 | 674 km |
| 99613<br>0.293 | IV · Light | 5 | 283 km |
USGS PAGER estimate
Who was exposed to shaking
Modelled from the ShakeMap and population data. Alert level green: low likelihood of casualties and damage.
- IV · Light shaking
- 7,300 people
- III · Weak shaking
- 151,000 people
- II · Weak shaking
- 361,000 people
Estimated shaking in nearby towns
- DillinghamPopulation 2,300IV · Light
- Sand PointPopulation 976IV · Light
- KodiakPopulation 6,100III · Weak
- Kodiak StationPopulation 1,300III · Weak
- Diamond RidgePopulation 1,200III · Weak
- HomerPopulation 5,000III · Weak
- AnchoragePopulation 292,000II · Weak
- Eagle RiverPopulation 25,000II · Weak
After the main event
Aftershocks within 100 km
USGS lists 80 M2.5+ earthquakes within 100 km in the following 30 days; the largest was M4.2 on October 11, 2021. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.
- 2.6113 km E of Chignik, Alaska4y ago · 8 km S of this epicenter
- 2.587 km ESE of Chignik, Alaska4y ago · 56 km SW of this epicenter
- 2.5117 km E of Chignik, Alaska4y ago · 5 km E of this epicenter
- 2.5111 km E of Chignik, Alaska4y ago · 5 km S of this epicenter
- 2.9113 km E of Chignik, Alaska4y ago · 2 km N of this epicenter
- 3.599 km E of Chignik, Alaska4y ago · 18 km SW of this epicenter
- 2.5154 km SSW of Akhiok, Alaska4y ago · 87 km SE of this epicenter
- 3.0156 km SSW of Akhiok, Alaska4y ago · 86 km SE of this epicenter
Regional history
Past M5+ earthquakes within 300 km
USGS lists 421 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 · 160 km SW of this epicenter
- 8.22021 Chignik, Alaska EarthquakeJuly 29, 2021 · 132 km SW of this epicenter
- 7.82020 Perryville, Alaska EarthquakeJuly 22, 2020 · 186 km SW of this epicenter
- 7.62020 Sand Point, Alaska EarthquakeOctober 19, 2020 · 269 km SW of this epicenter
- 7.454 km SE of Sand Point, AlaskaMay 31, 1917 · 253 km SW of this epicenter
- 7.14 km ENE of Karluk, AlaskaNovember 7, 1912 · 196 km NE of this epicenter
- 7.0113 km ESE of Chignik, AlaskaMay 4, 1923 · 52 km S of this epicenter
History · NOAA
Deadliest earthquakes in this area
NOAA’s significant-earthquake database lists 13 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700. The most severe:
- 1900M8.3
Kodiak Island
9 Oct 1900 · 209 km away · intensity VIII
Limited damage (under $1M)
- 1788M8.0
Unga Island
21 Jul 1788 · 232 km away · intensity VII
Limited damage (under $1M)tsunami
- 2020M7.8
Alaska
22 Jul 2020 · 186 km away · intensity VII
Limited damage (under $1M)tsunami
- 1970M6.0
Andreanof Islands
11 Mar 1970 · 211 km away · intensity V
Limited damage (under $1M)tsunami
- 1792
Kodiak Island
1792 · 291 km away · intensity VII
Limited damage (under $1M)tsunami
- 1938M8.2
Alaska
10 Nov 1938 · 144 km away · intensity VI
tsunami
NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.