M5.6 earthquake 181 km SE of Chignik, Alaska

Thursday, September 16, 2021, 17:51:55 UTC · 55.071°N, 156.499°W

A magnitude 5.6 earthquake struck 181 km SE of Chignik, Alaska on Thursday, September 16, 2021, 17:51:55 UTC, at a depth of 36.8 km (shallow). USGS received 2 “Did You Feel It?” reports, with a maximum reported intensity of II (weak). 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.

Magnitude: 5.6; Moderate · mww
Depth: 37 km; Shallow
Felt reports: 2; Max intensity II
Est. shaking: III; Weak (ShakeMap)
PAGER alert: Green; Impact estimate
Tsunami: No flag; USGS feed

Outlined circle: this earthquakeDashed circle: 75 km aftershock search areaColoured lines: estimated shaking intensity (USGS ShakeMap)

In context

How big was it, and why here?

Energy released

4 kilotons of TNT

That is about 1/4 of the Hiroshima bomb. It released about 8 times the energy of a magnitude 5.0.

Tectonic setting

64 km

from the nearest mapped plate boundary, between the North American and Pacific plates. Earthquakes cluster along boundaries like this one, where plates collide, pull apart or slide past each other.

Depth: shallow

37 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
99709<br>0.330	II · Weak	1	1193 km
99516<br>0.330	I · Not felt	1	780 km

I felt it — report to USGS ↗

USGS PAGER estimate

Who was exposed to shaking

Modelled from the ShakeMap and population data. Alert level green: low likelihood of casualties and damage.

II · Weak shaking: 2,500 people

Estimated shaking in nearby towns

Sand PointPopulation 976II · Weak
KodiakPopulation 6,100I · Not felt
Kodiak StationPopulation 1,300I · Not felt

After the main event

Aftershocks within 75 km

USGS lists 14 M2.5+ earthquakes within 75 km in the following 30 days; the largest was M5.1 on September 16, 2021. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.

Time after the mainshock on a logarithmic scale (first hours stretched), magnitude on the vertical axis. M2.5+ events from USGS.

Regional history

Past M5+ earthquakes within 300 km

USGS lists 342 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:

History · NOAA

Deadliest earthquakes in this area

NOAA’s significant-earthquake database lists 12 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700. The most severe:

1900M8.3
Kodiak Island
9 Oct 1900 · 292 km away · intensity VIII
Limited damage (under $1M)
2020M7.8
Alaska
22 Jul 2020 · 133 km away · intensity VII
Limited damage (under $1M)tsunami
1938M8.2
Alaska
10 Nov 1938 · 127 km away · intensity VI
tsunami
2021M8.2
Alaska Peninsula
29 Jul 2021 · 90 km away · intensity VIII
tsunami
1788M8.0
Alaska Peninsula
6 Aug 1788 · 287 km away
tsunami
1906M7.6
Aleutian Islands
23 Dec 1906 · 256 km away

NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.

Nearby trackers