M5.6 earthquake 115 km SSE of Perryville, Alaska

Sunday, October 31, 2021, 16:37:46 UTC · 54.906°N, 158.715°W

A magnitude 5.6 earthquake struck 115 km SSE of Perryville, Alaska on Sunday, October 31, 2021, 16:37:46 UTC, at a depth of 25.0 km (shallow). USGS received 5 “Did You Feel It?” reports, with a maximum reported intensity of III (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 October 11, 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: 5.6; Moderate · mww
Depth: 25 km; Shallow
Felt reports: 5; Max intensity III
Est. shaking: IV; Light (ShakeMap)
PAGER alert: Green; Impact estimate
Tsunami: Flag; Check tsunami.gov

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

99 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

25 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
99648<br>0.330	III · Weak	1	119 km
99709<br>0.330	III · Weak	1	1261 km
99565<br>0.330	III · Weak	1	156 km
99516<br>0.330	II · Weak	1	868 km
99571<br>0.330	II · Weak	1	179 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.

III · Weak shaking: 1,700 people

Estimated shaking in nearby towns

Sand PointPopulation 976III · Weak

After the main event

Aftershocks within 75 km

USGS lists 16 M2.5+ earthquakes within 75 km in the following 30 days; the largest was M4.3 on November 18, 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 305 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 9 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700. The most severe:

2020M7.8
Alaska
22 Jul 2020 · 20 km away · intensity VII
Limited damage (under $1M)tsunami
1938M8.2
Alaska
10 Nov 1938 · 68 km away · intensity VI
tsunami
2021M8.2
Alaska Peninsula
29 Jul 2021 · 73 km away · intensity VIII
tsunami
1788M8.0
Alaska Peninsula
6 Aug 1788 · 146 km away
tsunami
2020M7.6
Alaska
19 Oct 2020 · 68 km away · intensity IX
tsunami
1948M7.5
Alaska Peninsula
14 May 1948 · 154 km away

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

Nearby trackers