M4.2 earthquake 137 km SE of Perryville, Alaska

Sunday, October 17, 2021, 19:27:11 UTC · 55.052°N, 157.592°W

A magnitude 4.2 earthquake struck 137 km SE of Perryville, Alaska on Sunday, October 17, 2021, 19:27:11 UTC, at a depth of 33.9 km (shallow). The USGS PAGER alert is green: low likelihood of casualties and damage.

Magnitude: 4.2; Light · ml
Depth: 34 km; Shallow
Felt reports: 0; Did You Feel It?
Est. shaking: II; Weak (ShakeMap)
PAGER alert: Green; Impact estimate
Tsunami: No flag; USGS feed

Outlined circle: this earthquakeDashed circle: 50 km aftershock search areaColoured lines: estimated shaking intensity (USGS ShakeMap)

In context

How big was it, and why here?

Energy released

30 tons of TNT

That is about 1/500 of the Hiroshima bomb. A magnitude 5.0 releases about 16 times more energy.

Tectonic setting

87 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

34 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

No “Did You Feel It?” reports have been published for this event yet. If you felt it, you can add your report on the USGS website.

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.

Estimated shaking in nearby towns

Sand PointPopulation 976I · Not felt

After the main event

Aftershocks within 50 km

USGS lists 7 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M3.2 on November 8, 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 334 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 · 64 km away · intensity VII
Limited damage (under $1M)tsunami
1938M8.2
Alaska
10 Nov 1938 · 69 km away · intensity VI
tsunami
2021M8.2
Alaska Peninsula
29 Jul 2021 · 34 km away · intensity VIII
tsunami
1788M8.0
Alaska Peninsula
6 Aug 1788 · 217 km away
tsunami
2020M7.6
Alaska
19 Oct 2020 · 140 km away · intensity IX
tsunami
1948M7.5
Alaska Peninsula
14 May 1948 · 227 km away

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

Nearby trackers