M4.0 earthquake in Alaska Peninsula

Tuesday, August 15, 2023, 18:09:36 UTC · 54.778°N, 161.210°W

A magnitude 4.0 earthquake struck in Alaska Peninsula on Tuesday, August 15, 2023, 18:09:36 UTC, at a depth of 35.0 km (shallow). USGS received 1 “Did You Feel It?” report, with a maximum reported intensity of III (weak).

Magnitude: 4.0; Light · mb
Depth: 35 km; Shallow
Felt reports: 1; Max intensity III
Est. shaking: I; Not felt (ShakeMap)
PAGER alert: None; Not issued
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

15 tons of TNT

That is about 1/998 of the Hiroshima bomb. A magnitude 5.0 releases about 32 times more energy.

Tectonic setting

129 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
99612<br>0.330	III · Weak	1	86 km

I felt it — report to USGS ↗

USGS PAGER estimate

Who was exposed to shaking

USGS publishes PAGER impact estimates for significant earthquakes, usually within an hour. No PAGER estimate is available for this event.

After the main event

Aftershocks within 50 km

USGS lists 25 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M5.1 on August 30, 2023. 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 398 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, which together caused about 169 recorded deaths. The most severe:

1946M8.6
Unimak Island
1 Apr 1946 · 178 km away · intensity VI
168 deaths$26 million damagetsunami
2023M7.2
Alaska Peninsula
16 Jul 2023 · 46 km away · intensity VI
1 deathtsunami
2020M7.8
Alaska
22 Jul 2020 · 170 km away · intensity VII
Limited damage (under $1M)tsunami
1938M8.2
Alaska
10 Nov 1938 · 197 km away · intensity VI
tsunami
2021M8.2
Alaska Peninsula
29 Jul 2021 · 223 km away · intensity VIII
tsunami
1788M8.0
Alaska Peninsula
6 Aug 1788 · 28 km away
tsunami

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

Nearby trackers