M5.1 earthquake 154 km S of False Pass, Alaska

Thursday, September 9, 2021, 19:23:23 UTC · 53.470°N, 163.525°W

A magnitude 5.1 earthquake struck 154 km S of False Pass, Alaska on Thursday, September 9, 2021, 19:23:23 UTC, at a depth of 10.4 km (shallow). USGS received 5 “Did You Feel It?” reports, with a maximum reported intensity of III (weak). It is the strongest earthquake within 300 km since an M5.4 on June 18, 2021.

Magnitude: 5.1; Moderate · mww
Depth: 10 km; Shallow
Felt reports: 5; Max intensity III
Est. shaking: III; Weak (ShakeMap)
PAGER alert: None; Not issued
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

674 tons of TNT

That is about 1/22 of the Hiroshima bomb. It released about 1 times the energy of a magnitude 5.0.

Tectonic setting

38 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

10 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
99701<br>0.330	III · Weak	1	1529 km
99705<br>0.330	III · Weak	1	1560 km
99603<br>0.330	III · Weak	1	1014 km
99501<br>0.330	I · Not felt	1	1190 km
99685<br>0.330	I · Not felt	1	216 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 75 km

USGS lists 33 M2.5+ earthquakes within 75 km in the following 30 days; the largest was M4.7 on September 12, 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 445 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 10 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 168 recorded deaths. The most severe:

1946M8.6
Unimak Island
1 Apr 1946 · 46 km away · intensity VI
168 deaths$26 million damagetsunami
1965M6.5
Aleutian Islands: Fox Islands
2 Jul 1965 · 272 km away
Limited damage (under $1M)tsunami
1788M8.0
Alaska Peninsula
6 Aug 1788 · 236 km away
tsunami
1902M7.8
Aleutian Islands: Fox Islands
1 Jan 1902 · 292 km away
2020M7.6
Alaska
19 Oct 2020 · 284 km away · intensity IX
tsunami
1948M7.5
Alaska Peninsula
14 May 1948 · 201 km away

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

Nearby trackers