M4.2 earthquake 199 km ESE of Atka, Alaska

Sunday, September 25, 2011, 20:05:19 UTC · 51.676°N, 171.423°W

A magnitude 4.2 earthquake struck 199 km ESE of Atka, Alaska on Sunday, September 25, 2011, 20:05:19 UTC, at a depth of 41.5 km (shallow).

Magnitude: 4.2; Light · mb
Depth: 42 km; Shallow
Felt reports: 0; Did You Feel It?
Est. shaking: —; No ShakeMap
PAGER alert: None; Not issued
Tsunami: No flag; USGS feed

Outlined circle: this earthquakeDashed circle: 50 km aftershock search area

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

49 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

42 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

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 32 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M4.8 on October 8, 2011. 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 749 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 13 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 2 recorded deaths. The most severe:

1957M8.6
Alaska
9 Mar 1957 · 291 km away
2 deathsSevere damage ($5–24M)tsunami
1986M8.0
Aleutian Islands: Adak
7 May 1986 · 232 km away · intensity VI
Moderate damage ($1–5M)tsunami
1901M7.8
Aleutian Islands: Fox Islands
31 Dec 1901 · 38 km away
Limited damage (under $1M)tsunami
1902M7.8
Aleutian Islands: Fox Islands
1 Jan 1902 · 280 km away
1907M7.8
Aleutian Islands
2 Sep 1907 · 154 km away
1929M7.8
Aleutian Islands: Fox Islands
7 Mar 1929 · 124 km away · intensity V
tsunami

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

Nearby trackers