M2.8 earthquake 196 km E of Atka, Alaska

Wednesday, September 21, 2011, 15:40:48 UTC · 51.930°N, 171.364°W

A magnitude 2.8 earthquake struck 196 km E of Atka, Alaska on Wednesday, September 21, 2011, 15:40:48 UTC, at a depth of 25.7 km (shallow).

Magnitude: 2.8; Minor · ml
Depth: 26 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

239 kg of TNT

That is about 1/63,000 of the Hiroshima bomb. A magnitude 5.0 releases about 1,995 times more energy.

Tectonic setting

75 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

26 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 52 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 755 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 14 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 · 297 km away
2 deathsSevere damage ($5–24M)tsunami
1986M8.0
Aleutian Islands: Adak
7 May 1986 · 239 km away · intensity VI
Moderate damage ($1–5M)tsunami
1901M7.8
Aleutian Islands: Fox Islands
31 Dec 1901 · 58 km away
Limited damage (under $1M)tsunami
1965M6.5
Aleutian Islands: Fox Islands
2 Jul 1965 · 286 km away
Limited damage (under $1M)tsunami
1902M7.8
Aleutian Islands: Fox Islands
1 Jan 1902 · 269 km away
1907M7.8
Aleutian Islands
2 Sep 1907 · 133 km away

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

Nearby trackers