M5.5 earthquake 67 km SSE of Adak, Alaska

Wednesday, April 11, 2012, 09:00:09 UTC · 51.364°N, 176.097°W

A magnitude 5.5 earthquake struck 67 km SSE of Adak, Alaska on Wednesday, April 11, 2012, 09:00:09 UTC, at a depth of 20.8 km (shallow). It is the strongest earthquake within 300 km since an M5.5 on November 19, 2011.

Magnitude: 5.5; Moderate · mb
Depth: 21 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: 75 km aftershock search area

In context

How big was it, and why here?

Energy released

3 kilotons of TNT

That is about 1/6 of the Hiroshima bomb. It released about 6 times the energy of a magnitude 5.0.

Tectonic setting

76 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

21 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 75 km

USGS lists 12 M2.5+ earthquakes within 75 km in the following 30 days; the largest was M4.3 on April 11, 2012. 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 1,040 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, which together caused about 2 recorded deaths. The most severe:

1957M8.6
Alaska
9 Mar 1957 · 36 km away
2 deathsSevere damage ($5–24M)tsunami
1986M8.0
Aleutian Islands: Adak
7 May 1986 · 93 km away · intensity VI
Moderate damage ($1–5M)tsunami
1905M7.9
Andreanof Islands
14 Feb 1905 · 185 km away
1996M7.9
Andreanof Islands
10 Jun 1996 · 109 km away · intensity VI
tsunami
1996M7.3
Andreanof Islands
10 Jun 1996 · 54 km away
tsunami
1971M7.1
Andreanof Islands
2 May 1971 · 77 km away · intensity VI
tsunami

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

Nearby trackers