Reviewed by USGSAlaska22y ago
M4.2 earthquake 28 km ESE of Adak, Alaska
Saturday, November 20, 2004, 20:21:36 UTC · 51.813°N, 176.232°W
A magnitude 4.2 earthquake struck 28 km ESE of Adak, Alaska on Saturday, November 20, 2004, 20:21:36 UTC, at a depth of 80.1 km (intermediate).
- Magnitude
- 4.2
- Light · mb
- Depth
- 80 km
- Intermediate
- 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
126 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: intermediate
80 km
Intermediate-depth earthquakes usually break inside a plate that is sinking into the mantle. Shaking spreads over a wide area but is gentler at the epicenter than a shallow quake of the same size.
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 3 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M2.7 on December 5, 2004. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.
Regional history
Past M5+ earthquakes within 300 km
USGS lists 888 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:
- 8.61957 Atka, Alaska EarthquakeMarch 9, 1957 · 54 km SE of this epicenter
- 8.01986 Atka, Alaska EarthquakeMay 7, 1986 · 106 km E of this epicenter
- 7.977 km WSW of Adak, AlaskaJune 10, 1996 · 100 km W of this epicenter
- 7.65 km WNW of Adak, AlaskaFebruary 14, 1905 · 33 km W of this epicenter
- 7.4265 km WSW of Adak, AlaskaFebruary 4, 1965 · 291 km W of this epicenter
- 7.388 km WSW of Adak, AlaskaJuly 7, 1929 · 111 km W of this epicenter
- 7.346 km SSW of Adak, AlaskaJune 10, 1996 · 56 km SW of this epicenter
- 7.2238 km WSW of Adak, AlaskaMarch 22, 1905 · 262 km W of this epicenter
History · NOAA
Deadliest earthquakes in this area
NOAA’s significant-earthquake database lists 8 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 · 54 km away
2 deathsSevere damage ($5–24M)tsunami
- 1986M8.0
Aleutian Islands: Adak
7 May 1986 · 106 km away · intensity VI
Moderate damage ($1–5M)tsunami
- 1905M7.9
Andreanof Islands
14 Feb 1905 · 201 km away
- 1996M7.9
Andreanof Islands
10 Jun 1996 · 100 km away · intensity VI
tsunami
- 1996M7.3
Andreanof Islands
10 Jun 1996 · 56 km away
tsunami
- 1971M7.1
Andreanof Islands
2 May 1971 · 81 km away · intensity VI
tsunami
NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.