M8.1 earthquake 2009 Samoa Earthquake

Tuesday, September 29, 2009, 17:48:10 UTC · 15.489°S, 172.095°W

A magnitude 8.1 earthquake struck 2009 Samoa Earthquake on Tuesday, September 29, 2009, 17:48:10 UTC, at a depth of 18.0 km (shallow). USGS received 66 “Did You Feel It?” reports, with a maximum reported intensity of VII (very strong). No earthquake of this size or larger appears within 300 km in the USGS catalog since 1900.

Magnitude: 8.1; Great · mwc
Depth: 18 km; Shallow
Felt reports: 66; Max intensity VII
Est. shaking: VI; Strong (ShakeMap)
PAGER alert: None; Not issued
Tsunami: No flag; USGS feed

Outlined circle: this earthquakeDashed circle: 400 km aftershock search areaColoured lines: estimated shaking intensity (USGS ShakeMap)

In context

How big was it, and why here?

Energy released

21 megatons of TNT

That is about 1,415 times the Hiroshima bomb. It released about 44,668 times the energy of a magnitude 5.0.

Tectonic setting

28 km

from the nearest mapped plate boundary, between the Tonga and Pacific plates. Earthquakes cluster along boundaries like this one, where plates collide, pull apart or slide past each other.

Depth: shallow

18 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
Apia, Apia Urban Area, Samoa	VI · Strong	9	187 km
Iliʿili, Western, American Samoa	V · Moderate	6	193 km
Tafuna, Western, American Samoa	V · Moderate	5	197 km
Nuʿuuli, Eastern, American Samoa	VI · Strong	3	198 km
Pavaʿiaʿi, Western, American Samoa	VI · Strong	2	193 km
Pago Pago, Eastern, American Samoa	VI · Strong	2	203 km
96799<br>0.320	V · Moderate	2	199 km
Taputimu, Western, American Samoa	VII · Very strong	1	190 km
Matavai, Rest of Upolu, Samoa	VII · Very strong	1	171 km
Faleniu, Western, American Samoa	VII · Very strong	1	194 km
Afono, Eastern, American Samoa	VI · Strong	1	207 km
Atuʿu, Eastern, American Samoa	V · Moderate	1	203 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 400 km

USGS lists 291 M2.5+ earthquakes within 400 km in the following 30 days; the largest was M6.3 on October 14, 2009. 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 1,095 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 194 recorded deaths. The most severe:

2009M8.1
Samoa Islands
29 Sep 2009 · 0 km away
192 deaths$285 million damagetsunami
1917M8.3
Samoa Islands
26 Jun 1917 · 97 km away
2 deathsModerate damage ($1–5M)tsunami
1981M7.7
Apia; American Samoa: Pago Pago
1 Sep 1981 · 121 km away · intensity VI
Limited damage (under $1M)tsunami
1977M7.6
Apia
2 Apr 1977 · 134 km away · intensity VI
Limited damage (under $1M)tsunami
1975M7.8
Samoa Islands
26 Dec 1975 · 95 km away · intensity V
tsunami
1957M7.5
Samoa Islands
14 Apr 1957 · 97 km away

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

Nearby trackers