M6.1 earthquake 97 km NE of Hihifo, Tonga

Wednesday, April 21, 2010, 17:20:29 UTC · 15.271°S, 173.219°W

A magnitude 6.1 earthquake struck 97 km NE of Hihifo, Tonga on Wednesday, April 21, 2010, 17:20:29 UTC, at a depth of 35.0 km (shallow). USGS received 11 “Did You Feel It?” reports, with a maximum reported intensity of IV (light). It is the strongest earthquake within 300 km since an M6.1 on February 9, 2010.

Magnitude: 6.1; Strong · mwc
Depth: 35 km; Shallow
Felt reports: 11; Max intensity IV
Est. shaking: —; No ShakeMap
PAGER alert: None; Not issued
Tsunami: No flag; USGS feed

Outlined circle: this earthquakeDashed circle: 100 km aftershock search area

In context

How big was it, and why here?

Energy released

21 kilotons of TNT

That is about the energy of the Hiroshima bomb. It released about 45 times the energy of a magnitude 5.0.

Tectonic setting

53 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

35 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	IV · Light	7	226 km
Pago Pago, Eastern, American Samoa	III · Weak	2	294 km
Pavaʿiaʿi, Western, American Samoa	III · Weak	1	287 km
Tafuna, Western, American Samoa	II · Weak	1	290 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 100 km

USGS lists 11 M2.5+ earthquakes within 100 km in the following 30 days; the largest was M5.3 on April 22, 2010. 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 1,386 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 8 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 · 123 km away
192 deaths$285 million damagetsunami
1917M8.3
Samoa Islands
26 Jun 1917 · 35 km away
2 deathsModerate damage ($1–5M)tsunami
1981M7.7
Apia; American Samoa: Pago Pago
1 Sep 1981 · 37 km away · intensity VI
Limited damage (under $1M)tsunami
1977M7.6
Apia
2 Apr 1977 · 199 km away · intensity VI
Limited damage (under $1M)tsunami
1975M7.8
Samoa Islands
26 Dec 1975 · 137 km away · intensity V
tsunami
1957M7.5
Samoa Islands
14 Apr 1957 · 35 km away

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

Nearby trackers