M7.6 earthquake 191 km S of ‘Ohonua, Tonga

Thursday, March 19, 2009, 18:17:40 UTC · 23.043°S, 174.660°W

A magnitude 7.6 earthquake struck 191 km S of ‘Ohonua, Tonga on Thursday, March 19, 2009, 18:17:40 UTC, at a depth of 31.0 km (shallow). USGS received 42 “Did You Feel It?” reports, with a maximum reported intensity of VII (very strong). It is the strongest earthquake within 300 km since an M7.8 on October 14, 1997.

Magnitude: 7.6; Major · mwc
Depth: 31 km; Shallow
Felt reports: 42; Max intensity VII
Est. shaking: V; Moderate (ShakeMap)
PAGER alert: None; Not issued
Tsunami: No flag; USGS feed

Outlined circle: this earthquakeDashed circle: 200 km aftershock search areaColoured lines: estimated shaking intensity (USGS ShakeMap)

In context

How big was it, and why here?

Energy released

3.8 megatons of TNT

That is about 252 times the Hiroshima bomb. It released about 7,943 times the energy of a magnitude 5.0.

Tectonic setting

9 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

31 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
Nukuʿalofa, Tongatapu, Tonga	VI · Strong	8	220 km
Wellington, Wellington, New Zealand	III · Weak	3	2254 km
Vaini, Tongatapu, Tonga	V · Moderate	2	213 km
Neiafu, Vavaʿu, Tonga	V · Moderate	2	493 km
Apia, Apia Urban Area, Samoa	IV · Light	2	1065 km
Pangai, Haʿapai, Tonga	VII · Very strong	1	360 km
Haveloloto, Tongatapu, Tonga	V · Moderate	1	219 km
Suva, Central, Fiji Islands	IV · Light	1	903 km
Auckland, Auckland, New Zealand	III · Weak	1	1840 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 200 km

USGS lists 76 M2.5+ earthquakes within 200 km in the following 30 days; the largest was M5.6 on April 6, 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 1,162 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 10 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 1 recorded deaths. The most severe:

1977M7.2
Tonga Trench
22 Jun 1977 · 128 km away · intensity IV
1 death$1 million damagetsunami
1937M8.1
Fiji Islands
16 Apr 1937 · 296 km away
1948M7.9
Tonga Islands
8 Sep 1948 · 237 km away
tsunami
1975M7.8
S Of Tonga
11 Oct 1975 · 211 km away
1982M7.7
Tonga Trench
19 Dec 1982 · 172 km away
tsunami
1997M7.7
Tonga Islands
14 Oct 1997 · 241 km away
tsunami

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

Nearby trackers