M4.9 earthquake 194 km S of ‘Ohonua, Tonga

Tuesday, April 7, 2009, 19:23:29 UTC · 23.078°S, 174.701°W

A magnitude 4.9 earthquake struck 194 km S of ‘Ohonua, Tonga on Tuesday, April 7, 2009, 19:23:29 UTC, at a depth of 50.9 km (shallow). It is the strongest earthquake within 300 km since an M5.6 on April 6, 2009.

Magnitude: 4.9; Light · mb
Depth: 51 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: 50 km aftershock search area

In context

How big was it, and why here?

Energy released

338 tons of TNT

That is about 1/45 of the Hiroshima bomb. A magnitude 5.0 releases about 1 times more energy.

Tectonic setting

11 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

51 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 50 km

USGS lists 6 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M4.7 on April 28, 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 1,172 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 · 125 km away · intensity IV
1 death$1 million damagetsunami
1937M8.1
Fiji Islands
16 Apr 1937 · 295 km away
1948M7.9
Tonga Islands
8 Sep 1948 · 242 km away
tsunami
1975M7.8
S Of Tonga
11 Oct 1975 · 206 km away
1982M7.7
Tonga Trench
19 Dec 1982 · 167 km away
tsunami
1997M7.7
Tonga Islands
14 Oct 1997 · 239 km away
tsunami

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

Nearby trackers