A stong 7.4Mw tremor rattled Nuku'alofa before dawn this morning for over a minute starting at 6:19am. Two aftershocks, including 6.6Mw, were felt in Nuku'alofa after 10:00am.

Early information from NOAA stated that a 7.4 magnitude earthquake was centred south of the Fiji Islands at a depth of 208 km. The coordinates were at 23.1 South and 176.6 west.

NOAA did not expect a tsunami to be generated based on the earthquake and historical data.

The earthquake shook homes and rattled windows in the capital. No damage has been reported.

The United States Geological Service (USGS) located the epicentre of the first earthquake at:

1. 282km (175mi) SW of Vaini, Tonga
2. 287km (178mi) SW of Nuku`alofa, Tonga.

USGS stated that this morning's Mw7.4 earthquake, (May 23, 2013) southwest of Vaini, Tonga, occurred as a result of normal faulting at a depth of approximately 170 km. "At the location of this earthquake, the Pacific and Australia plates are converging at a rate of approximately 73 mm/yr in an east-west direction, resulting in the westward subduction of the Pacific plate beneath Tonga at the Tonga-Kermadec trench. The depth and faulting mechanism of the May 23rd earthquake indicate it ruptured a fault within the subducting Pacific lithosphere rather than on the shallower thrust interface between the two plates.

"The Tonga-Kermadec arc has frequent moderate-to-large earthquakes, and has hosted over a dozen M6.5+ earthquakes within 500 km of the May 23 earthquake over the past 40 years. Most of these also occurred at intermediate depths; the largest was an Mw 7.7 earthquake in October of 1997."

"None are known to have caused significant damage. Intermediate-depth (70-300 km) and deep-focus (depth > 300 km) earthquakes are distinguished from shallow earthquakes (0-70 km) by the nature of their tectonic setting, and are in general less hazardous than their shallow counterparts, though they may be felt at great distances from their epicenters. The Tonga-Kermadec slab in the region of the May 23 2013 earthquake is seismically active to depths of over 650 km." 

http://earthquake.usgs.gov/earthquakes/eventpage/usb000h3k3#summary [2]

An Mw 6.6 aftershock was also recorded 84km (52mi) to the north west of Nuku'alofa, Tonga. The location was 20.561°S 175.730°W depth=103.2km (64.2mi) at  a local Tonga time 10:07:40 at epicenter.