Major Earthquake Near Tonga

A shallow, major earthquake close to magnitude 8 struck south of the Tongan islands this morning.

Friday 20th March 2009

The magnitude 7.9 earthquake, which struck at 7:18 a.m. New Zealand Daylight Time, was located 215 km south-south-east of the Tongan capital Nuku’Alofa at a depth of 34 km. The epicentre of the undersea quake was 1845 km north-east of Auckland, 490 km east-south-east of Ndoi Island, Fiji according to the U.S. Geological Survey.

Analysis by Geoscience Australia placed the quake a little further north at a depth of 28 km. Their seismologists calculated the quake as magnitude 7.6, suggesting it would have caused damage in an area within 180 km of the epicentre, and would have been felt up to 1300 km away.

In New Zealand, the quake was felt lightly on the Bay of Plenty coast, at Poverty Bay and Mahia Peninsula on the East Coast of the North Island, and at several locations in Wellington.

Twelve minutes after the quake, the Pacific Tsunami Warning Centre (PTWC) issued a tsunami warning for Tonga, Niue, the Kermadec Islands, Samoa, American Samoa, Wallis and Futuna, and Fiji. Estimating the quake at magnitude 7.7 at a depth of 48 km, it was determined that the undersea earthquake was sufficiently shallow and powerful to have generated a tsunami wave. Estimates of arrival time for tsunami waves at Tonga were given as 7:45 a.m. NZDT, Niue at 8:04, the Kermadecs at 8:12 a.m. and Samoa at 8:34.

A further bulletin was issued at 8:32 a.m., confirming the tsunami warning for the island states, noting that sea level readings confirmed that a tsunami was generated. The quake was still estimated at magnitude 7.7, but further analysis suggested it was shallower than first thought, at about 10 km.

By 9:08 a.m. NZDT, data from the wave gauge at Niue had been analysed, showing that a tsunami wave of only 4 cm had been recorded. This allowed another bulletin to be issued, cancelling the warning for nearby islands. The PTWC had earlier estimated that the arrival time at Niue would probably have been 8:04 a.m. and the actual arrival time was very close to this, being just 7 minutes later at 8:11 a.m.

Since this morning’s quake, there have been remarkably few aftershocks. Only two events, a magnitude 5.3 event at 9:34 a.m. and a magnitude 4.9 quake at 3:11 p.m. have been reported near Tonga.

[Compiled from data supplied by the U.S. Geological Survey and its contributing agencies, the U.S. National Oceanic & Atmospheric Administration’s Pacific Tsunami Warning Centre, and Geoscience Australia.]

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