Almost a quiet quake year
NZPA
Last updated 12:57 05/01/2010

If it were not for Fiordland's massive 7.8 magnitude earthquake and aftershocks, 2009 would have been a quiet year for earthquakes in New Zealand, according to a GNS Science summary of principal quakes for the year.

A total of 42 earthquakes of magnitude 5.0 or greater were recorded and 392 earthquakes were reported to the GeoNet website - between 150 and 500 earthquakes are reported in a typical year.

The largest earthquake to occur in New Zealand since the 1931 Napier earthquake, struck remote Dusky Sound in Fiordland at a depth of 30km on July 16, followed by a series of aftershocks.

Were it not for this sequence, this would have been a quiet year for earthquakes in New Zealand, GNS Science said.

"The Dusky Sound earthquake was interesting because it was an earthquake that occurred at the boundary of the two plates. Normally earthquakes occur in one plate or the other plate but this has occurred where the two plates meet, and they are reasonably rare," said GeoNet data manager Kevin Fenaughty.

"We have got some very good data from a very rare earthquake. The physics are quite different to any others we have seen down there."

However, the resulting damage was much less than expected for an earthquake of its size, as the rupturing caused by the earthquake moved upwards and to the south, directing most of the energy offshore.

Walls were cracked and there was some damage in Southland and there were reports of ground slumping around Invercargill.

The earthquake was felt throughout the South Island and mainly on the western side of the lower North Island, with scattered reports from as far north as Auckland, and also from Sydney.

A small tsunami was detected and a one metre wave measured at Jackson Bay in south Westland.

The biggest aftershock was a magnitude 6.1 that occurred 20 minutes after the main shock, and there have been thirty of magnitude 5.0 to 5.9 spread over the faulting area, with the latest a magnitude 5.0 on December 13.

A magnitude 5.4 earthquake centred 90km south of Opunake, in Taranaki, and at a depth of 170km occurred 16 minutes after the Fiordland earthquake, and was felt from Taranaki to the West Coast.

Two deep earthquakes of magnitude 6.1 were the other largest earthquakes of the year. The first, on March 22 at a depth of 160km and centred 30km northwest of Whakatane, was felt from Coromandel to Christchurch; the second, on September 1 at a depth of 280km and centred 120km north of Whakatane, was felt from Gisborne to Nelson.

The most strongly felt earthquake in the Wellington region was a shallow magnitude 5.2 on August 28.

On the morning of June 27, two shallow earthquakes centred about 5km northwest of Turangi and of magnitudes 4.3 and 4.4 were reported felt strongly in the area at the southern end of Lake Taupo.

These were the biggest of a swarm of several hundred earthquakes that commenced in late May, and as a result residents of Waihi village were advised to evacuate because of a risk of landslide activity.

At the time the Dusky Sound earthquake was the largest recorded worldwide for the year, however it was overtaken by the magnitude 8.1 Samoan quake on September 30, which, while it was not felt here, generated a tsunami for which northern coastal areas were on alert for some hours.

The tsunami killed 183 people in Samoa, 34 in American Samoa and nine in Tonga.

There were five earthquakes outside the New Zealand region that were reported felt. The most significant, on March 20, was a shallow magnitude 7.6 quake centred in the Tonga Islands region, about 2000km northeast of Auckland, that was felt on the eastern side of the North Island and in Nelson.

From here.