One big one gives the year a shake-up

If it had not been for Fiordland's 7.8 magnitude earthquake and aftershocks, last year would have been a quiet year for earthquakes in New Zealand, says a GNS Science summary of principal quakes for the year.

Forty-two 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 in New Zealand since the 1931 Napier quake struck remote Dusky Sound in Fiordland at a depth of 30km on July 16, and was followed by a serious of aftershocks.

"The Dusky Sound earthquake was interesting because it was an earthquake that occurred at the boundary of the two plates," said GeoNet data manager Kevin Fenaughty

"Normally earthquakes occur in one plate or the other but this has occurred where the two plates meet, and they are reasonably rare.

"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."

But the damage was much less than expected for an earthquake of its size as the rupturing caused by the quake moved upwards and to the south, directing most of the energy offshore. Walls were cracked and piles damaged 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, although scattered reports of feeling it came from as far north as Auckland, and from Sydney.

A small tsunami was detected and 1m waves measured at Jackson Bay in south Westland.

The biggest aftershock was a magnitude 6.1 that occurred 20 minutes after the main shock. There have been 30 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 big 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 felt strongly at the southern end of Lake Taupo.

These were the biggest of a swarm of several hundred earthquakes that began 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, but it was overtaken by the magnitude 8.1 Samoan quake on September 30, which 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.

- NZPA