article: Jan 16 2003 - Big Quakes Distant and Deep - 2002 Another Lucky Year

New Zealand got by last year without any damaging earthquakes mainly because the major jolts were distant or occurred deep underground.

Felt earthquakes in 2002. Green are likely to have been felt, yellow had the potential to cause contents damage, red had the potential to cause structural damage.

The three largest quakes were centred offshore, the Institute of Geological and Nuclear Sciences Limited (GNS) said.

The largest, a magnitude 7.7 quake on 17 August in the Fiji region, was felt in some parts of Auckland as a swaying motion. Two other offshore earthquakes of greater than magnitude 6 were felt in the North Island.

A magnitude 6.5 quake on September 26 at 1:33 am, located 530 km north of East Cape was felt over much of the North Island. On October 24 at 8:15 am a magnitude 6.6 earthquake located 800 km north-east of White Island was felt on the east coast of the North Island.

GNS seismologist Mark Chadwick said New Zealand had experienced nearly two decades without a seriously damaging earthquake.

Longterm averages indicated that sooner or later a large, shallow earthquake would occur under or near a populated area and cause damage, he said.

In terms of the number, size, and distribution of earthquakes last year was an average one. Earthquakes above magnitude 5 could result in significant damage to buildings and cause injuries and deaths in built-up areas, Dr Chadwick said.

As in the previous few years, the great depth of several onshore earthquakes in 2002 prevented any serious damage. On August 17 at 8:21 pm a magnitude 6 earthquake located at a depth of 150 km occurred near Whakatane and was widely felt in the North Island.

Several other deep earthquakes greater than magnitude 5 occurred under the North Island, but were only mildly felt because of their depth. In Fiordland, where deep earthquakes also occur, the largest quake during 2002 was of magnitude 5.6 and occurred on 10 May at 7:25 pm at a depth of 140 km.

The North Island's east coast, as in previous years, was the scene of considerable earthquake activity.

The largest was a magnitude 5 quake, which occurred on 11 April at 1:04 pm 80 km east of Tokomaru Bay. A series of mainly smaller but more numerous earthquakes occurred in the Gisborne region. The largest of these occurred on 15 January at 12:19 am and was of magnitude 4.7.

Most of these quakes occurred at depths between 30 and 60 km. Another group of earthquakes occurred along the Wairarapa coast. The largest occurred on 17 October at 2:49 pm and was located 60 km south of Waipawa at a depth of 20 km, measuring magnitude 4.8.

As usual many smaller, but shallow earthquakes were felt in the central volcanic region of the North Island. The largest of these was a magnitude 4.8 event, which occurred 30 km south-west of Taupo on 16 April at 10:13 pm.

In the South Island, earthquake activity continued near the location of the December 2001 magnitude 6.1 Jackson Bay earthquake on the West Coast. The largest of these quakes was a magnitude 5.8 and occurred 50 km south-west of Haast on 24 February at 7:37 pm.

Earthquake activity also continued near Hanmer Springs, where the largest earthquake occurred 30 km north of Hanmer Springs on 30 August at 8 pm and measured magnitude 4.8.

There was also earthquake activity on the West Coast of the South Island near Karamea, the largest of which was a shallow magnitude 5.5 quake located 20 km south-east of Karamea on 5 May at 1 am.

An earthquake was felt on Stewart Island on 3 January at 9.50 pm, even though it was located 360 km south-west of the island and had only a moderate magnitude of 5.

During December two moderate earthquakes were felt in central New Zealand. The first was of magnitude 5 and occurred on December 7 at 7 pm and was located 10 km north-west of Waikanae at a depth of 40 km.

The second occurred on Christmas Eve at 8:42 pm, was of magnitude 5.1 and was located 40 km south of Seddon at a depth of 15 km. This earthquake was preceded by a magnitude 3.5 foreshock, and followed by a series of small aftershocks.

GNS records about 16,000 earthquakes in New Zealand each year, although only about 1 percent are big or shallow enough to be felt by humans.

GNS is into the second year of a 10-year, $50 million programme to upgrade the equipment that monitors earthquakes and volcanoes in New Zealand. Called GeoNet, the upgrade will provide more accurate and more rapid information about earthquakes, as well as early signs of volcanic unrest. The project is funded by the Earthquake Commission (EQC).