article: Jan 1 2008 - Principal earthquakes of New Zealand in 2007

Despite the occurrence of two earthquakes of magnitude over 6.5, 2007 was statistically speaking an average year for earthquakes in New Zealand, with 26 events of magnitude 5.0 or greater. A total of 504 events were reported felt through the GeoNet website during the year.

Felt earthquakes in 2007. Green are likely to have been felt, yellow had the potential to cause contents damage, red had the potential to cause structural damage.

The largest and most destructive earthquake in 2007 occurred on the evening of December 20 and was centred offshore, 50 km south-east of Gisborne. The magnitude 6.8 earthquake, the most damaging in New Zealand since the Edgecumbe earthquake in March 1987, was at a depth of 40 km and was felt from Auckland to Dunedin and on the Chatham Islands. At least three buildings collapsed in Gisborne city, and others suffered severe structural damage. Services, including electricity, gas, water and sewerage, were also affected. Because the earthquake was centred within the subducting Pacific plate, few aftershocks followed, with the largest being a magnitude 4.8. GeoNet received over 3,000 felt reports, and preliminary Earthquake Commission estimates for the cost of damage to residential property are around $30 million.

Early on October 16, a magnitude 6.7 earthquake centred 60 km west of Milford Sound caused landslides in the Milford Sound area, and minor to moderate damage, mostly goods falling from shelves, throughout the lower South Island. The depth of the event was 24 km, and it was caused by stresses due to the subduction of the Australian plate beneath the Pacific plate. There were several significant aftershocks, the largest being of magnitude 6.2, which also caused goods to fall from shelves. The Earthquake Commission reports the cost of claims so far at about $1.5 million, and there were close to 1,600 felt reports for the main event.

The largest deep earthquake, a magnitude 5.9 event on November 30, was also in the Fiordland region, centred 50 km north-west of Te Anau at a depth of 115 km. This was felt from Stewart Island to Christchurch, with a few scattered reports of goods off shelves.

Other widely felt earthquakes of interest include a magnitude 5.4 event on January 18, at a depth of 50 km and centred 20 km north of Taihape, which was felt throughout the lower North Island. On May 13, a magnitude 5.4 event near Nelson, with a depth of 90 km, was felt from Marton to Christchurch. On August 25, a magnitude 5.1 event at a depth of 50 km and centred 40 km west of Foxton, was felt from the King Country through to North Canterbury. On October 4, a magnitude 5.6 earthquake centred 40 km north of Hanmer Springs at a depth of 70 km, was felt from Hamilton to Ashburton, with over 2,230 felt reports submitted. And on November 7, a magnitude 5.4 earthquake, 60 km deep, centred 30 km south of Saint Arnaud, was felt from New Plymouth to Christchurch, with 930 felt reports received.

Four earthquakes located outside the New Zealand region were reported felt. The two most significant of these occurred on September 30 and were both shallow events centred about 400 km south-west of Stewart Island. The first had a magnitude of 7.3, the second four hours later was of magnitude 6.4. Both were felt from Stewart Island to Christchurch, with goods off shelves reported at several localities for both. There was a total of over 1,200 felt reports for these events.

Seismic swarm activity in the region close to Matata in the central Bay of Plenty, which began in January 2005, continued intermittently during 2007, with about 1,200 events of depth about 5 km. Six of magnitude 4.0 or greater were located there in the period from March to May. In late September and October there was another period of intense activity during which five events of magnitude greater than 4.0 occurred. The biggest event of the sequence was magnitude 4.7 on September 30, and was felt throughout the Bay of Plenty. A total of about 190 of these swarm events have been reported felt throughout the year. The activity is due to spreading of the earth’s crust in the region, gradually pulling apart at the rate of about one cm per year.

Unusual earthquakes throughout the year include three small shallow earthquakes centred in the Hauraki Gulf east of Orewa that occurred on February 21, the biggest of magnitude 4.5. These were felt widely throughout Auckland and the Coromandel Peninsula, with a few reports of items falling from shelves. Because of its location close to a major urban area, close to 2,000 felt reports were submitted for this relatively minor event.

Another small but unusual event, a magnitude 4.1 earthquake, 10 km deep and centred 20 km south-east of Dunedin, was felt throughout eastern Otago, with some reports of goods falling off shelves in Dunedin. A total of over 630 felt reports were received for this event.