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Big finish to the earthquakes of 2007 - 09/01/2008

Whilst most of the large earthquakes of 2007 were distant from populated centres or too deep to cause harm, the big shake of December 20 was close enough to cause significant damage in Gisborne city.

Earthquakes of 2007 map

It occurred at 8:55 in the evening and was the most damaging in New Zealand since the Edgecumbe earthquake in March 1987. The magnitude 6.8 earthquake was centred offshore, 50 km south-east of Gisborne, at a depth of 40 km and was felt from Auckland to Dunedin and also 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.

The other major event of the year occurred early on October 16, a magnitude 6.7 earthquake centred 60 km west of Milford Sound. It 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 quake 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.

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.

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 earthquakes at a depth of 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 of magnitude greater than 4.0 occurred. The biggest earthquake 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 centimetre per year.

Three small shallow earthquakes centred in the Hauraki Gulf, east of Orewa, 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.

GNS Science seismologist, Brian Ferris, said that despite the occurrence of two earthquakes of magnitude over 6.5, 2007 was statistically speaking an average year for earthquakes in New Zealand.

“There were 26 earthquakes of magnitude 5 or greater, and a total of 504 events were reported felt via the GeoNet website during the year. Whilst that is about twice the average number, many of these ‘extra’ quakes were part of the ongoing Matata swarm.”