Geological Summary for New Zealand Area, December 2007

Earthquake activity increased during December 2007. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

GeoNet, the USGS (NEIC) and GNS Science reported 53 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during December 2007. The magnitude distributions were as follows:
M7 to 7.9 (1), M6 to 6.9 (1), M5 to 5.9 (6), M4 to 4.9 (21) M3 to 3.9 (20).
An additional 4 events in the magnitude 2 range were deemed worthy of mention.

Gisborne residents were given a foretaste of what was to come later in the month, when a magnitude 7.8 earthquake struck north of Raoul Island on the evening of December 9th. The 150 km-deep quake was located 375 km north of Raoul Island and was felt throughout the North Island and as far south as Christchurch. On the East Coast, the quake was felt as a strong swaying motion which lasted long enough for people to react and head for doorways and other cover.

Only one aftershock, a magnitude 4.8 quake on the 17th, has been identified amongst the 8 other quakes ranging in magnitude between 4.7 and 5.2 that occurred throughout the Kermadec Islands during December.

A pair of quakes with magnitudes 4.3 and 4.0 struck 40 km north of Karamea on December 12th. The shallow quakes were at depths of 20 and 12 km.

On December 16th a pair of quakes struck 20 km south-west of Wanganui. The quakes with magnitudes 3.7 and 4.0 were at depths of 30 km.

Christmas shoppers in Gisborne had lucky escapes when a magnitude 6.8 earthquake struck just before 9 p.m. on December 20th. The off-shore quake was located 50 km south-east of Gisborne at a depth of 40 km and caused considerable damage. Buildings lost parapets, some partially collapsed and houses sustained damage. Fires were quickly brought under control and only a dozen minor injuries were reported. One death was associated with the quake, that of an elderly woman who had a heart attack during the 40-odd seconds of shaking that overturned furniture and filing cabinets and hurled contents from cupboards.

By the 6th of January 2008, disaster insurer EQC had received claims totalling over $13 million from the quake, with nearly $12 million coming from over 2,000 claims in the Gisborne area.

The Matata earthquake swarm recommenced activity after a quiet period in November. Thirteen quakes with magnitudes between 2.8 and 4.1 were reported at offshore locations between 10 and 20 km of the town during December.

Regular reporting of the status of New Zealand’s volcanoes ceased at the end of June 2007, with the closure of the Hazard Watch service. Only one Alert Bulletin was issued during December 2007, following continued earthquake activity near White Island. Minor changes to the volcano’s hydrothermal system were noted as well as weak volcanic tremor and small volcanic earthquakes. However, the changes were not significant, and the alert status for the volcano remained at 1.

At the end of December 2007, New Zealand’s active volcano status can be summarised as follows:
Raoul Island (Alert Level 0).
White Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 1).
Mt Ruapehu (Alert Level 1).

[Compiled from data supplied by GNS Science, US Geological Survey, GeoNet, and their contributing agencies.]

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