Geological Summary for New Zealand Area, October 2007

Earthquake activity increased during October. White Island and Mt. Ngauruhoe remained at Alert Level 1, Mt. Ruapehu was reduced to Alert Level 1.

Geonet, the USGS (NEIC) and GNS Science reported 55 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during October 2007. The magnitude distributions were as follows:
M6 to 6.9 (4), M5 to 5.9 (12), M4 to 4.9 (18) M3 to 3.9 (15).
An additional 6 events in the magnitude 2 range were deemed worthy of mention.

Earthquake activity along the plate boundary from Fiordland to Macquarie Island in the Southern Ocean increased during October 2007. Two distinct series of events, one located near the Auckland Islands the other near Milford Sound, contributed most of the earthquake and damage reports.

Activity near the Auckland Islands began at the end of September, with magnitude 7.3 and magnitude 6.8 earthquakes on the 30th. These quakes were located nearly 500 km south-west of Invercargill, 200 km north-west of Auckland Island. Three aftershocks with magnitudes between 4.9 and 5.4 were reported on the 2nd and 12th of October before activity waned.

A new area of activity commenced on October 16th, with a magnitude 6.7 quake located 60 km west of Milford Sound. By October 25th, 13 more quakes with magnitudes between 4.1 and 6.2 had been reported in the area, attracting insurance claims totalling more than $1.8 million from the southern part of the South Island.

Also in the South Island, a pair of 70 km deep quakes struck near Hanmer Springs on October 4th and 7th. The magnitude 5.6 and 4.7 quakes did not cause serious damage.

Three events with magnitudes between 2.8 and 5.1 were reported near Reefton toward the end of the month. The mainshock-aftershock sequence began with the magnitude 5.1 event on October 24th, and all of the quakes were at depths of 9 km or less.

In the North Island, a short swarm of very shallow quakes struck 30 km south-west of Taupo between the 1st and the 10th. The quakes, at depths of 5 km or less, had magnitudes between 2.6 and 4.5.

A magnitude 5.7 quake located 20 km south of Tauranga on October 4th was felt in Christchurch, Nelson and Wellington, but was only lightly felt in Tauranga because of its great depth of 280 km.

A further 7 events in the Matata earthquake swarm were reported during October 2007. The quakes, all at depths of 5 km or less, had magnitudes between 3.0 and 4.0.

A 220 km-deep quake of magnitude 5.0 near Raetihi on October 25th, was followed by two smaller events of magnitude 3.5 and 3.4 at depths of 15 km on the 30th and 31st. The wide variation in depth between the larger and smaller events mean that this was not a normal mainshock followed by aftershocks, but symptomatic of stresses in the area changing after nearby earthquakes.

Earthquake activity in the Kermadec Islands was centred 450-530 km north-west of Raoul where four events with magnitudes between 4.8 and 6.6 were reported. All of the quakes were very deep, at depths of 480 km or more.

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.

Two alert bulletins were issued for Mt. Ruapehu during October, following the eruption of September 25th and a volcanic earthquake on September 29th. By October 9th, investigation showed that the crater lake temperature had risen 6 ºC to 19 ºC and there had been a small rise in background volcanic tremor. Gas levels had changed slightly, with sulphur dioxide unchanged but carbon dioxide slightly increased.

As activity appeared to be declining, the alert status of Mt. Ruapehu was reduced to 1 on October 9th.

At the end of October 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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