Geological Summary for New Zealand Area, October 2006

Earthquake activity continued at a low but steady level during October 2006. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

Geonet, the USGS (NEIC) and IGNS reported 33 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during October 2006. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (3), M4 to 4.9 (6) M3 to 3.9 (21).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.

Earthquake activity in the Kermadec Islands was substantially lower than in recent months, with only 2 events being reported more than 100 km south of Raoul Island. The magnitude 4.8 and 5.0 quakes were at depths of 230 km and 35 km respectively.

Between the 2nd and 4th of October, a swarm of shallow earthquakes struck near the town of Fox Glacier. The 10 earthquakes ranged in magnitude between 2.7 and 4.7 at depths between 5 and 10 km. Most of the earthquakes were felt in the nearby area which is 30 km north-west of Mt. Cook.

The swarm was followed by a cluster of four earthquakes in the Fjordland area. The quakes, which struck at different locations in an area west and south of Te Anau, ranged in magnitude between 3.7 and 3.9 at depths between 5 and 12 km. The sequence ran from the 4th to the 22nd of October.

Earthquakes struck north and south of the Bay of Plenty town of Kawerau between the 13th and 29th. All of the earthquakes, which had magnitudes between 3.1 and 3.8, were within 30 km of the town at depths between 10 and 30 km.

Other earthquakes struck at widely dispersed locations in both the North and South Islands from Morrinsville to Ranfurly.

The magnitude 5.4 quake which struck at a depth of 240 km, 50 km north-east of Collingwood just after 11 p.m. on the 9th of October was widely felt in the northern South Island and southern North Island.

The southern ocean was quiet except for a magnitude 5.8 earthquake which struck at a depth of 10 km on October 16th. The epicentre was 265 km west of Auckland Island.

Vulcanologists reported that most of the nation’s volcanoes were quiet during October 2006. Their 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).

Activity at Raoul Island continued its decline during October 2006, with a low level of hydrothermal activity occurring in the Green Lake area.

Seismic activity at White Island was low during the month. The crater lake was 2-3 metres below overflow at the beginning of the month, and dropped further by month’s end.

Mt. Ngauruhoe continued to experience about 20 to 30 small earthquakes during October, activity that is significantly higher than normal for the volcano. However, measurements of temperature and volcanic gas concentrations near the summit remained normal. The increased activity within 1 km of the surface on the volcano’s northern flank began in May, with the largest earthquakes reaching magnitude 1.

Following a brief period of volcanic tremor late in September, Mt. Ruapehu experienced a moderate volcanic earthquake of magnitude 2.8 at 10:27 p.m. on October 4th, triggering the lahar warning system. A minor eruption was suspected, but bad weather prevented vulcanologists visiting the volcano until the 7th.

Observations showed that the crater lake had abruptly risen by a metre, and its temperature had climbed 15 °C to 22.5 °C. The minor eruption was confirmed with evidence that waves had risen 5 metres above the lake surface, but not high enough to flow over the tephra dam.

Activity then eased but, as the month progressed, the crater lake temperature rose to reach 27 °C by the end of the month. Seismic activity remained normal apart from minor tremor and 3 volcanic earthquakes which were recorded during the week ending the 27th.

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

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