Geological Summary for New Zealand Area, November 2006

Earthquake activity continued at a low but steady level during November 2006, with an increase in the number of magnitude 4 and 5 events compared with October.

White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1, Raoul Island remained at Alert Level 0.

Geonet, the USGS (NEIC) and IGNS reported 31 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during November 2006. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (6), M4 to 4.9 (13) M3 to 3.9 (10).
An additional 2 events in the magnitude 2 range were deemed worthy of mention.

A series of 4 earthquakes in the Kermadec Islands commenced on November 1st. Epicentres marched steadily south and focal depth increased as the earthquakes of magnitude 5.0, 4.6, 4.8 and 5.3 struck.

A magnitude 4.4 quake 30 km west of Hastings on the 3rd was followed by a swarm of smaller earthquakes nearby. The quakes of magnitude 3.9, 4.0 and 3.7 struck 40 km south of Hastings on the 8th and 22nd.

Deep earthquake activity near Rotorua resumed during November. Four quakes with magnitudes between 4.0 and 5.6 struck between the 11th and 29th.

A series of 3 earthquakes trending northward through northern Canterbury struck between the 18th and 22nd. The first event was a magnitude 3.5 quake near Rangiora, 30 km north of Christchurch. It was followed by a magnitude 4.3 quake 70 km northwest of Christchurch and a magnitude 4.1 near Hanmer.

North and South Island areas near Cook Strait experienced a series of four earthquakes of increasing strength and depth between November 13th and 18th. The first two events were shallow with a magnitude 2.9, 11km deep event 10 km south-east of Wellington followed by a magnitude 3.5, 30 km deep event near Tawa. Activity continued moving westwards with a magnitude 4.6, 50 km deep quake followed by a magnitude 5.6, 70 km deep event, both located in the Marlborough Sounds. The magnitude 5.6 quake, which was widely felt, was centred 40 km north-east of Picton and caused goods to tumble from shelves in some locations.

The south-western South Island was quiet, with only one magnitude 4.3 event near Te Anau on the 16th. However, this was preceded by a magnitude 5.5 event 260 km west of Auckland Island.

New Zealand experienced small tsunami waves from a magnitude 8.1 earthquake in the Kuril Islands north of Japan during November. The Pacific Tsunami Warning Centre issued a bulletin after the quake stating that a Pacific-wide tsunami was unlikely, but sea-waves from the earthquake bounced around the Pacific Ocean for several days.

Hazardwatch advise that Hawaii and parts of the California Coast in the United States, where waves reached about 1.5 metres, experienced strong surges. Some damage was sustained, totalling about US$1 million, whilst small tsunami waves were recorded throughout the Pacific Ocean.

Hazardwatch also reported that the first wave reached New Zealand within 14 hours of the earthquake, but the largest waves struck 54 hours after the quake, peaking at more than half a metre, as large volumes of water ebbed and flowed every 10-15 minutes. On the 18th of November, a 58 cm wave was recorded at Timaru, illustrating that the waves had been reflected from land masses in the Pacific Ocean to come ashore on the opposite side of the South Island from where a direct wave might have been expected to land.

Vulcanologists reported the nation’s volcanoes to be quiet during November 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).

Following its eruption earlier in the year, activity at Raoul Island has returned to a low level. Throughout November hydrothermal activity in Green Lake has continued a slow decline, and fumarole activity and up-welling in the lake is very minor.

Seismic activity at White Island was low, and the hot lake’s level steadily declined during November as the water temperature rose to 60 °C.

Earthquake activity at Mt. Ngauruhoe continued at its elevated level throughout the month, with the size and number of quakes relatively constant at 10-20 events per day.

Seismic activity at Mt. Ruapehu was normal during November. The crater lake temperature declined by about 6 °C during the month to 21.8 °C, as the water level rose by about half a metre. During the latter half of November, micro-earthquakes were detected and were probably caused by ice cracking as it thawed.

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

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