Geological Summary for New Zealand Area, December 2008

Earthquake activity continued at an increased level. White Island and Mt. Ruapehu remained at Alert Level 1, Mt. Ngauruhoe was reduced to Alert Level 0.

GeoNet, the U.S. Geological Survey and GNS Science reported 51 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during December 2008.

The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (13), M4 to 4.9 (11) M3 to 3.9 (19).
An additional 7 events in the magnitude 2 range were deemed worthy of mention.

December the 18th was a busy day for seismologists, with a pair of shallow fourth magnitude quakes near Hanmer Springs quickly followed by a deep magnitude 5.8 event near White Island. A fourth magnitude quake then struck near Murupara mid-morning. During the evening hours earthquakes of third magnitude were recorded near Porirua and Hastings.

As usual, most activity occurred in the Kermadec Islands, where a dozen earthquakes were reported by the U.S. Geological Survey. Activity near Raoul ranged between 355 km north of the volcanic island and 220 km south. Five quakes with magnitudes between 4.8 and 5.6 at depths between 10 and 100 km were reported.

The southern part of the Kermadec chain was more active with 7 quakes with magnitudes between 4.7 and 6.8 recorded within 215 km of L’Esperance Rock. The largest event was the magnitude 6.8 earthquake located 190 km east of L’Esperance Rock on the evening of December 9th. The quake was only 14 km deep.

Residents of the East Coast felt tremors on the 12th with an 8 km-deep magnitude 4.0 quake near Ruatoria just after midday and a 30 km-deep magnitude 4.4 quake near Gisborne at 7:45 that evening.

A deep magnitude 5.3 quake located 30 km north-east of Te Puke on December 2nd was felt as far south as Blenheim but did not attract a lot of public attention. Only 6 felt reports were lodged with the GeoNet site following this early evening event

The magnitude 5.8 earthquake located 80 km north-east of White Island on December 18th was felt at many locations in the Bay of Plenty and at Gisborne, Hawke’s Bay, Palmerston North and Wellington. The quake, which struck at 9:33 in the morning, was 130 km deep.

Another moderately strong earthquake struck the Bay of Plenty area the following afternoon. The magnitude 5.1 quake at 1:21 p.m. on the 19th of December was located 20 km south-west of Rotorua and attracted 464 felt reports. Shaking was felt from Whangamata to Nelson, strongly at Pahiatua and Wanganui.

Seven more earthquakes were felt as the Matata earthquake swarm continued. Magnitudes ranged between 2.7 and 4.1 with all the events at depths of 5 km or shallower.

A magnitude 5.1 quake which struck under the Tasman Sea 40 km west of Levin at 8:49 a.m. on the 27th was widely felt in the lower North Island and upper South Island. The 50 km-deep earthquake was felt strongly at several locations in Horowhenua, Kapiti Coast and Wellington, attracting nearly 1100 reports from the public. Reports flowed in from the Bay of Plenty to Taranaki and as far south as Banks Peninsula, with one report being filed from Birkenhead in Auckland.

Residents of the spa town of Hanmer Springs had an unpleasant start to the day on December the 18th with two strong tremors, seventeen minutes apart. The shallow magnitude 4.9 quake at 8:55 a.m. was only 12 km deep, allowing it to cause light damage at Hanmer. 145 felt reports were lodged with GeoNet, with the shaking being felt from Banks Peninsula to Wellington and on the South Island’s West Coast.

The quake was located 10 km north of Hanmer Springs, with a slightly shallower magnitude 4.1 aftershock at 9:12 a.m. being located even closer – within 5 km of the town.

Residents of Haast experienced five shallow earthquakes during December. The quakes ranged between magnitude 3.1 and 4.9 and were at depths between 5 and 15 km. The largest event was the shallow magnitude 4.9 earthquake located 10 km north-east of Haast on the evening of December 5th. This quake was felt along the western coast of the South Island and in Otago.

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. GNS Science now only issues bulletins which record significant changes in volcanic behaviour.

One Alert Bulletin was issued by GNS Science during December 2008, announcing that the alert level for Mt. Ngauruhoe had been lowered from 1 to 0. During the latter half of the year, volcanic earthquake activity at the central North Island volcano had declined to a background level of a few events per week. Measurements of volcanic gases taken during the period of elevated earthquake activity had not shown any significant change, therefore it was considered that the volcano was no longer showing signs of unrest.

At the end of December 2008, 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 0).
Mt Ruapehu (Alert Level 1).

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

One Response to “Geological Summary for New Zealand Area, December 2008”

  1. Darren says:

    Well done!
    Even better than GeoNets summary!

Leave a Reply for Darren