Geological Summary for New Zealand Area, February 2007

Earthquake activity continued at a low level. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

Geonet, the USGS (NEIC) and IGNS reported 38 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during February 2007. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (4), M4 to 4.9 (9) M3 to 3.9 (16).
An additional 9 events in the magnitude 2 range were deemed worthy of mention.

The strongest on-shore earthquake for February struck 40 km east of Turangi on the morning of February 5th. The 60 km deep magnitude 5.3 quake was felt throughout the central and southern North Island and in Blenheim and Richmond. The earthquake’s epicentre was 50 km south of Lake Taupo where a magnitude 4.5 event struck at a depth of 150 km forty minutes earlier.

Two days later, residents of the southwest North Island, Nelson and Marlborough felt the effects of a magnitude 4.8 earthquake centred 30 km west of Porirua at a depth of 50 km early on the afternoon of Wednesday 7th. Goods were thrown from shelves in Johnsonville, Lower Hutt and Porirua.

The earthquake swarm activity near Matata continued at two centres near the town during February. Three shallow quakes with magnitudes between 2.9 and 3.7 struck 10 km north of Matata while six earthquakes with magnitudes between 2.5 and 3.0 struck within 5 km of the town. Another centre of activity developed with a magnitude 3.4 quake centred 10 km north-east of Matata. All of the quakes, which occurred between the 8th and 28th, were at depths of 7 km or less. The larger earthquakes in the swarm were felt at Edgecumbe, Thornton and Whakatane as well as at Matata itself.

Four earthquakes were reported in the Kermadec Islands, three within 170 km of L’Esperance Rock and one 70 km east of Raoul Island. The quakes, which were at depths between 10 km and 640 km, ranged between magnitude 4.8 and 5.5.

Aucklanders were shocked by an unexpected series of three earthquakes on Wednesday February 21st. Earthquakes near Auckland are not common, but in other respects the sequence seems to have been a classic foreshock-mainshock-aftershock sequence. The quakes were all centred 40 km north-east of Auckland, in the Hauraki Gulf. The magnitude 3.7 foreshock struck at 8:24 p.m. and was followed by the magnitude 4.5 mainshock 36 minutes later. The sequence was completed by a magnitude 3.8 aftershock at 11:23 p.m. The shallow nature of the earthquakes (6-15 km) meant that the quakes were felt by many in the Auckland area and Coromandel, with some reports of items being shaken from shelves.

Bay of Plenty residents felt another swarm of very shallow earthquakes which struck on the 25th and 26th of February. The four quakes which had magnitudes between 2.5 and 3.8 were centred 10 km north of Whakatane at depths of 5 km.

Residents of Wellington, Marlborough and Nelson felt their third quake for the month when a magnitude 4.5 quake struck near French Pass in the northern Marlborough Sounds just before midday on Monday February 27th. The 110 km deep quake was centred 50 km north of Picton, and was felt as far away as Christchurch.

Vulcanologists reported the nation’s volcanoes to be slightly more active during February 2007.

Raoul Island volcano remained quiet during the month, with minor hot water and steam discharges occurring in the Green Lake area.

White Island’s crater lake continued warming and evaporating during February. The lake hit a record temperature of 74°C prompting GeoNet to issue an alert bulletin on February 15th. With the lake level having dropped by 6 metres due to evaporation it was signalled that minor eruptive activity might occur at the volcano as pressure in the geothermal system changed. The volcano showed no sign of surface deformation, but weak volcanic tremor, which commenced during January, continued.

By the 23rd, the crater lake had fallen further, dropping 1.2 metres in 10 days, with the rate increasing. The lake was then at or below its 2004 level, having shrunk in volume by about 20%.

Mt Ngauruhoe became quieter during February with the rate of volcanic earthquakes dropping from its elevated rate to about 10-20 per day by the end of the month.

Mt Ruapehu’s crater lake rise stabilised during the early part of February when the input from snowmelt and rain was balanced by seepage through the tephra dam. Minor to moderate volcanic tremor also declined, remaining at a very low level during the latter part of the month. The lake’s temperature increased slightly and was last reported to be 28°C on the 14th. With the lake level stable at about 1.5 metres below the top of the debris dam it is uncertain when the anticipated Mt Ruapehu lahar flow will occur.

At the end of February, New Zealand’s 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, Hazard Watch and their contributing agencies.]

One Response to “Geological Summary for New Zealand Area, February 2007”

  1. Chris says:

    I’ve been wondering about the Lahar threat on Ruapehu lately – that threat seems to have died down…well, from a public perspective anyway. I would imagine the threat is still very much there….

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