Geological Summary for New Zealand Area, March 2007

Earthquake activity continued at a low level, apart from a swarm of quakes near Matata. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

Geonet, the USGS (NEIC) and IGNS reported 58 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during March 2007. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (4), M4 to 4.9 (12) M3 to 3.9 (32).
An additional 10 events in the magnitude 1 and 2 range were deemed worthy of mention.

The swarm of earthquakes striking within 10 km of the Bay of Plenty town of Matata dominated earthquake reports during March 2007. During the month, 30 quakes with magnitudes between 1.9 and 4.1 struck at depths between 1 and 8 km. This marked a dramatic increase in the swarm’s activity which had been occurring since December 2006.

Apart from the Matata swarm, the remaining 28 earthquakes were spread across the reporting region. A series of 5 earthquakes with magnitudes between 4.7 and 5.7 were reported within 180 km of Raoul Island at depths between 35 and 130 km.

A pair of quakes with magnitudes of 3.7 and 3.5 struck 10 north-west of Carterton on March 19th and 21st at depths of 15 and 20 km.

A series of 3 quakes struck within 30 km of Karamea between March 4th and 6th. The quakes ranged in magnitude between 3.2 and 4.7 and were at depths between 20 and 30 km.

A pair of earthquakes struck 60 and 80 km west of Te Anau on March 4th and 11th. The quakes, with magnitudes 4.5 and 3.7 were at depths of 70 and 10 km.

The month’s most unusual earthquake was a man-made event reported to the Geonet “felt earthquake” programme by several members of the public. The quake which registered as magnitude 2.0 was recorded on March 2nd at a depth of 0 km, and resulted from a quarry blast near Horokiwi. Felt reports were sent in from Wellington, Newlands and Petone.

No seriously damaging quakes were reported, but three events caused items to fall off shelves. The magnitude 4.7 quake near Karamea on March 6th generated reports of items off shelves at diverse locations, according to Hazardwatch. The following day, the magnitude 4.6 quake 20 km west of Palmerston North generated several reports of items off shelves around the Manawatu region.

The magnitude 4.1 quake in the Matata swarm which struck on March 30th generated reports of items falling off shelves at Matata, Pikowai and Thornton. This quake was the largest of the swarm of 30 reported during the month.

Vulcanologists reported the nation’s volcanoes to be lightly active during March 2007.

Raoul volcano experienced low levels of hydrothermal activity and remained at Alert Level 0.

White Island’s crater lake remained at 74 ºC during March, as the hot lake continued to evaporate. Steam plumes were seen above the island, but no eruptive activity occurred. Minor tremor occurred throughout most of the month.

Mt Ngauruhoe continued to exhibit small volcanic earthquakes during March. Earthquake rates were less than 20 per day for most of the month, but rose to 20-30 per day again by the month’s end.

Mt Ruapehu became the centre of attention during March, when the long-anticipated lahar flowed from its crater lake. Volcanically, the crater lake remained warm at 28 ºC during the early part of the month, with seismic activity at a low level. Following the tephra dam collapse and lahar on March 19th, the crater lake dropped to 24 ºC but sulphur slicks were seen in combination with increased seismic activity immediately after. It was thought that increased hydrothermal activity might result from the lake emptying, but these concerns were allayed when activity soon returned to pre-lahar levels, with the crater lake flowing freely through a 40 metre-wide breach in the tephra dam.

Initial analysis indicated the lahar was smaller than the 1953 Tangiwai flow but, as more information came to hand, the 2007 Ruapehu lahar was declared as a larger event in terms of volume. Thorough management by the Department of Conservation, GNS Science, local and regional councils, Police and other government agencies dramatically limited damage. The debris flow was kept within natural or man-made channels, and minimal public inconvenience resulted. Careful planning also allowed many aspects of the dam collapse and debris flow to be recorded with cameras and scientific instruments for later study.

The decision to allow the lahar to occur naturally after the tephra dam built up during the 1996 Ruapehu eruption was highly contentious. However, on the day, those charged with seeing the eventual debris flow to its conclusion achieved a highly commendable outcome.

At the end of March, 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, Hazard Watch and their contributing agencies.]

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