Geological Summary for New Zealand Area, May 2006

Earthquake activity lower than April. Raoul Island, Ruapehu, White Island remain at Alert Level 1. Change at Mt Ngauruhoe.

Geonet, the USGS (NEIC) and IGNS reported 29 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during May 2006. The magnitude distributions were as follows:
M7 to 7.9 (1), M6 to 6.9 (none), M5 to 5.9 (5), M4 to 4.9 (13) M3 to 3.9 (9).
An additional event in the magnitude 2 range was deemed worthy of mention.

A major earthquake located outside the area normally covered by this report was felt in several locations early in May. The magnitude 7.9 earthquake struck at 3:26 a.m. NZ Time on Thursday May 4th, and was located 160 km north-east of Nuku’alofa, Tonga. The earthquake was felt at Auckland, Whakatane, Minginui, Napier, Wanganui and Wellington according to Geonet.

The size and location of the earthquake triggered a precautionary tsunami warning for parts of New Zealand’s coast, but this was withdrawn at 5:36 a.m., less than an hour before its anticipated arrival time at Gisborne.

The earthquake heralded movement on the interface between the Pacific and Australian plates, and was followed by a magnitude 7.4 quake located 290 km south-south-west of Raoul Island at 10:39 on the evening of Tuesday May 16th. This event was felt widely in the North island and as far south as Dunedin and in the Chatham Islands. In Gisborne, people reported a pronounced swaying movement which lasted almost a minute.

Geonet estimated that this quake could have been magnitude 7.6, and reported items falling off shelves at scattered localities in the North Island.

A magnitude 5.0 earthquake located 50 km west of Paraparaumu on May 15th was the only other event during May which caused items to topple from shelves. Geonet reported that this 5:32 a.m. quake was felt in the lower half of the North Island and in the upper South Island, with items falling off shelves in the Wellington and Marlborough regions.

A series of 4 shallow earthquakes occurred near Whakatane, White island and Tauranga during May. The quakes ranged between magnitude 2.8 and 3.6 and occurred at depths between 5 and 12 km.

Three earthquakes of magnitude 3.6 to 5.0, with depths ranging between 30 and 60 km struck with 50 km of Wellington between May 6th and 15th.

A cluster of 3 earthquakes occurred at varying depths within 70 km of Milford between May 9th and 30th. The quakes ranged between magnitude 4.0 and 4.4 and were located at depths between 12 and 130 km.

Three events in the southern part of Hawke’s Bay were recorded between May 20th and 29th. The earthquakes were between magnitude 3.1 and 4.1, at depths between 10 and 40 km.

The earthquakes which have been occurring deep under the North Island continued at a reduced rate during May. The earthquakes were between magnitude 4.0 and 4.4 at depths between 145 and 165 km along a line from 10 km ESE of Rotorua to 20 km west of Wanganui.

Earthquake activity near Raoul Island was much reduced when compared with data supplied for April. Eight earthquakes scattered across an area within 470 km of Raoul were reported during May. Of these, 4 quakes at depths between 32 and 410 km were reported at locations between 75 and 470 km south-west of the island. The magnitudes of these events ranged between 4.6 and 7.4.

Vulcanologists reported the nation’s volcanoes to be quiet during May 2006. Their status at month’s end can be summarised as follows:
Mt Ruapehu (Alert Level 1).
White Island (Alert Level 1).
Raoul Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 0).

Mt Ruapehu was reported to be seismically and geothermally quiet until the 26th when weak volcanic tremor was reported, and the crater lake’s temperature was noted to have risen to 29°C. The lake level rose to about 1.7 metres above the bedrock at the base of the debris dam as a result of rainfall, but was still about 6 metres below overflow.

White island was very quiet during May, with its lake level continuing to fall.

Raoul Island was also quiet during May with the level of Green Lake slowly falling but still a few metres above its level before the March 17th eruption.

Activity at the Tongariro/Ngauruhoe complex increased mid-May, with the number of low frequency earthquakes near Mt Ngauruhoe increasing from less than 5 to more than 20 per day by the end of the month. The earthquakes appeared to be occurring at depths of about 4 km and prompted closer investigation.

Leave a Reply