Geological Summary for New Zealand Area, April 2006

Geonet, the USGS (NEIC) and GNS Science reported 61 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during April 2006. The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (21), M4 to 4.9 (30) M3 to 3.9 (7).
An additional 2 events in the magnitude 2 range were deemed worthy of mention.

Earthquake activity in the Kermadec Islands dominated the earthquake listings during the early part of April 2006, at a time when mainland New Zealand was seismically quiet.

There were 36 earthquakes within 140 km of Raoul Island between April 1st and 23rd. Most were shallow at 10 km depth, but events in the sequence ranged between 2.8 and 72 km deep, with magnitudes between 4.5 and 6.5.

The deep earthquake activity under the North Island and Bay of Plenty continued at a slightly reduced rate, with 5 events between the 8th and 23rd. The earthquakes were centred between 105 km south-west of Rotorua and 410 km north-east of the city. Magnitudes ranged between 4.1 and 5.3 with the quakes occurring at depths between 160 km and 250 km.

A pair of earthquakes of magnitude 4.2 and 4.1 occurred at the shallow depth of 5 km at a location 170 km NNE of Rotorua on the 22nd and 23rd.

The deep earthquake activity near Cook Strait continued with another 4 events during April. The earthquakes ranged in magnitude between 3.8 and 4.7 and occurred at depths between 40 km and 137 km. The quakes were within 140 km of Wellington with an additional shallow (30 km) earthquake of magnitude 3.3 striking 20 km northwest of Wellington on the 17th.

A pair of very shallow earthquakes occurred near Harhari in the South Island on the 27th and 28th of April. The earthquakes were magnitude 4.1 at depths of 7 and 10 km.

Vulcanologists report the nation’s volcanoes to be quiet. Their status can be summarised as follows:
Mt Ruapehu (Alert Level 1).
White Island (Alert Level 1).
Mt Tongariro (Alert Level 0).
Raoul Island (Alert Level 1)

Mount Ruapehu was quiet throughout the month, with seismic activity reported to be at a very low level. The crater lake has cooled to about 25°C, with rain causing the water level to slowly rise to 1.5 metres above the bedrock at the base of the dam, about 6 metres below the top, according to GNS Science.

White Island was also quiet with a few very small local earthquakes. The crater lake level continued dropping during April and was about 2.2 metres below overflow with a temperature of 58°C at the end of the month.

Mt Tongariro, which exhibited increased earthquake activity earlier in the year, has faded from website reports.

Raoul Island in the Kermadecs, which erupted on March 17th claiming one life, was featured in two alert bulletins during April. The island was officially classified as a frequently active volcano in an alert bulletin from Brad Scott, Volcano Surveillance Co-ordinator for GNS Science on April 3rd.

By the 7th of April, seismic activity at the volcano had declined, and Green Lake in the main crater stopped rising. GNS Science reported that seismic activity had returned to normal on the 13th and Green Lake had started falling. On the 21st, it was reported that hydrothermal activity at the island had declined significantly and an alert bulletin on the 26th reduced the volcano’s alert status from 2 to 1. At month’s end, the level of Green Lake was reported as continuing to fall.

Some of the Department of Conservation staff who were evacuated from Raoul Island when the volcano erupted have returned to the island. However, an exclusion zone exists within the main vent, and volcanic observations are being carried out from vantage points overlooking the area.

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