Geological Summary for New Zealand Area, July 2006

Earthquake activity continued at a low rate, but an increase in magnitude has been apparent. Raoul Island, White Island, Mt Ngauruhoe and Mt Ruapehu remained at Alert Level 1.

Geonet, the USGS (NEIC) and GNS Science reported 36 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during July 2006. The magnitude distributions were as follows:
M6 to 6.9 none, M5 to 5.9 (7), M4 to 4.9 (22) M3 to 3.9 (7).

Deep earthquake activity in the subducting Pacific Plate continued during the month along a line from the Kermadec islands, under the North Island to the Tasman Sea west of Fjordland.

Earthquake activity in the Kermadec Islands was scattered across an area within 300 km of Raoul Island during July. The quakes ranged in magnitude between 4.6 and 5.3 at depths between 24 km and 285 km.

Further south, 7 events with magnitudes between 4.0 and 5.8 were reported along a line north-east of Rotorua at depths ranging between 116 km and 220 km. The magnitude 5.6 earthquake of July 2nd was felt along the eastern North Island and the magnitude 5.8 event of the 9th was felt at Gisborne, Napier, Palmerston North, and Whakatane according to Hazardwatch.

Three quakes with magnitudes between 4.0 and 4.4 struck at depths between 170 km and 240 km under the North Island to the south-west of Rotorua.

A magnitude 5.5 event occurred at a depth of 180 km on the 23rd. It was located 50 km east of Collingwood (150 km north-west of Wellington) and was felt throughout the lower North Island and upper South Island according to Hazardwatch.

Just over 12 hours later a magnitude 4.0 quake struck at a depth of 140 km off Fjordland. The quake was located 105 km west of Queenstown and, as it occurred very early in the morning, does not seem to have generated felt reports.

Shallower earthquakes were reported at scattered locations in both the North and South Islands during July.

Three quakes were reported for Hawke’s Bay between the 8th and 22nd, with magnitudes between 4.0 and 5.4 at depths between 30 and 50 km. The magnitude 5.4 event of July 8th was felt widely in Hawke’s Bay and the central North Island.

Two quakes struck off the Wairarapa coast on the 14th and 16th. The first was magnitude 5.2, 260 km east of Porangahau at a depth of 33km. The second was magnitude 4.1, 70 km east of Castlepoint at a depth of 30 km.

Two small events occurred east of Seddon on the 10th and 20th. The first was magnitude 4.1 at a depth of 9km; the second was magnitude 3.2 at a depth of 10 km. The first quake was felt in Marlborough and Wellington, the second was felt in Ward.

The northern part of Canterbury experienced two magnitude 3 earthquakes during July. Both were near Rangiora and locally felt.

Vulcanologists report that the nation’s volcanoes were quiet during July. Their status can be summarised as follows:
Raoul Island (Alert Level 1).
White Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 1).
Mt Ruapehu (Alert Level 1).

Raoul Island remained at Alert Level 1 throughout the month due to upwelling while springs remained active in Green Lake, despite the lake’s level slowly falling. Activity has declined slightly during July.

White Island remained quiet throughout most of the month, apart from an increase in the number of micro-earthquakes during the week to July 21st. The lake level rose under the influence of rain which also helped to erode a barrier stopping the lake flowing into a drainage channel.

Mt Ngauruhoe continued to experience volcanic earthquakes during July. The quakes, which are occurring about 1 km below the northern flank of the volcano, were recorded at a rate of 10-20 per day at the beginning and end of the month with a peak rate of 20-30 per day around the 21st.

Mt Ruapehu was quiet as the crater lake temperature cooled to 20 °C during the first half of the month. Several days of moderate volcanic tremor occurred during the last two weeks of July.

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

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