Geological Summary for New Zealand Area, February 2006

Geonet, the USGS (NEIC) and IGNS reported 32 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during February 2006. The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (2), M4 to 4.9 (18) M3 to 3.9 (11).
An additional 4 events in the magnitude 2 range were deemed worthy of mention.

The first cluster of earthquakes commenced activity of the lower North Island’s west coast on February 1st. Four earthquakes between 40 and 50 km deep occurred off-shore from Porirua, Paraparaumu and Waikanae. Magnitudes ranged between 3.3 and 3.8, with the last event occurring on the 9th.

A pair of earthquakes struck further north near Weber in Wairarapa. The first was magnitude 4.0, 50 km deep on the 3rd; the second was magnitude 4.4, 40 km deep on the 5th.

A cluster of four earthquakes with magnitudes between 4.0 and 5.1 struck within 35 km of Rotorua between the 6th and 18th. All were deep, ranging between 158 and 205 km in depth.

A cluster of three shallow events occurred halfway between the Bay of Plenty and the Kermadecs during the latter half of February. Magnitudes ranged between 4.2 and 4.5 and all were shallow at between 6 and 33 km depth. Activity continued into March with a 4th event (mag 4.2) in the area.

Toward the end of the month, southern Hawke’s Bay hosted two additional members of the loose cluster of three earthquakes that occurred between Hastings and Waipukurau during January. The first was a magnitude 4.0 event on the 21st, followed by a magnitude 3.2 shake 3 days later. All five earthquakes in this lengthy sequence have been between 20 and 50 km deep.

The only swarm of earthquakes during February occurred near Taupo. Geonet gave few details, but the largest member of the swarm, at magnitude 1.8 and only 5 km deep, was felt in Taupo during the early hours of Monday 20th.

February’s largest earthquake was a deep magnitude 6.0 event 80 km north-east of Collingwood on the 16th. Its depth of 180 km and close proximity to the lower North and upper South Islands resulted in it being felt from Hamilton to Christchurch. Geonet received a few reports of items falling from shelves around Wellington and Marlborough.

One of the magnitude 5 events occurred near the Kermadecs, while the other was deep under Rotorua on February 18th – probably too deep to have been felt.

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).

Mt Ruapehu was reported to be seismically quiet at the beginning of the month. The crater lake temperature rose to 30°C and then dropped back slightly to 28°C by month’s end . Volcanic tremor began increasing during the last week of February.

Emissions of sulphur dioxide were reported to be rising at White Island at the beginning of February, but seismic activity remained low. By mid-month, seismic activity had risen slightly and a team visiting the volcano found minor landslides probably due to a storm on the 10th.

The pattern of frequent small earthquakes continued at Mt Tongariro.

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