Geological Summary for New Zealand area, December 2005

Geonet, the USGS (NEIC) and IGNS reported 48 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during December 2005. The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (6), M4 to 4.9 (27) M3 to 3.9 (14).
An additional 5 events in the magnitude 2 range were deemed worthy of mention.

There were 2 earthquake swarms, 2 foreshock sequences, and 2 areas of complex activity involving clusters of earthquakes and associated foreshock sequences. Two earthquakes caused minor damage in the North Island.

A complex series of deep earthquakes near the Bay of Plenty began on December 2nd with a magnitude 4.1 earthquake SSW of Rotorua at a depth of 155 km which had an associated aftershock of magnitude 4.0 on December 13th. Between the 3rd and 14th a cluster of 3 earthquakes at a depths between 220 and 300 km with magnitudes between 4.0 and 4.7 occurred about 100km NNE of Rotorua. An additional 2 events East of Rotorua at depths of 66km and 62 km with magnitudes 4.1 and 4.6 occurred on the 12th and 29th.

Simultaneously, a foreshock sequence of three earthquakes occured Northeast of Gisborne at depths between 20 and 33 km. The first event was on December 5th, magnitude 4.0 followed by a magnitude 4.1 on the 18th and the main shock of 4.4 on the 20th. A nearby earthquake of magnitude 4.3, 84 km deep occurred 140 km North of Gisborne on the 17th.

Another complex sequence of events played out along the Kermadec Islands throughout December. It started with a magnitude 4.1 event 173 km deep 390 km South of L’Esperance Rock on December 3rd. This event may have been a foreshock for a magnitude 4.8 quake at similar depth but slightly north on December 17th.

In the meantime, a cluster of 6 earthquakes 200-odd km North East of L’Esperance Rock at depths between 10 and 45 km commenced with a magnitude 6.4 event on December 7th. Other magnitudes were 4.8, 5.1, 4.8, 5.2 and the last event of magnitude 4.9 on the 19th. Two shallow events – one of mag 5.2 to the south on the 4th and one of mag 5.3 to the north-east of L’Esperance Rock on the 31st complete the sequence of 11 earthquakes in the area.

Closer to home, three further members of the swarm 10 km East of Seddon (magnitudes 3.9, 3.6 and 3.7) occurred on the 18th and 19th and a nearby event of magnitude 2.9 occurred on the 7th.

A late magnitude 2.4 member of the unusual swarm of quakes near Auckland’s Waiheke Island in November occurred on December 14th.

Two deep earthquakes occurred near Nelson – a magnitude 4.1 at 70 km depth on the 10th and a magnitude 4.3 at 180 km depth on the 31st.

Another interesting pair of earthquakes occurred near Arthur’s Pass on the 4th and 8th being magnitude 3.6 and 3.5 and very shallow. Similar small earthquakes have occurred nearby during 2005.

A pair of earthquakes offshore at Castlepoint on the 23rd at 30 km depth were magnitude 4.2 and 3.6.

The next day another pair of offshore earthquakes further north (30 km East of Porangahau) at 12 km depth were magnitude 4.2 and 4.0 occurring 12 minutes apart on Christmas Eve.

The first of the damaging earthquakes was a magnitude 5.0 event 25 km deep between Pahiatua and Pongaroa early on December 13th. It appears to have had only 1 reported aftershock of magnitude 3.9 on the 17th. The main quake threw items from shelves in some parts of Manawatu and Wairarapa.

The second damaging earthquake was a magnitude 4.5 event 30 km deep near Porirua on the evening of the same day. Reports of items thrown from shelves came from many parts of the Wellington region, with the greatest effect being felt at nearby Titahi Bay. More information on this particular tremor can be found under “Earthquakes – Observations.” No aftershocks have been reported.

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

At Mt Ruapehu, seismic activity remained low throughout the month, with the crater lake at a warm 39 degrees C and the water level below the base of the tephra dam.

At White Island seismic activity remained low, with the crater lake levels dropping during the first part of the month. Toward month’s end, rain caused the lake to rise slightly to about 0.9 metres below overflow.

The frequent small earthquakes which recently started at Mt Tongariro continued throughout the month. On December 3rd, an alert bulletin was issued when a magnitude 2.4 earthquake on the mountain triggered an additional series of small earthquakes. It was suspected that the earthquake had increased pressure within the volcano’s hydrothermal system and vulcanologists expected the activity to wane over time as had been seen at similar sites in the world. This has indeed been the case, as the additional activity died away after a few days.

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