Geological Summary for New Zealand area, January 2006

Geonet, the USGS (NEIC) and IGNS reported 35 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during January 2006. The magnitude distributions were as follows:
M5 to 5.9 (5), M4 to 4.9 (21) M3 to 3.9 (9).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.

Seismological activity was noticeably quieter in January, compared with December. There were no distinct earthquake swarms, but the interesting sequence of deep earthquakes near the Bay of Plenty continued, as did the cluster of deep earthquakes further north in the Kermadec Islands.

The sequence of deep earthquakes offshore to the north of Rotorua and Gisborne continued with two distinct phases. The first phase involved a series of 7 earthquakes migrating southward along a line from 190 km north-east of Rotorua to 70 km south-west of the city between the 2nd and the 17th. As the epicentres progressed southward, the depths increased from 120 km to 280 km on the 15th, and then began rising to 157 km. One anomalous event defied this pattern at 12 km depth on the 5th.

The second phase of the sequence commenced activity on the 22nd at 190 km north of Rotorua but lacked the steady southward trend of the first phase. Epicentres in this series of 5 earthquakes ranged between 185 km north-east and 50 km south-west of Rotorua and depths ranged between 33 km and 252 km.

Earthquakes in both phases of the sequence ranged in magnitude between 5.5 and 4.0. The final event in the sequence was a magnitude 5.5 quake at 170 km depth located 30 km north-west of Taupo on the 28th. This earthquake was widely felt because of its magnitude and depth, and Geonet advise that it was felt from diverse locations in Hawkes Bay, Manawatu, Kapiti Coast and Wellington.

Activity in the cluster of earthquakes in the Kermadec Islands eased from its January peak, with 5 events between magnitude 4.5 and 5.2 occurring between the 17th and 30th. A subset of 3 events occurred between 150 km and 180 km north-east of L’Esperance Rock with magnitudes between 4.9 and 5.2 and depths between 10 km and 43 km.

A cluster of 3 earthquakes in southern Hawkes Bay occurred on the 7th, 23rd and 28th of January. Magnitudes ranged between 3.6 and 4.4 and depths between 20 km and 50 km. The magnitude 4.4 event on the 23rd which was located 10 km north of Waipukurau was one of the month’s 3 damage-causing earthquakes, with minor damage being noted by Geonet.

The other notable sequence of earthquakes was located within 5 km of Upper Hutt. When looked at in isolation, the pair of earthquakes seem to be a classic foreshock and mainshock pair. The first event was magnitude 3.3, 30 km deep on the 1st. The second was magnitude 4.3, 35 km deep on the 13th. However, these seem to be members of a long-term swarm that has been happening near Upper Hutt at various intervals since April 2004.

Only minor damage resulted from the three damaging earthquakes during January according to Geonet. They were:
January 7th, mag. 4.1, 30 km deep, 10 km west of Porirua. Items off shelves.
January 23rd, mag. 4.4, 20 km deep, 10 km north of Waipukurau. Unspecified minor damage.
January 29th, mag. 3.9, 5 km deep, 20 km east of Arthurs Pass. Unspecified minor damage.

Vulcanologists report that activity at the nation’s volcanoes is little changed from December. Their status can be summarised as follows:
Mt Ruapehu (Alert Level 1).
White Island (Alert Level 1).
Mt Tongariro (Alert Level 0).

Mt Ruapehu’s crater lake cooled to 28° C during the first half of the month, with the lake level rising slightly due to snow melt. Seismic activity remained at a low level.

White Island’s crater lake dropped slightly during the month, and seismic activity remained low. Surface features such as small collapse holes, mud pots and dry areas developed during January. Sulphur dioxide emissions increased slightly.

The numerous small earthquakes beneath Mt Tongariro continued at the increased level observed in recent months.

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