Geological Summary for New Zealand Area, April 2008

Earthquake activity increased slightly. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

Geonet, the U.S. Geological Survey and GNS Science reported 37 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during April 2008.

The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (6), M4 to 4.9 (9) M3 to 3.9 (17).
An additional 4 events in the magnitude 2 range were deemed worthy of mention.

A burst of strong earthquake activity occurred in the South Pacific outside the area normally covered by this report. During just over 4 hours overnight on the 9th and 10th of April, nine shallow earthquakes struck within 90 km of the island of Tanna, Vanuatu. Quake magnitudes ranged between 4.9 and 7.3, with three of the quakes registering between 6.0 and 6.3. Activity abruptly ceased just after 4 a.m. but resumed early that afternoon. Eight more quakes with magnitudes between 4.9 and 5.9 struck over the next two days as the centre of activity drifted south, closer to the Loyalty Islands.

A magnitude 7.3 quake struck to the south of New Zealand at half-past midday on Saturday April 12th. The quake was located 110 km south of Macquarie Island (755 km south-west of Auckland Island) at a depth of 10 km. With the epicentre located nearly 1400 km south-west of Dunedin, the quake was not felt in the South Island. A tsunami wave was not generated by the undersea quake.

Closer to New Zealand, eight earthquake events occurred in the Kermadec Islands, with seven events with magnitudes between 4.5 and 5.8 to the north of Raoul Island and a magnitude 4.9 event south of L’Esperance Rock. All of the events were deep at between 90 and 595 km in depth.

A magnitude 4.5 quake struck 220 km north of East cape at a depth of 234 km on the morning of April 27th.

The Matata earthquake swarm eased during April with only five quakes being reported by GeoNet. The quakes were all within 10 km of the Bay of Plenty town with magnitudes between 2.4 and 3.1 at depths of 4 to 5 km.

Hawke’s Bay residents felt three local 3rd magnitude earthquakes during April. Two shallow quakes of magnitude 3.0 and 3.7 struck to the south of Hastings on the 2nd and 9th and a deeper magnitude 3.8 quake struck to the east of Napier on the 8th.

Two shallow quakes of magnitude 3.2 and 3.3 struck to the north of Ohakune on the 6th and 14th.

Three quakes struck near Paraparaumu during the month. A pair of quakes, one deep the other shallow struck on the 4th and 5th. The offshore quakes were magnitude 4.3 and 3.6. On the 18th another offshore quake of magnitude 3.4 struck 10 km west of Paraparaumu at a depth of 50 km.

In the South Island, a pair of quakes occurred near Seddon on the 9th of April. The magnitude 3.7 and 3.5 events were separated by 16 minutes and were only 9 and 8 km deep respectively.

Early in the month a pair of earthquakes struck near Arthur’s Pass on the morning and evening of April 3rd. The quakes had magnitudes of 4.1 and 3.8 and were very shallow at depths of 8 and 7 km.

Two offshore quakes of magnitude 4.7 and 5.3 near Milford Sound made themselves felt on the 4th and 13th. The first event was deep at 90 km whilst the second was at the shallow depth of 12 km. They were followed by a magnitude 6.1 quake located 430 km south-west of Invercargill on the morning of the 27th. The 12 km-deep quake was felt in the southern part of the South Island, but not strongly.

Regular reporting of the status of New Zealand’s volcanoes ceased at the end of June 2007, with the closure of the Hazard Watch service. GNS Science now only issues bulletins which record significant changes in volcanic behaviour.

One alert bulletin was issued during April 2008, reporting elevated unrest at Mt. Ruapehu. The bulletin, issued on the 23rd reported increasing temperatures deep within the hydrothermal system and a small increase in the background level of volcanic tremor at the volcano. Following the moderate eruption of September 25th 2007, Ruapehu’s crater lake had warmed by about 20 ºC, peaking at 37.5 ºC on December 21st. The temperature remained high, fluctuating between 34 ºC and 36.8 ºC.

Since March hydrogen sulphide gas had been measured at Ruapehu, and the level of sulphur dioxide had increased since the September eruption. The alert level for the volcano was left at 1 (signs of unrest), but it was pointed out that if an eruption was to occur, it could do so without warning.

At the end of April 2008, New Zealand’s active volcano status can be summarised as follows:
Raoul Island (Alert Level 0).
White Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 1).
Mt Ruapehu (Alert Level 1).

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

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