Geological Summary for New Zealand Area, October 2009

Earthquake activity continued at a lower rate. White Island and Mt. Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science reported 22 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during October 2009.

The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (2), M4 to 4.9 (10) M3 to 3.9 (7).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.

No significant earthquakes were reported in the Kermadec Islands during October. A shallow magnitude 4.7 quake struck 200 km south of L’Esperance Rock (695 km east-north-east of Auckland) on October 3rd. The 20 km-deep quake was not felt in New Zealand.

A deep magnitude 4.6 quake struck near Te Araroa on the 11th. The earthquake was felt in the Bay of Plenty, Gisborne and Hawke’s Bay.

Three very shallow earthquakes were recorded just off the coast near Tauranga on the 29th and 30th of October. The quakes ranged between magnitude 2.6 and 2.8 and were felt nearby.

Early on the morning of Thursday 15th, a deep magnitude 5.5 earthquake struck 10 km south-west of Matata. The 150 km-deep quake was felt in the Bay of Plenty, along the eastern coast of the North Island, and in the Wellington region.

That evening a magnitude 4.4 earthquake struck 10 km south-west of Havelock North at a depth of 60 km. The timing meant that more of the public were aware of the event, and shaking was reported from Hawke’s Bay, westward through the centre of the North Island to Wanganui.

A 40 km-deep magnitude 4.8 earthquake located under Cook Strait 20 km north-west of Wellington just after 6 p.m. on the 10th of October attracted a lot of interest. The quake was centred off the coast near Porirua and was felt from Ohakune south to Nelson and Marlborough. Strong shaking was reported throughout the Wellington region, and slight damage experienced at Lower Hutt, Pukerua Bay and Wellington.

The relatively small earthquake was surprisingly noisy and vigorous, prompting GeoNet to issue a report on the event explaining why it was so widely felt, attracting more than 2500 felt reports.

The interface between the Australian and underlying Pacific Plate lies at about 30 km in the area and the quake occurred in the upper levels of the Pacific Plate. The rocks in both plates are very strong at this point, allowing seismic waves to travel very efficiently. The bending of the Pacific Plate triggered the quake which was relatively low in energy, hence of a relatively low magnitude, but strongly felt because of the nature of the rock nearby.

Four earthquakes with magnitudes between 4.7 and 5.1 were reported near Fiordland during October. Two events were located on-shore 80 km west of Te Anau 19 minutes apart on the 8th, and two were located off the southern coast of Fiordland on the 1st and 12th. All four were shallow, being at depths of 7 to 25 km.

Other South Island quakes were a magnitude 3.6 event near Arthur’s Pass on the 2nd, a magnitude 3.4 quake south of Seddon on the 5th and a magnitude 4.0 event south-west of Haast on the 3th. The Haast quake was very shallow, at 5 km, but no felt or damage reports were lodged with GeoNet.

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.

No Alert Bulletins were issued by GNS Science during October.

At the end of October 2009, 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 0).
Mt Ruapehu (Alert Level 1).

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

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