Geological Summary for New Zealand Area, February 2010

Earthquake activity remained steady. White Island and Mt. Ruapehu remained at Alert Level 1. A tsunami warning was issued for coastal communities. Slow-slip events occurred near Gisborne and Hawke’s Bay.

GeoNet, the U.S. Geological Survey and GNS Science reported on 31 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during February 2010.

The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (9), M4 to 4.9 (3) M3 to 3.9 (14).
An additional 5 events in the magnitude two range were deemed worthy of mention.

Earthquake activity in the Kermadec Islands was quiet with only three events reported. Two shallow quakes with magnitudes of 5.9 and 5.2 were reported near Raoul Island, and one deep event of magnitude 5.2 near L’Esperance Rock.

A magnitude 5.1 earthquake struck 180 km east of Gisborne just before midnight on February 9th. The quake was at a depth of 10 km and instruments indicated that it should have been felt in Gisborne and Hawke’s Bay areas, but no reports appear to have been filed with GeoNet.

The locking between the Pacific and Australian tectonic plates was temporarily released off the eastern coast of the North Island during January, and two slow-slip events were detected off the coast near Tolaga Bay and Mahia Peninsula. Apart from three small earthquakes near Tolaga Bay toward the end of January, the movement went largely unnoticed by locals and the area had moved eastward by 100 mm by the time the activity ceased mid-February. The plate interface near Mahia Peninsula had moved by 60 mm when that activity also ceased mid-February.

A shallow magnitude 4.5 earthquake struck 40 km east of Napier on the afternoon of February 5th. The 25 km-deep quake was felt at several locations in Hawke’s Bay.

A deep offshore earthquake located in the South Taranaki Bight just after 6 a.m. on the 19th was widely felt from Taranaki to Nelson and Marlborough. The magnitude 5.0 quake was centred 80 km south-west of Hawera at a depth of 170 km.

An early morning magnitude 5.0 quake centred very close to Paraparaumu on the Kapiti Coast on the 13th attracted nearly 800 felt reports from the public. The quake was 60 km deep and was felt from southern Hawke’s Bay to Marlborough with several Wellington suburbs experiencing what was considered as “strong” shaking.

Some South Island residents were woken just after midnight on the 10th by a magnitude 4.5 quake centred 50 km north of Hanmer Springs. The depth of the quake (110 km), which struck at 11 minutes after midnight, allowed it to be widely felt in Nelson, Marlborough and North Canterbury districts.

A very shallow magnitude 5.1 quake centred 40 km west of Milford Sound late on the 5th of February attracted a lot of interest. The 10:48 p.m. quake was only 5 km deep and was felt throughout Fiordland, Southland and Otago, with the public filing 271 felt reports with GeoNet.

Two magnitude five events struck the southern part of the South Island at the end of the month. An offshore magnitude 5.5 quake centred 170 km west of Invercargill at 5:38 p.m. on the 25th was only 12 km deep. This was felt throughout Fiordland and Southland and in parts of Otago. On the 28th, at 6:14 p.m., a deep magnitude 5.1 quake struck onshore 140 km north-west of Invercargill. This quake was was felt throughout Fiordland, Southland and Otago, creating unease in the wake of the tsunami warning from a massive earthquake off the coast of Chile.

A magnitude 8.8 undersea earthquake centred north of Concepcion, Chile at 7:34 New Zealand time on the evening of February 27th generated a tsunami that was locally devastating and caused damage as far away as Japan. In New Zealand, a tsunami warning was issued and some coastal events were cancelled while some commercial shipping vacated ports and put out to sea. Waves up to 1.5 metres above normal were recorded at Pitt Island in the Chathams the following morning while waves of about a metre were recorded at Gisborne, Sumner, Timaru and Whitianga. At Lyttelton a wave of 1.9 metres was recorded.

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 February 2010.

Notes on a survey of White Island volcano reported that the crater lake had fallen by about one to one-and-a-half metres and upwelling seemed to be less vigorous than when last observed. Several new mud pots were observed on the main crater floor, and spa and stream flows appeared to be normal.

At the end of February 2010, 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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