Geological Summary for New Zealand Area, August 2009

Earthquake activity continued at an increased rate. White Island and Mt. Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science reported 48 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during August 2009.

The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (10), M4 to 4.9 (19) M3 to 3.9 (16).
An additional 2 events in the magnitude 2 range were deemed worthy of mention.

Reported earthquake activity in the Kermadec Islands was lower than normal but included a deep magnitude 6.3 quake located 360 km north of Raoul Island on August 19th. Quakes of magnitude 4.7 near Raoul on the 4th and magnitude 5.2 south of L’Esperance Rock on the 11th brought the total of Kermadec events to three.

Activity extended south of the Kermadecs toward the North Island with a magnitude 5.6 quake located 200 km north-east of Te Araroa just before dawn on the 22nd. The 140 km-deep quake was felt in Gisborne.

A series of three shallow earthquakes struck off the coast north of Gisborne during August. A magnitude 4.4 quake struck 60 km north-east of Te Araroa on the 13th and was felt near Ruatoria, and a magnitude 5.3 event struck 455 km north-north-east of Gisborne (400 km south-south-west of L’Esperance Rock) on the evening of the 16th but was not felt onshore. A magnitude 4.4 quake struck 270 km east-north-east of Gisborne on the morning of the 25th but also was not felt onshore. The three quakes were shallow events at depths of 25 to 35 km.

Two very deep earthquakes were recorded near Tauranga on the 16th and 18th. The magnitude 3.6 quake of the 16th was felt in Wanganui and Hawke’s Bay, and the deeper magnitude 5.4 event on the morning of the 18th was felt from Patea to Wellington.

Two quakes with magnitudes of 3.6 and 3.9 struck 20 km south-west of Wanganui on the 15th and 16th of August. The off-shore quakes were only 12 km deep and were felt from Wanganui to Levin and inland at Feilding.

A cluster of four earthquakes struck near Porangahau in southern Hawke’s Bay during August. The quakes ranged in magnitude between 3.6 and 4.5 at depths of 30 to 50 km and were felt nearby. The magnitude 4.5 and 3.6 events which struck 6 minutes apart on the evening of the 26th were also felt at several locations in the Horowhenua and Hawke’s Bay areas.

Two earthquakes struck under Cook Strait, 20 km south of Wellington early on the morning of the 28th. The mainshock was magnitude 5.2 at a depth of 30 km, followed an hour and forty minutes later by an aftershock of 4.3 slightly deeper at 35 km. The mainshock was felt strongly throughout the Wellington region and in the Wanganui, Nelson, Marlborough and Wairarapa regions, attracting nearly two-and-a-half thousand felt reports. Slight damage was reported from five locations between Plimmerton and Wellington.

A pair of earthquakes near Takaka on the 15th and 20th were very shallow at 5 km depth. The magnitude 3.3 and 3.2 events were felt nearby and at Motueka.

Aftershocks associated with the magnitude 7.6 Dusky Sound earthquake of July 15th continued at an irregular rate. Bursts of activity on the 4th and 5th and on the 25th included the largest aftershocks for August. Twelve aftershocks with magnitudes between 4.2 and 5.9 were reported by GeoNet and the U.S. Geological Survey. All of the events were shallow, being at depths of between 5 and 15 km.

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 August.

At the end of August 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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