Geological Summary for New Zealand Area, November 2008

Earthquake activity increased slightly during November 2008, but magnitudes were generally lower than events recorded during October. White Island, Mt. Ngauruhoe, and Mt. Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science reported 45 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during November 2008.

The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (7), M4 to 4.9 (12) M3 to 3.9 (20).
An additional 5 events in the magnitude 2 range were deemed worthy of mention.

Earthquake swarms were reported near Matata, Masterton and Taupo, and clusters of quakes were recorded in the Kermadec Islands and Fiordland.

Two distinct quake sequences occurred in the Kermadec Islands during November, with shallow activity near Raoul Island and a pair of deep events near L’Esperance Rock. Three shallow quakes struck 115 km south or south-east of Raoul Island on the 3rd, 6th and 25th. The quakes had magnitudes between 5.6 and 6.0 and were located at depths of 10 to 45 km. Two earthquakes with magnitudes of 4.9 and 5.2 struck west and south of L’Esperance Rock on November 5th and 10th. These were deep events at depths of 443 and 111 km respectively.

Two deep quakes struck within 20 km of Rotorua, the first being a magnitude 5.7 event just before 1 a.m. on the 9th. A magnitude 5.2 quake followed just after 3 a.m. on the 15th. The quakes were at depths of 200 and 160 km respectively.

The Matata earthquake swarm, which commenced in 2005, continued during November 2008, with five very shallow quakes with magnitudes between 3.2 and 4.3 being felt. The largest quake in the Matata sequence this year had also been a magnitude 4.3 quake which struck on the 24th of January, and both events were widely felt in the Bay of Plenty area.

A pair of shallow earthquakes struck 30 km south of Gisborne on the 14th and 24th. The magnitude 3.5 and 4.0 events were at depths of 30 km.

A cluster of eight shallow earthquakes struck near Masterton on the afternoon of Saturday November 15th. The quakes, which occurred over a period of about five hours, were located in three distinct areas 10 km north-west of Masterton. The activity began with a magnitude 4.1 earthquake at 1:22 p.m. and two events of magnitude 3 and five of magnitude 2 followed.

All of the earthquakes were at similar depths of between 23 and 28 km. The second largest earthquake of the Saturday series, a 28 km-deep magnitude 3.4 quake at 1:26 p.m., was slightly removed from the other two centres of activity. Only the two largest quakes were reported as felt by the public, as was a magnitude 4.0 quake late the following night.

A swarm of very light earthquakes was reported 10 km north of Taupo around the 18th. The largest event was a magnitude 1.9 quake, 3 km deep, which was felt at Wairakei.

On the 30th of November a pair of quakes with magnitudes 3.1 and 3.8 struck near Seddon. The earthquakes were located 30 km south-east of the township at depths of 15 and 12 km. A deeper magnitude 4.0 quake struck 30 km east of Seddon at a depth of 60 km on the 11th.

Five earthquakes with magnitudes between 3.3 and 5.0 struck Fiordland during November. Four of the events were located 40 to 70 km north-west of Te Anau, with three being shallow at depths between 15 and 25 km. Fortunately the magnitude 5.0 quake was 120 km deep, lessening its impact.

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 November.

At the end of November 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, the U.S. Geological Survey, GeoNet, and their contributing agencies.]

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