Geological Summary for New Zealand Area, March 2009

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

GeoNet, the U.S. Geological Survey and GNS Science reported 43 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during March 2009.

The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (8), M4 to 4.9 (14) M3 to 3.9 (17).
An additional 4 events in the magnitude 2 range were deemed worthy of mention.

A massive magnitude 7.9 earthquake struck the southwest Pacific at 7:18 a.m. on March 20th. The quake was located 215 km south-south-east of the Tongan capital Nuku’Alofa, an area not usually covered by this summary. However, while the epicentre was 1845 km north-east of Auckland, light shaking was felt in the Bay of Plenty, Poverty Bay, Mahia Peninsula and Wellington.

The undersea quake was finally determined to be 34 km deep and sparked tsunami warnings for several Pacific island nations. The Pacific Tsunami Warning Centre issued two bulletins containing calculated arrival times for several Pacific locations, with a third bulletin at 9:08 a.m. cancelling the warnings and confirming the arrival of a small wave of 4 cm at Niue. The actual arrival time at Niue was 8:11 a.m. New Zealand time, just 7 minutes later than the forecast time published in earlier bulletins.

Further analysis of wave data showed that a 17 cm wave was recorded at Rarotonga in the Cook Islands, 27 cm at the Santa Cruz Islands, and 10 cm at Iqueque (Chile), Honolulu (Hawaii) and Napier.

Five earthquakes with magnitudes between 4.6 and 5.3 were recorded near Raoul Island in the Kermadecs during March. Most were less than 70 km deep. Only two significant quakes with magnitudes of 5.0 and 5.2 were recorded near L’Esperance Rock in the southern Kermadec Islands.

The Matata earthquake swarm was more active during March with six quakes reported by GeoNet. Magnitudes ranged between 2.6 and 3.6 at depths of 5 km. All were centred off-shore, 10 km north-west of Matata. The increased swarm activity followed a deep magnitude 5.8 earthquake, also located 10 km north-west of Matata, at a depth of 160 km on the morning of Sunday the 22nd. This quake generated a strong response from the public with 583 reports of shaking filed from across the North Island, the upper South Island and Canterbury.

An off-shore quake of magnitude 5.5 on March 15th was less widely felt. The quake was located 100 km north of White Island at a depth of 170 km and attracted the attention of the public in coastal areas of the Bay of Plenty, and from Gisborne to northern Hawke’s Bay.

A pair of magnitude four earthquakes struck off-shore near Gisborne on the 13th and 27th. The quakes of magnitude 4.3 and 4.2 were 30 km deep and located 50 km south-east of the city.

Three earthquakes located 10 km west of Twizel on March 28th consisted of a magnitude 4.9 mainshock followed by two aftershocks of magnitude 3.4 and 3.2. The quakes were only 15 km deep, and it was expected that damage would have been sustained in Twizel. 283 felt reports were filed by members of the public from Dunedin, Queenstown, Greymouth, Christchurch and areas in-between. Strong shaking was reported from Fairlie, Lake Ohau, Omarama and Twizel.

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 March.

At the end of March 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, the Pacific Tsunami Warning Centre, and their contributing agencies.]

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