Geological Summary for New Zealand Area, July 2009

Earthquake activity increased sharply. New Zealand experienced its biggest earthquake in 80 years. White Island and Mt. Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science reported 63 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during July 2009.

The magnitude distributions were as follows:
M7 to 7.9 (1), M6 to 6.9 (2), M5 to 5.9 (23), M4 to 4.9 (22) M3 to 3.9 (12).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.

July was a busy month for geological activity in New Zealand. The month began with the settlement of Waihi on the shore of Lake Taupo evacuated after the Turangi earthquake swarm and heavy rain raised fears that the Hipaua thermal area (known as the “steaming hills”) above the village might slide into the lake. About 30 residents of Waihi were evacuated on the 29th of June and State Highway 41 which runs through the area was closed.

In 1846, sixty people were killed and the village of Te Rapa was obliterated when a huge landslide swept through the area. Heavy rain had caused a temporary lake to form on a blocked steam which ran through a gorge above the lake. After three days the temporary dam broke releasing a torrent of rocks and mud which careered down the slope and into Lake Taupo carrying the occupants of the village with it.

The village of Waihi was then established near to crop fields which were developed on the site of the debris. In 1910 a series of slips and a large landslide again swept through the valley. Thought to have been triggered by a small hydrothermal eruption, the landslide was larger than the 1846 event, and the debris generated a 3 metre wave which flowed across Lake Taupo to the distant shore. Fortunately, the initial slips were noisy and the occupants of Waihi were able to evacuate to safety before the main landslide passed through. One man was overwhelmed by the debris and died.

During June 2009, residents of Waihi had noticed that birds had left the area, the waterfall in the settlement had moved, some streams had dried up and others had started where there were none before. The commencement of the earthquake swarm at Lake Rotoaira (near Turangi) and a period of heavy rain at the end of the month increased the risk of another landslide event.

However, the Turangi swarm quickly abated and examination of the slopes above Waihi allowed the state of emergency to be lifted at 7 p.m. on the 2nd of July and residents were allowed to return. Those who did return may have had second thoughts when another burst of earthquake activity occurred on the 11th. Three very shallow quakes with magnitudes between 2.8 and 3.0 were reported near Turangi, but the activity then ceased.

Earthquake activity in the Kermadec Islands was lower than usual with only two events of magnitude 4.8 and 5.6 being reported near Raoul Island.

On July 13th, a 30 km-deep magnitude 5.1 quake struck 90 km north of Te Araroa. The quake was felt on the Bay of Plenty coast from Whakatane to Ohope.

Residents of the town of Otane in Hawke’s Bay reported feeling small earthquakes during the latter part of June, and GeoNet reported a magnitude 3.7 event on June 26th. This was followed by a magnitude 3.8 quake on July 6th, a magnitude 3.6 event on the 16th, a magnitude 3.9 quake the next day, and a magnitude 4.3 quake on the 19th. The swarm activity then ceased.

A pair of earthquakes struck 40 km south-west of Nelson on July 3rd. The magnitude 4.2 and 4.6 quakes were centred 20 km south of Wakefield, 20 km south-west of Brightwater, at a depth of 90 km. The quakes struck at 4:25 a.m. and 5:16 a.m. waking sleepers from Marahau to Nelson and at Westport. The larger event was also felt in Takaka, Marlborough and in some Wellington suburbs.

Much of New Zealand was shaken by three powerful earthquakes in less than twenty minutes on the evening of July 15th. A shallow magnitude 7.6 earthquake struck off the Fiordland coast at 9:22 p.m., followed sixteen minutes later by a deep magnitude 5.4 quake under the Tasman Sea in the South Taranaki Bight. Then a shallow magnitude 6.1 aftershock struck off Fiordland at 9:41 p.m.

In the immediate confusion, the Fiordland event was reported as magnitude 6.6 by GeoNet but 8.2 by the U.S. Geological Survey; and the Taranaki Bight event went unreported, as seismologists tried to sort out the confused traces from overloaded seismometers. The public responded with felt reports of the Fiordland mainshock from several locations in the North Island creating further confusion. Attention then turned to the risk of tsunami, as the true magnitude of the Fiordland quake became apparent. The experience was a sobering illustration of the confusion that will occur when the Alpine Fault releases its accumulated strain.

GeoNet seismic network on July 15
Seismologist’s nightmare or delight? GeoNet’s national seismograph network shows the traces of the three earthquakes that struck over a 19 minute period on the evening of July 15th.

Work has been carried out subsequently to try and map out the impact of the three quakes. It would seem that most of the felt reports from the North Island relate to the magnitude 5.4 quake at 9:38 p.m. This quake was located 80 km south of Opunake (90 km north of French Pass, 160 km north-west of Wellington) at a depth of 160 km. The great depth allowed the event to be widely felt and it was probably felt from Taranaki to Westland.

The much larger Fiordland quake was located under the Tasman Sea 100 km south-west of Te Anau (160 km north-west of Invercargill) at a depth of 12 km. The magnitude 7.6 quake was felt from Taranaki to Southland, causing moderate levels of damage in Fiordland, Southland and Otago. It generated a tsunami that was measured at 1 metre at Jackson Bay but only 5 cm in the Tasman Sea, allowing tsunami warnings to be cancelled at 11:51 p.m.

While electricity supplies were knocked out in southern parts of the South Island, the quake caused less damage than would normally be expected from such a large event. Investigation shows that the rupture was a subduction thrust event, starting at a depth of 30 km and continuing upwards and to the south, focusing the energy offshore. As the Australian Plate slid forward the energy was released more slowly, generating lower frequency shaking and rolling, rather than higher frequency, sharp shaking movements. The slower release and low frequency shaking explain the low number of landslides and, coupled with the remote location of the epicentre, limited damage to man-made structures. Even with the low frequency behaviour, had the quake struck near a populated centre, the damage would have been massive.

Two magnitude 5 aftershocks followed the magnitude 6.1 aftershock before midnight. A magnitude 5.9 shock struck at 1:50 a.m. setting the pattern for seven more 5th magnitude quakes before the second largest aftershock of magnitude 6.0 mid-morning on July 17th. Several magnitude 4 and 5 aftershocks were felt daily until the strength declined to magnitude 4 events on the 20th. A burst of activity generated four magnitude 5 quakes on the 30th. In all two aftershocks of magnitude 6, twenty of magnitude 5 and sixteen of magnitude 4 were reported by GeoNet and/or USGS before the end of the month.

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.

One Alert Bulletin was issued by GNS Science during July. The bulletin issued for Ruapehu volcano on July 14th reported a small magnitude 2 volcanic earthquake beneath the crater lake on the evening of July 13th. The lake level rose by 15 cm but the temperature was unchanged at 20 ºC. Despite poor observing conditions, a helicopter flight indicated no obvious changes in the volcano and found that the lake was overflowing. It was calculated that 20 million litres of water had entered the lake from the hydrothermal system below, and no change was made to the volcano’s alert status.

At the end of July 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, and the Taupo District Council.]

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