Confusion over Massive Landslide in Southern Alps

Confusion surrounds the large landslip at Passchendaele Peak in Mt Aspiring National Park.

Early media reports suggested that the landslip occurred late on the afternoon of Thursday January 11th 2007, when a helicopter pilot reported seeing a plume of dust in the area.

It was speculated that a 4th magnitude earthquake recorded in the vicinity may have either caused the slip or resulted from it. However, GNS Science databases do not show the mysterious magnitude 4 quake for the afternoon of the 11th. Instead, two earthquakes of magnitude 2.9 and 2.6 were recorded in the approximate area of the landslide at 3:15 p.m. and 4:13 p.m. on Thursday, with indicative depths of 5 km.

It has also been speculated that a magnitude 4.4 quake centred 40 km north-east of Milford Sound on December 7th may have triggered the landslide. The earthquake epicentre was about 10 km away from the landslide site.

The landslide has occurred at a remote location within New Zealand’s second largest national park. Geologists are preparing an expedition to investigate the slip which can only be reached by air or on foot, as there are no roads in the area.

Landslides caused by earthquakes are not unusual in New Zealand, which hosts the second largest landslide known to exist on the planet. The landslide, named Green Lake landslide after the lake which occupies one corner, lies deep in Fjordland National Park near Lake Monowai. The slide probably occurred 12-13,000 years ago when a 9 kilometre-long section of the Hunter Mountains collapsed, depositing 27 cubic kilometres of rock debris across an area of 45 square kilometres.

The landslide, which is now covered by lush rain forest, is so massive that it was not recognised until 1976. It may have resulted from a powerful earthquake of at least magnitude 7.5 possibly on the Alpine Fault which lies 75 km northwest of Green Lake. The earthquake probably triggered the collapse of the crest of the range which broke away along the Mt Cuthbert Fault. [Awesome Forces, Te Papa Press, 1998, edited by Geoff Hicks and Hamish Campbell.]

Lake Waikaremoana, inland from Gisborne, was formed 2,200 years ago when an earthquake shook a 3 km-long wedge of stone from the Ngamoko Range, damming the Waikaretaheke River. [Awesome Forces].

Smaller quake-driven landslides have occurred during historical times. The magnitude 8.1 Wairarapa quake of 1855 caused wisespread slippages in the lower North Island. The magnitude 7 quakes at Arthur’s Pass in 1929, Hawke’s Bay (1931) and Inangahua (1968) also caused notable landslides which blocked roads, rivers and rail links.

However, the greatest cause of landslides in New Zealand remains heavy rain.

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