Boulders plummet near bus
Car-sized rocks blocking the Haast Pass highway between Otago and Westland are believed to have tumbled from Clarke Bluff in front of a bus.
Haast police constable Paul Gurney said the boulders apparently came down in front of a 40-seater vehicle bound for Haast.
Gurney said the rockfall followed five localised quakes felt within a few kilometres of Haast township.
“Very loud bangs, like an explosion, with a short, sharp shock. All very shallow.”
GNS recorded earthquakes around Haast including a magnitude 2.3 on Monday, a 3.6 last Wednesday and a 2.4 on August 5.
A 5.5 quake struck the lower South Island on Monday night, after the rockfall on Monday afternoon.
The highway was expected to remain closed until at least midday today.
Clarke Bluff is a narrow point on State Highway 6, just west of the Gates of Haast.
A magnitude 5.5 earthquake that sent debris rolling down South Island mountains struck on a “dog’s breakfast” of tectonic systems — but there’s no indication it had any effect on the big-risk Alpine Fault.
Nearly 7000 people between Southland and Canterbury reported feeling Monday’s 10.35pm quake, which was recorded 12km deep about 15km north of Milford Sound.
There have been about 60 aftershocks — 16 of them registering at magnitude three or higher.
GNS Science seismologist John Ristau said the quake was a case of “reverse faulting”. This happened when the Earth’s crust was being compressed, and one block of crust moved up and over the other.
Ristau said it was unclear whether the quake had struck on any preexisting fault, because GeoNet’s monitoring coverage wasn’t as refined in Milford Sound’s remote wilderness.
“The best we can give is an approximate location, within a few kilometres,” he said.
The Milford area is one of New Zealand’s — and the world’s — most seismically active regions.
“At the southern end of Fiordland, you have the Australian plate subducting beneath the Pacific plate, but as you start to get further north, along the West Coast, we transition into the Alpine Fault, and the plates start to slide past each other,” he explained. “And so this earthquake was almost in that transition area — that’s where things start to get really complicated. It’s a like a little dog’s breakfast of tectonics going on there, and we really can’t calculate what the impact of stresses would be.”
It followed another 5.5 quake at Fiordland’s Big Bay in June, which might have been the most significant shake observed on the South Island’s high-risk, 600km-long Alpine Fault in nearly two decades.
However, in both cases, the impact on the big quake-maker would have been negligible.
“It’s always good to remember that Fiordland is the most seismically active area in New Zealand, and earthquakes of this size happen there on a regular basis, so there’s nothing really unusual about this.”