Monster Fiordland quake of nine years ago was rude wake-up call

It was the biggest New Zealand quake in nearly a century, beginning with a seismic lurch, then exploding with an energy-packed snap.

But the 7.8 magnitude Fiordland earthquake nine years ago near Dusky Sound has since been overshadowed by bigger events.

A seismologist nonetheless says the monster jolt — the equivalent of 500 million tonnes of TNT, 25,000 times the energy release of the atomic bomb dropped on Nagasaki in 1945 — proved a rude wake-up call after a relatively sleepy period quake-wise.

The main shock, striking at 9.22pm on Wednesday, July 15, 2009, was a large reverse faulting triggered by the Australian tectonic plate pushing beneath the Pacific plate, upon which Fiordland lies.

Scientists now know it was the largest of this type of quake in New Zealand, making it very important for understanding how this type of hazard might threaten us.

While the rupture began about 30km deep, it quickly travelled upward and to the south, driving most of its energy offshore and moving from a lurch to a snap. But the relatively slow release of its energy meant its motion unfolded in a rolling pattern, which likely explained why there was little damage to buildings.

Cracks in walls were reported in a single property in Invercargill, although there were also cases of ground slumping causing some property damage, and items falling from cupboards and shelves.

A metre-high tsunami registered on the tide gauge at Jackson Bay, near Haast, while another instrument southwest of NZ picked up a wave that could have been set off by an undersea landslide.

Over our recorded history, the occurrence of big quakes tended to happen in clusters.

GNS Science seismologist Dr John Ristau said the period from the mid19th century to the mid-20th century saw at least 10 quakes larger than 6.8, which struck near populated areas and caused heavy damage.

A few over 6.5 had been recorded around Fiordland, but hadn’t affected Dusky Sound quake damage. July 15, 2009 7.8 magnitude Located near Dusky Sound many people. “As a result, people born in the second half of the 20th century didn’t have as much experience with large earthquakes in New Zealand and may not have realised the potential. “The Dusky Sound earthquake acted as a bit of a wake-up call to New Zealanders that earthquakes are a serious threat.” For those who hadn’t got the message, plenty of other reminders followed: two 6.6 quakes in the Cook Strait and Marlborough areas in mid-2013, the 7.1 East Cape quake in 2016, the 2014 6.2 Eketahuna earthquake, 2016’s 7.8 Kaiko¯ ura earthquake, the most complex ever recorded, and the Canterbury sequence triggered by the 2010 7.1 Darfield event and later killing 185 people in the 2011 Christchurch earthquake. “The Darfield earthquake showed what a much smaller earthquake can do if it occurred near a major population centre,” Ristau said. While any large quake ramped up the probability of another one in the immediate area, that likelihood faded over time and the fall-out effects from the event had likely passed.

But it had still probably been enough to change the stress pattern on the Alpine Fault, he noted. “The probability of the Alpine Fault rupturing anytime soon is lower than it was immediately after Dusky Sound, but it is still higher than it was prior.”

The start of the Canterbury quakes less than a year later meant scientists studying it quickly moved on, and Ristau acknowledged it was sometimes forgotten about.

“When Darfield occurred I even had people ask me when was the last time New Zealand had an earthquake that big, and were surprised to find out it was only a little over a year earlier and was about nine times larger,” he said.

One reason for the relative lull was that seismicity typically came in ebbs and flows. “There have also been no significant earthquakes since Kaiko¯ura to cause large numbers of aftershocks, and the Kaiko¯ura aftershock sequence has quieted down quite a bit from the first half of 2017.

“One day there will be another large earthquake. We just don’t know where, when . . . or how big.”