Kelvin Berryman explains how the tectonic plate boundary creates faults and earthquakes in New Zealand. Video / GNS
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, which erupted nine years ago today in the remote and bushy wilderness near Dusky Sound, has been over-shadowed by bigger events that came in its wake.
A seismologist nonetheless says the monster jolt - which unloaded the equivalent of 500 million tonnes of TNT, 25,000 times that of the energy release of the atomic bomb dropped on Nagasaki in 1945 - proved a rude wake-up call after a relatively sleepy period in our earthquake history.
The main shock, striking at 9.22pm on Wednesday, July 15, 2009, was a case of large reverse faulting, and triggered by the Australian tectonic plate pushing beneath the Pacific plate, upon which Fiordland lies.
Scientists now know it was the largest - and one of very few examples - of this type of quake in New Zealand, making it very important for understanding how this specific hazard might threaten us.
While the rupture began at about 30km depth, the quake 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 rather than violent shaking, which likely explained why there was little damage to buildings around the region.
Cracks in walls were reported from just a single property in Invercargill, although there were also cases of ground slumping causing some property damage, and items falling from cupboards and shelves throughout Southland, as far north as Wanaka and Hawea and east to Dunedin.
A small, metre-high tsunami registered on the tide gauge at Jackson Bay, near Haast, while another instrument southwest of the country picked up a wave that could have been set off by an undersea landslide.
More people further up the country might have felt the rumble, had another 5.5 quake not hit south of Taranaki soon after.
A forgotten monster
In all, the event was powerful enough to shunt the southwest of the South Island slightly to the west-southwest.
It was also the biggest shake since the Buller and Hawke's Bay earthquakes of 1929 and 1931.
Over our recorded history, the occurrence of big quakes tended to happen in clusters - like those in the mid-1800s or from the late 1920s to the early 1940s - which made the probability of an event higher after a large shake than after one.
The quake left behind evidence of landslides and rockfall. Photo / GNS Science
GNS Science seismologist Dr John Ristau said the period from the mid-19th Century to the mid-20th Century saw at least 10 quakes larger than 6.8, which struck near populated areas and caused heavy damage.
The period since then had only included a handful, notably the 1968 7.1 Inangahua quake, the 6.5 Edgecumbe Earthquake in 1987, and the 6.7 event that rocked Gisborne in 2007.
A few over 6.5 had been recorded around Fiordland, but hadn't affected 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 of severely damaging earthquakes," Ristau said.
"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, 7.1 East Cape quake in 2016, the 2014 6.2 Eketahuna earthquake, 2016's 7.8 Kaikoura Earthquake, the most complex ever recorded, and the Canterbury sequence set off by the 2010 7.1 Darfield event.
A small landslide from the Dusky Sound earthquake. Photo / GNS Science
"At the time Dusky Sound occurred, seismologists said that New Zealand had dodged a bullet as the effects were not as bad as they could have been given the remote location, lack of high frequency energy produced – which meant less shaking that would damage buildings, and most of the energy was directed to the south of the South Island," Ristau said.
"The Darfield earthquake showed what a much smaller earthquake can do if it occurred near a major population centre."
How the Herald reported the event in 2009. Photo / File
Incidentally, the Dusky Sound quake was the first major shake to occur since the introduction of GeoNet and the huge upgrade of its seismic and geodetic network.
That meant scientists had much more data to sift through than they'd gained from the last time a large-scale quake hit there, six years earlier.
"We could calculate detailed slip models which showed definitively that the earthquake was on the boundary of the Australian and Pacific plates, how the slip was distributed, and how much slip there was."
Most importantly, scientists could also reveal its impact on nearby faults such as the island-straddling Alpine Fault, which posed one of New Zealand's largest geological threats and packed the potential to unleash a monster that could kill thousands.
This graphic shows horizontal motions caused by the Dusky Sound earthquake. Photo / GNS Science
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.
However, Ristau noted, it had still probably been enough to change the stress pattern on the Alpine Fault, along with the wider subduction zone itself.
"For example, the southern offshore extension of the Alpine Fault received an increase in stress bringing it closer to failure, and this doesn't change even years after the earthquake," he said.
This map shows the peak ground accelerations (PGA) recorded for the Dusky Sound earthquake. The quake had a total energy release equivalent to 500 million tonnes of TNT. Source / GNS Science
"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 of the landslides carved through Fiordland. Photo / GNS Science
"But its impact shouldn't be forgotten as it reminded everyone that New Zealand is earthquake country, which has been borne out by the number of impactful earthquakes that have happened in the nine years since."
2018 in earthquakes
Meanwhile, an analysis of the first six months of the year showed earthquake activity had been "fairly typical", he said.
"There is still an increase in activity along the northeast coast of the South Island due to Kaikoura aftershocks, although much less than in first six months of 2017," Ristau said.
"Also, Fiordland seems to be a bit quieter than usual; however, this is likely due to the normal ebbs and flows in seismic activity rates."
This image shows the motion-measuring focal mechanisms for the earthquakes in Fiordland. The mainshock is shown in yellow and offset from its true location. Photo / GNS Science
The amount of seismic activity was probably still above what it was before the Canterbury sequence, as there had been several significant earthquakes and aftershock sequences that kicked off with Dusky Sound.
It was difficult for scientists to crunch the exact numbers, as GeoNet's analysis system was changed in 2012 and auto-locations of smaller events in the old database weren't reliable enough to review.
But one reason for the relative lull was that seismicity typically came in ebbs and flows.
"There have also been no significant earthquakes since Kaikōura to cause large numbers of aftershocks, and the Kaikoura aftershock sequence as quieted down quite a bit from the first half of 2017," he said.
This map shows where the largest quakes have hit in the first six months of 2018. Source / GNS Science
"The decrease in activity doesn't really tell us anything; but it's a reminder that we can't predict earthquakes and can only talk about statistical probabilities and long-term seismicity rates.
"One thing we can guarantee is that seismicity will increase again, and one day there will be another large earthquake.
