On shaky ground Star News

The scars on the ground tell us that Dunedin is also an earthquake country, Paul Gorman writes.

It cuts a slice of the southeastern corner of the South Island that can be seen from space.

The Actor Fault has been responsible for three earthquakes of at least 7.0 magnitude in the past 10,000 years, and winds its way through the rolling countryside between the settlements of Tyre Mouth and Toko Mouth. But it also extends offshore near Dunedin, and ends somewhere around Green Island.

There’s even a subdivision and a freshly paved road across the cliff where the fault comes ashore through a field in Tayree Beach, across the road from the school. Under the terms of their Resource Approval, buildings must be returned 20 meters from either side of the rift.

The fault can be easily seen on Google Maps, as a single macrocarpa tree guarded its arrival on land from an underwater reef extending to Taieri Island-Moturata, which the rift pushed out of the sea.

Akator is an independent mistake, judging by his recent history. Its ruptures in the Holocene epoch – the past 10,000 years – were irregular, and the last two major earthquakes occurred about 1,000 and 700 years before the present – the blink of an eye when it came to geological time.

This means there was a gap of about 9,000 years and then two devastating earthquakes within about 300 years. The early Maori settlers in the area may have felt the last.

The head of the seismology department at the University of Otago, Professor Mark Stirling, says such a recent rupture makes it an “effective mistake”.

Nobody can know whether it will be another 10,000 years before the next big mistake occurs or whether the error is much closer to reappearing.

Actor’s three earthquakes are estimated to have magnitude 7.0, making them comparable to the 7.1-magnitude Darfield earthquake on September 4, 2010, which started the sequence of the last Canterbury earthquake.

But there is a difference. While the epicenter of the Darfield earthquake was about 40 kilometers from central Christchurch, the Actor Fault ran about 12 kilometers from central Dunedin.

Akator isn’t the only local bug people need in Dunedin.

The Tetri Fault defines the eastern and southeastern edges of the Tayy Plain and has pushed the ridges between the plain and the coast.

GNS geologist and geomorphologist David Barrell says the Tetri fault is located 5 kilometers from central Dunedin and has caused major earthquakes at least twice in the past 38,000 years.

If there are three segments of a tetri fault, as is thought, it could be a future earthquake of 7.0 magnitude if one segment breaks, or it could reach 7.7 magnitude in the least likely event of the entire fault rupture at once, he says.

Research conducted two years ago also revealed a structure now known as the Kaikurai Fault, running roughly the length of the canyon of the same name.

GNS Science seismologist Dr. Pilar Villamor says it is not yet clear if the Kaikurai fault is active, but it is considered “active”.

Early estimates indicate that it generates large earthquakes every 30,000 to 1 million years with a magnitude of 6.9, although this could be higher, and possibly as high as 7.6, if another fault such as Akator occurred at the same time.

The Hyde Fault error is also likely a concern to Dunedin.

Professor Stirling says this is located 50 km from the city. The last rupture is 10,000 to 20,000 years old, and the average period of frequency between earthquakes is believed to be around 12,000 years.

Has the potential to trigger an earthquake of magnitude 7.0 or greater.

Located southwest of Taieri Beach in a secret area of ​​approximately 10 kilometers, the Akatore Fault passes through the Akatore River, mudflats and forests until it reveals itself in all its glory in a small field full of objects crossed by the meandering Big Creek.

This unobserved field is a geologist’s hidden treasure for seismologists. The fault here is clearly visible as a steep cliff, about 4 meters high, that lies directly across Big Creek Road.

The bug actually looks more like a railroad bridge, and you would be forgiven for expecting there are tunnels at either end where you meet the hills.

“This is one of the best examples of a reverse error exposed in New Zealand,” says Professor Stirling.

The high side, closer to the sea, is solid rock, but the flat field from which it arose consists of several meters of swampy sediments, below which are pebbles with schist in the basement of Otago.

The slope is the “sum of three earthquakes” in the past 10,000 years.

“This is about 4 meters high. When we go deeper into this, we can see the gradual displacement of the three earthquakes that formed this.” [fault rupture] About 2 million per event. “

It’s a slippery walk up the rift through a rush-filled grazing field, with a few large bovine pats in the swamp adding to the excitement.

The all-wheel drive appears and makes the circuit work easier for us. Beyond the steps of the Swiss couple Regula and Thomas Fischli, who have owned 365ha Little Fish on the Roof Farm since 2002.

When they bought the drug, Thomas says, they had no idea an important piece of science was piling up in one of their fields thousands of years ago.

“It was one of the treasures that we did not know about,” Thomas says. “A hidden treasure.”

Professor Stirling says the Actore Fault dips 45 ° to 50 ° underground toward the southeast. Over millions of years, it created a low-lying coastal ridge.

“While this hike has continued, Big Creek has managed to keep up with the pace of the decline as the hills rise, and this is called a former gorge.

“When a single earthquake occurs on the Actor fault, Big Creek is blocked by the slope of the fault, thus causing the current to create a swamp. Ultimately, the current will cut through the slope and drain the swamp. This has happened over and over … and you have this sequence of swamp deposits Inside, there are bits of wood. “

When you look at the marshy vegetation around the rift slope, it’s clear that the landscape hasn’t fully recovered from the recent earthquake, he says.

“We are still seeing the aftermath of the last earthquake.”

Edward Ellison the Elder in Otago (Ngai Tahoe and Te Attiawa) believes that the first Maori in coastal Otago could sense the Actor earthquake.

“The closest people to arrive were her whites, which probably coincides with this event about 700 years ago. Ngai Tahi did not arrive until 200 years or so ago, and Katie Mamoy was 100 years earlier.

“So we’re talking about the very first people, and they had the most amazing building stories of South Island.

“Our ancestor, Rakaihautu, is credited with digging the Great Lakes using the ko, or digging stick. This is how they remember events and paint their stories in the landscape.

“This is a wonderful story about creation, and it leads me to believe that there may or may not be a connection.”

After dating, it was believed that Kati Mamoe’s boss, Te Rakitauneke, had a pet taniwha.

On his voyages, he left Taniwa behind, and went to search for Te Rakitawniki, and on those voyages Taniwa, called Matamata, is said to storm the port of Otago.

“ As he uprooted the hole where the Mosgiel is now and then also, the winding nature of the Taieri River that descends across the Taieri plains is said to have been created by taniwha this slope down the plains in search of his master.

The southeast corner of the South Island, during the period of European settlement, was very quiet when it came to seismic activity.

But it is worth noting that the low level of earthquakes depends on only a few hundred years. It is only relatively low when compared to the more active parts of New Zealand, such as Fiordland, Wellington, or East North Island.

The first seismograph for New Zealand was built in 1884, and it is only since the mid-1960s that the levels of background energy emitted in small and medium-sized earthquakes have been recorded consistently.

Dr Ben McKee, a natural hazards analyst at Otago Regional Council, says a potential major earthquake is just one of many hazards that could affect lives and livelihoods across the region, as many coastal Otago residents realize.

“In particular, ground shaking due to earthquakes is a potential region-wide hazard. It is important for people living in this environment to be aware of the potential risks.”

GNS Science’s Mr Barrell says the Titri Fault is about 90 km long. It starts near Milton and continues around Wingatoy as a superficial crack. It then runs northeast as a fault buried below Three Mile Hill and Flagstaff, and ends north of Swampy Summit.

“Data from trench-digging and dating investigations near Milton indicate that the Tetre Fault has ruptured at least twice in the last approximately 38,000 years. The former rupture occurred sometime from 38,000 to 28,000 years ago, followed by another sometime before about 18,000 years ago. Year.

Scientists have calculated a range of approximate frequency durations of the Titri Fault earthquakes from about 5,000 to more than 40,000 years.

“We are still analyzing data from the work of Tetri’s investigations, and by the end of this year, there may be further subtle improvements to our assessment of Titri’s fault behavior – some approximate values ​​become ranges within narrow uncertainties – but the main conclusions are expected to stand still,” he says. .

Dr. Vilamor says there are still “great doubts” about the Kaikurai Fault.

“ From the evidence gathered, we have yet to prove with confidence whether she is dormant, inactive or active.

“With the limited information we have, we have estimated that the collapse times range from 30,000 to 1 million years, [but] Determining rupture durations and rupture size in low-seismic regions such as Dunedin and coastal Otago is a scientific challenge.

“There is a suggestion that local errors are related and require ‘teamwork’ to relieve the stresses associated with plate tectonics, where one fault is active for a period and then turns off while activity is passed to another fault for a while.”

While Dunedin is a low-seismic-risk region, it is not risk-free, she says.

“Although earthquakes are rare in this region, it makes sense to be prepared. The aim of our error studies is to try to build a more complete picture of Dunedin’s earthquake risk, because we want the best possible information available to decision makers.”

A three-year study to build a more complete picture of the earthquake risk in Dunedin used topographical and geological maps, geophysical information and seismic imaging to detect subsurface deformation and gravitational measurements across the region.

“ The data from the provisional deployment of 18 seismometers across the Kaikurai Fault did not reveal any small earthquakes.

“ Measurements of current distortion with ground-based geodesic stations and from satellite imagery barely reveal movement across the city – we were only able to measure 1 mm per year of shrinkage between GPS stations at the University of Otago and the GPS station in Alexandra.

“The low geodesic rates confirm the low fault activity rates in the region. [But] In our studies, we always realize that a “absence of evidence” is not a “evidence of absence”.

“We think it is safest to treat errors as ‘guilty until proven innocent’ in terms of being a potential danger,” says Dr. Vilamor.

Dunedin may be sitting in a low-risk seismic zone, but there is a feeling that rioters are gathering everywhere.

Drawing on a profession of experience here and abroad, Professor Stirling presents a thought-off scenario.

“Upon seeing Canterbury, Kaikoura and some international earthquakes, I would say, yes, if the Actor earthquake happened and ruptured its entire length, there would be very significant damage to the heritage buildings of Dunedin.”

The question is, is Dunedin ready for the day that happens?

More information on Dunedin Errors can be found in this GNS report: https://tinyurl.com/y5urabto. This series of online features and content is made possible by a grant from Aotearoa-New Zealand Science Journalism Fund and EQC. Online content produced and edited by ODT videographer Rudi Adrian.

Written by Paul Gorman

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