Paleo earthquakes light up New Zealand earthquake gates

A new study on the New Zealand Alpine Rift reveals details about past earthquakes and the potential for a large rift in the future.

Written by Fionna MD Samuels, Ph.D. Taleb, earthquake news expert at Temblor

Quotation: Samuels, FMD, 2021, Paleo earthquakes light up earthquake gates in New Zealand, Temblor, http://doi.org/10.32858/temblor.177

The New Zealand Alpine Fault cuts 530 miles (850 km) along the western edge of Te Waiponamo – South Island – and marks the boundary between the Australian Plates and the Pacific Ocean. Few oral histories of Maori and tree rings document the most recent rupture along this rift – likely its magnitude 8.1 in 1717. This rift unleashes steady blows from large earthquakes through time, and scientists know the area was due to another event. “Using a credible Alpine Fault scenario is the best way we can imagine what that event might have been – it gives us the power of a hypothesis,” says Caroline Orkiston, science pioneer at Alpine Fault Magnitude 8 (AF8). National program that connects scientists with the public to prepare for the next Alpine earthquake.

The impact of the Alpine Fault became visible from space by the snow from a blizzard. Credit: Jack Declouiter, NASA (Public Domain)

A study recently published in Nature Geosciences by an international team of geologists combines detailed Paleozoic information, error engineering and physics-based computer modeling to improve the likely timing and magnitude of the next earthquake in the Alpine region. They found that the rift has a 75% chance of a surface rupture earthquake of 7.0 magnitude or more in the next 50 years, significantly higher than the previously calculated ratio of 29% (Biasi et al., 2015). They also found that if ruptured, in fact, this looming earthquake has an 82% chance of recording as an event of magnitude 8.0 or greater.

Ancient logs are at the heart of the lake floor

Nicholas Barth, a geologist at the University of California, Riverside and co-author of the study, says the increased chance of an earthquake on the Alpine fault is the result of more detailed data from the ancient era. The new detail comes from drilling sediment cores from four lakes located along the central section of the Alpine Rift, Barth explains.

Lake Cannery, one of the lakes from which sediment cores were excavated to identify past fractures in the Alpine Rift. Credit: Aaron Rees (CC BY-SA 4.0)

Lead author Jimmy D. Howarth, of Victoria University in Wellington, developed the protocol presented in the paper to use lake sediment cores to determine the exact date of past earthquakes, Barth says. Specifically, they identified torpedoes, which are characteristic underwater deposits that have formed due to collapsing slopes, sometimes caused by earthquakes. Howarth and colleagues dated the fossilized leaves found in sediment layers over turbidites that fell into the lake during or shortly after the earthquake, thus restricting the timing of turbidite precipitation and, by inference, the earthquake.

From the cores, Howarth and colleagues determined that the rift bursts every 249 years, more or less than 58 years. Given the time since the last earthquake in 1717, they calculate three-quarters of the probability of a rupture of 7.0 degrees or greater in the next 50 years. Moreover, the likelihood will rise every year until the earthquake strikes.

Combining this new Paleochemical Lake data set with previously published data from swamp areas in the southern section of the rift, Howarth and colleagues created one of the most complete seismic records of its kind. Researchers found evidence of 20 earthquakes over 4,000 years and nearly 190 miles (300 km) from the fault. Additionally, each primary sampling site acts as a mile-long rupture marker; If the core samples from different locations had the same ancient earthquake, the fault must have at least cut the distance between them. This minimum tear length specifies the lower limit for size estimation.

Lake sediments’ cores are used to date ancient earthquakes. Credit: Jimmy Howarth

Earthquake gates

“There is this funny twisting of the fault level at the boundary between the middle and south sections,” Barth says, explaining that the angle of the rift changes from shallow in the north to steeper in the south. “It’s been in the back of my mind for years – do earthquakes care about this spot or are they torn through it?”

The newly created paleo earthquake record shows that about half of the ancient earthquakes ended near the bend, with rupture lengths indicating that they were most likely events of 7.0 magnitude. The remaining earthquakes tore through turns with a magnitude of 8.0 or higher, and released more than 30 times the energy. Moreover, the pattern is not random but appears periodically, says Barth, explaining that this twisting acts as an earthquake gate – a site on a fault that either stops the earthquake or allows it to pass. After a certain number of earthquakes of magnitude 8 or higher pass through the gate, they close and limit the maximum fracture length so that subsequent earthquakes are likely to be 7 on the Richter scale.

“I like to think of it as someone directing traffic in a building area,” explains Barth. Sometimes you get a green banner with the cars behind you, [and] Other times you get a red stop sign until conditions change. ”Understanding the condition of the gate may be the key to estimating the magnitude of the next earthquake.

Barth runs along the Alpine Fault. Credit: Andre Meer

Use modeling to determine gate state

Howarth and his team turned to a physics-based earthquake simulation on the alpine fault to produce 100,000 years of man-made earthquakes. This gave the researchers an accurate picture of potential ancient earthquakes to compare with a much shorter amount of old data they had gathered. Only the model that had the most realistic Alpine fault geometry successfully produced artificial earthquakes that mimicked actual Paleozoic patterns.

Because the code has mathematically simplified the intense physical equations that govern earthquake rupture, Barth says they can model very long records of earthquakes. After finding a model that matched observations from the Paleolithic record, Howarth and colleagues then extrapolated their model into the future, finding an 82% chance that the next earthquake would be at least an 8.0 magnitude event. “We think our study shows that this modeling approach can produce realistic records of earthquakes and provide new insights into future earthquakes that we can expect,” says Barth.

New predictions bring preparedness into public focus

Residents of New Zealand are aware of earthquakes, with the country witnessing thousands of them every year. However, there are a few events that are powerful enough to be felt, and even less so, that they cause harm.

“The new science has certainly aroused interest in the dangers posed by the Alpine Rift, as it often does when the error is in the public eye,” says Orkiston. With the higher risk this error represents, she says, “[it’s] More relevant and important for our community to prepare. ”

Franz Josef, New Zealand, is one of the cities built along the Alpine Rift. Credit: Gregg Auburn (CC BY-2.5)

References

Upton, p. Cochran, Yu. Orchiston, C. Howarth, J; Bitinga, J; Townend, J. Tercentenary of 1717 A.D. Alpine Fault Earthquake: Advances in Science and Understanding Hazards. New Zealand Journal of Geology and Geophysics 2018, 61 (3), 247-250. https://doi.org/10.1080/00288306.2018.1512389.

Biasi, GP; Langridge, RM; Berryman, K. Clark, KJ; Cochran, UA Maximum – probability and conditional recurrence parameters of a terrestrial earthquake rupture earthquake on the Southern Alps fault, South Island, New Zealand frequency parameters and conditional probability of a ground earthquake – rupture. Bulletin of the American Seismological Society 2015, 105 (1), 94-106. https://doi.org/10.1785/0120130259.

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