View of the coastal plain between the sea and the mountains looking northeast towards Waiau Franz Josef, 1984. Photo by Lloyd Homer, courtesy GNS Science.
Magnificent mountains with precipitous snowy peaks may be what Te Tai Poutini, the West Coast of Te Waipounamu, the South Island of Aotearoa, is most famous for. But the flatter land at the foot of the mountains is what’s most valuable to residents, farmers, and travellers of the West Coast. A paper by Kiwi earth scientists highlights the landscape changes expected for this precious, habitable land after large earthquakes.
Almond, P.C., Berryman, K., Villamor, P., Read, S., Alloway, B.V. & Tonkin, P. (2023) Alluvial fan response to Alpine Fault earthquakes on the Westland piedmont, Whataroa, Aotearoa-New Zealand. Earth Surface Processes and Landforms, 48(9), 1804–1829.
Between the Southern Alps and the Tasman Sea there’s only a narrow stretch of accessible and fertile land on which coasters live and make a living. Any road tripper travelling between Greymouth and Haast can observe the sharp change in slope between the steep flanks of the main range and the gently sloping lowlands that accommodate the towns, roads, and farms of the West Coast.
The change in slope marks the meeting of the Pacific and Australian tectonic plates. Better known as the Alpine Fault, the demarcation in the landscape shows us clearly which side of the fault has been uplifted in earthquakes to form mountains (the southeast side) and which has not (the northwest, seaward side). So how and when did the gently sloping land on the seaward side of the Alpine Fault form? This was the subject of a study recently published in Earth Surface Processes and Landforms.
View of the coastal plain between the sea and the mountains looking northeast towards Waiau Franz Josef, 1984. Photo by Lloyd Homer, courtesy GNS Science.
Erosion of unvegetated mountainsides is a gradual and continual process but it can be exponentially sped up by widespread land sliding after high rainfall or ground shaking. Remember the 2016 Kaikōura earthquake and the tens of thousands of landslides triggered by the shaking? Similarly, tens of thousands of landslides are likely to occur in the next magnitude 8 earthquake on the Alpine Fault. So, what happens to all the material brought down in landslides?
The landslide debris gets swept up by streams and rivers and starts filling channels and valleys in the weeks, months, and years following an earthquake. When it reaches the edge of the mountain range, unconfined by valley sides, it spills out forming big fans and plains and starts building up the level of the land in a process that can continue for decades. Much, but not all, of the sediment ultimately makes its way to the sea. Alluvial fans and plains are made by repeated deposition of boulders, gravel, sand, and silt – basically all the eroded debris coming out of the mountains.
The Hapuku River landslide in the seaward Kaikōura range was one of the many landslides that occurred in the 2016 Kaikōura earthquake. Photo by Dougal Townsend, courtesy GNS Science.
Outcrops created by erosion from the Whataroa River showing the extensive deposition of sediment in range-front fans over the last 1-2 thousand years. Image modified from Almond et al., 2023.
Soil scientists and earthquake geologists from Te Whare Wānaka o Aoraki Lincoln University and Te Pū Ao GNS Science teamed up to look at the history recorded in these range front alluvial fans. Conveniently, the Whataroa River has cut into the edges of some of the range front fans revealing cross sections of sediment layers dating back over 1500 years. Most of the visible thickness (>70 metres) of the fans consists of gravels and sands. But these masses of rocky grey debris are punctuated by stripes of dark brown, organic soils.
“Soils mark the stable periods in history, like the time we’re in now, when there is little to no sediment being added to the fans and vegetation can thrive”, explains lead author, Peter Almond. “We were interested in the timing of soil burial and how much time these soil units represent.”
Radiocarbon dating of tree rings from trees found in their original growth position in the soils, and pieces of wood in the gravels, as well as chemical estimates of soil residence times, enabled a timeline of fan building to be developed.
“Soils mark the stable periods in history, like the time we’re in now, when there is little to no sediment being added to the fans and vegetation can thrive”, explains lead author, Peter Almond. “We were interested in the timing of soil burial and how much time these soil units represent.”
The researchers identified four episodes of fan building over the last 1000 years and the timing of these aggradation events correlates with earthquakes on the Alpine Fault or in the Southern Alps.
“What we’re seeing in these fans are large pulses of sediment coming out of the ranges as a result of earthquake-induced landslides in the catchments”, says Kelvin Berryman, second author on the paper. “We are confident that large earthquakes are the main drivers for building these landforms in this setting.”
In other regions, high rainfall events can build similar fans, but on the West Coast, where annual rainfall is already high, it seems that earthquakes do most of the initial work in releasing sediment to the coastal plain.
The authors believe that their case study fans are representative of much of the central West Coast, “We consider it very likely that there will be a substantial response to all major Alpine Fault and Southern Alps earthquakes in these and other similar fans.”
Sediment spilling out of the Southern Alps in the Waiho River valley near Waiau Franz Josef. Photo by Lloyd Homer, courtesy GNS Science.
“What we’re seeing in these fans are large pulses of sediment coming out of the ranges as a result of earthquake-induced landslides in the catchments”, says Kelvin Berryman, second author on the paper. “We are confident that large earthquakes are the main drivers for building these landforms in this setting.”
In demonstrating that these fans were primarily built after earthquakes rather than storms, the timing of sediment build-up becomes clear. With the central section of the Alpine Fault rupturing, on average, every 250 years, region-wide outpouring of sediment may only happen every few hundred years but, when it does happen, it’s likely to continue for decades.
From the outcrops they could observe, the researchers calculated that, at a minimum, 2-4 metres of sediment would be added to fan surfaces over 130-400 years. But rates of up to 10 metres in <129 years were also observed. These are rough estimates, but high-resolution records from West Coast lakes paint a similar picture: that three times the amount of sediment enters the lakes in the 50 years after a large earthquake than in quiet periods.
Landslides at Gaunt Creek in ~1918 and Mount Adams in 1999 provide examples of what can happen on the plains once landslide debris is transported out of the mountains. Downstream of these landslides, rivers flooded and permanently changed course, the land surface built up with gravel and buried roads, fences, and farmland. These were one-off, isolated landslides affecting single catchments. After an Alpine Fault earthquake, most catchments along the central West Coast would be impacted.
Landslide and rock avalanche into the Poerua River near Mount Adams. Photo by Lloyd Homer, courtesy GNS Science.
The Otira zigzag on State Highway 73 illustrating the challenge of maintaining transport routes in the face of sediment that is on the move. This section of road is now bypassed by the Otira viaduct. Photo by Lloyd Homer, courtesy GNS Science.
Given the narrow habitable zone and the single state highway linking settlements, there are likely to be numerous locations where fan building, and river aggradation impacts daily life for decades after a large earthquake.
The vulnerability of State Highway 6 is well known – it crosses the fault and traverses steep, unstable terrain. However, this study has brought to light another post-earthquake hazard to contend with: the road traverses about 130 kilometres of alluvial fans and plains so it’s likely to be buried by sediment on a frequent basis. And if roads are being buried, then so is fertile soil. This will make farming untenable in many of the locations where it currently provides livelihoods for many people.
“We consider it very likely that there will be a substantial response to all major Alpine Fault and Southern Alps earthquakes in these and other similar fans.”
This alluvial fan study has highlighted some of the less obvious realities of living on a plate boundary – the challenge is not only about surviving large earthquakes and their immediate aftermath, but also about finding ways to live in a landscape that will be changing and adjusting to earthquake-induced disruption for decades.
Almond, P.C., Berryman, K., Villamor, P., Read, S., Alloway, B.V. & Tonkin, P. (2023) Alluvial fan response to Alpine Fault earthquakes on the Westland piedmont, Whataroa, Aotearoa-New Zealand. Earth Surface Processes and Landforms, 48(9), 1804–1829.
Thanks to Lincoln University for leading this research and Resilience to Nature's Challenges for funding the translation of the findings into this online article.
