Lessons Learnt from the Kaikoura Quakes
We were treated to a very informative and fascinating insight into the findings to date following the Kaikoura quake which was described by our speaker, Clark Fenton, as one of the largest earthquakes since the arrival of Pakeha on our shores.
A Senior Lecturer in Engineering Geology at University of Canterbury, Clark has been actively involved in ongoing research following the M7.8 Kaikoura earthquake on the 14th of November 2016. A geological engineer from Scotland, he is very experienced in researching earthquakes which included many years as a consultant in the greater San Francisco area.
In terms of history, the meting of ice in the planet’s surface many eons ago resulted in the Earth’s crust started “shuffling” as the tectonic plates had lost a lot of weight on top of them. “This sometimes results in high impact quakes often occurring in unexpected areas” Clark noted with Sweden having recorded some of the largest ones in the past.
Large quakes like the Kaikoura one, which luckily resulted in few deaths, occur roughly every 100 years or more. “The key approach when working in this field is to look into the geological record which drives a lot of the research and exploration” explained Clark.
In terms of the actual work, the process typically involves:
- An immediate response which involves gathering information on ground motion, ground deformation and structural performance.
- Short term activities for 1-2 years looking at resilience & recovery and hazard cascades including burst dams.
- Intermediate term activities including landscape evaluation and slope stability.
- Longer term activities including lessons learnt, disaster planning and anticipating the next quake.
The Kaikoura began at 00:02 am at a depth of 15 (+/- 4 km) with an epicentre 15 kms North of Culverden and continued for approximately 120 seconds.
“Geonet was rapidly overloaded”, explained Clark, “but luckily RNZ was able to keep us well informed.”
It started in the South and propagated northwards like a zipper. Most of the energy goes with the quake, noted Clark, with Wellington suffering the most structural damage as that is where most of the energy went while Christchurch got very little energy from it. Waiau, on the other hand experienced 2-3x vertical gravity movement which was twice as much of Christchurch.
One of the greatest impact features of the quake and the resulting aftershocks was slope failures (i.e. landslides) which resulted in significant road closures.
“Our initial attempts to try and figure out what caused it was initially hampered by not being able to get about much”, noted Clark, “and as a result we focused initially on the Kaikoura South area.” This included walking along ruptures plus talking to farmers who wanted to show visitors their “better” fault(s).
This covered a large area which was researched by mostly walking around. Farm fences were especially useful in enabling researchers to map how the land was displaced which proved especially useful in planning future below ground pipelines.
One interesting difference with this quale was that there was little liquefaction as the land was mostly bedrock. The biggest impact was slope failure with over 30,000 landslides having been identified to date. “With respect to buildings, wooden timber framed houses on block bases held up the best” noted Clark.
In 2017, storms and rains re-activated quake triggered landslides which continued in 2018-19. “This is an ongoing issue”, he explained, “as it results in far greater amounts if slip occurring compared to what slipped during the initial quake(s) which delays recovery.”
One feature Clark mentioned was the Leader River dam which, by 2017, resulted in more water that entered the dam starting to make its way downstream. This resulted in more erosion which further changes the evolving nature of the river both now and will continue to do so in the future.
Leader River Dam
“The Clarence River was lifted 9 metres”, noted Clark. As a result it now has to flow through prime paddock which was lost. Additional impact is also caused by the increasing amount of sediment that ends up flowing into the sea which also affects these ecosystems including potential damage to bridges.
Clark and noted that not every slope fails but instead leave large open fissures on the landscape. One issue with these, he explained, is that this allows water to enter which can weaken them resulting in potential future landslides. An additional challenge for farmers is the loss of water quality cause by pumps failing due to the increase in sediment.
“Over time things will eventually settle”, explained Clark, “but this will take a long time.”
In summary, Clark views that slope stability is the current main focus which is particularly relevant in New Zealand as we have lots of road corridors that are at the bottom of steep slopes.
With respect to the future, the longer term focus for those working in this area is to increase the understanding of the faults themselves which includes looking at the geological record with respect to how often they have occurred in the past and what we ca can expect in the future.
In closing Clark noted that: “Faults seem to talk to each other so we have to look at them as a collection”.
We thanked Clark for his insights into one of the largest quakes we have experienced in recent times and the longer term nature of these events that will affect us for decades to come.
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