Earthquakes can change the course of rivers – with devastating results

opinion

The 2016 Kaikoura earthquake in New Zealand stopped the Wayau Tua/Clarence River in its tracks. Within hours, the river flooded out of its channel and changed course. In the seven years since the 7.8-magnitude earthquake, the river has completely abandoned the course it was taking.

This isn’t the first time this kind of thing has happened.

Floods and earthquakes are among the most common natural disasters worldwide. A great deal of work has been done to understand its dangers – but relatively little to determine at the same time how it occurs.

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it’s a problem. Tens of thousands of active faults run under river channels around the world and in New Zealand. Where faults and rivers intersect, earthquake hazards and river floods intertwine as well.

Our new research shows that when a fault deforms the Earth’s surface, it can cause a river over it to suddenly flood out of its designated channel. This could endanger unsuspecting communities.

The 2016 Kaikoura earthquake caused a peak ground acceleration of 3.23 g – making it one of New Zealand’s largest earthquakes in recent history. photo/NZTA

In some cases, sudden shifts in the river—also known as overturning—may cause the river to create a new channel within the landscape.

There are many examples of this phenomenon throughout history, including the rupture of the Reelfoot Fault in 1812, which dammed the mighty Mississippi River for several hours. The same earthquake also permanently dammed the Reelfoot River, creating Lake Reelfoot.

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Earthquakes occur due to the sudden movement of faults. When a fault erupts at the surface, it can move one side of the fault vertically past the other. This can cause a large mass of land to be permanently raised or lowered.

When faults run beneath rivers, this vertical movement can produce a fault—a wall of rock and/or soil—that impedes the river’s ability to continue flowing in its usual channel.

This is what happened in Kaikoura in 2016. The Papatea Fault ruptured and created a 6.5m high barrier within the Waiau Toa/Clarence River channel, stopping the river in its tracks and quickly and permanently changing the path it takes.

But can we anticipate this kind of thing before it happens?

prediction shifts

The data from the Kaikoura earthquake provided an opportunity to test whether these types of shifts in river flows and potential flooding can be “predicted” in advance. Turns out, that might be possible.

We built two flood models intended to reproduce the Waiau Toa/Clarence River diversion. The first model used topographic data obtained in the aftermath of the 2016 Kaikoura earthquake, which contains the real Papatea fault wound. The second model simulates variability using pre-earthquake topography, modified by the artificial Papatya fault.

Both models performed well, accurately reproducing the flood patterns that occurred in 2016. This indicates that changes in river flood patterns after surface-shredding earthquakes can be predicted in advance.

However, it is impossible to predict the exact amount of surface displacement a fault might produce at a rupture, or the exact conditions of river flow when this occurs. Alternatively, flood modeling can be used to explore scenarios in advance using historical flow information and historical fault data.

Applying this to the Papatea Fault rupture, we found that abrupt shifts in river flow may not occur immediately if the river is low.

Better layout

This is important, as it indicates that flooding can be delayed after a superficially ruptured earthquake if the affected river is lower. However, the river may still change course later on, as the flow rate increases.

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Creating flood models in advance may allow planners to identify key areas along the river that are at risk. They can then put in place measures that will reduce the impact of flooding, such as dams.

New Zealand’s location above plate boundaries means that earthquakes are a common natural hazard. Flood risks are also increasing in frequency and intensity.

Kaikoura is not the only community that can be affected by a combination of earthquakes and floods.

Dust from a powerful aftershock hangs over the Clarence River. Image/Mark Mitchell

Many of New Zealand’s active faults lie beneath rivers located near populated areas, or critical infrastructure. Examples include the Wellington Fault, which lies under the Hutt River, the Tytree Fault and the Tyree River Junction which borders Dunedin Airport and many towns.

However, we usually don’t think about how these rivers might change after an earthquake ruptures the surface, meaning nearby populations and infrastructure remain exposed and unprepared. The unique combination of earthquakes and floods is rarely considered in current flood management strategies or earthquake response plans.

It is essential that current earthquake response plans take into account the impact of active faults supporting river systems. Existing flood models that neglect their existence may underestimate the extent, longevity, and patterns of post-earthquake floods.

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Our modeling provides a way forward. With some knowledge of fault location and mode of rupture, interactions between surface-ruptured earthquakes and river flood hazards can be explored early on.

Erin McEwan is a PhD student in Geosciences, University of Canterbury

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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