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Earthquakes can change the course of rivers – with devastating results. We may now be able to predict these threats
By Erin McEwan, University of Canterbury
New Zealand’s 2016 Kaikōura earthquake stopped the Waiau Toa – also known as the Clarence River – in its course. Within hours, the river flooded outside its channel and changed course. In the seven years since the magnitude 7.8 earthquake, the river has completely abandoned the path it used to take.
This is not the first time this sort of thing has happened.
Flooding and earthquakes are some of the most frequent natural disasters globally. A great deal of work has been done to understand their risk – but relatively little to determine how they can occur at the same time.
This is a problem. Tens of thousands of active faults run under river channels around the world and in New Zealand. In places where faults and rivers intersect, earthquake and river flood hazards are also intertwined.
Our new research shows that when a fault deforms the earth’s surface, it can cause an overlying river to suddenly flood outside its established channel. This can put unsuspecting communities at risk.
In some cases, the sudden river shifts – also known as avulsion – may even cause the river to establish a new channel within the landscape.
There are many examples of this phenomenon throughout history, including the 1812 Reelfoot fault rupture, which dammed the mighty Mississippi river for several hours. The same earthquake also permanently dammed the Reelfoot river, creating Reelfoot Lake.
Earthquakes occur due to sudden movement on faults. When a fault ruptures to the surface, it can shift one side of the fault vertically past the other. This can cause a large block of land to be permanently uplifted or depressed.
Where faults run under rivers, this vertical movement can produce a fault scarp – a wall of rock and/or soil – that obstructs the river’s ability to continue flowing in its usual channel.
This is what happened in Kaikōura in 2016.
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