"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. The Papatea Fault ruptured and
created a 6.5 meter high barrier within the channel of the Waiau Toa, stopping
the river in its course and rapidly and permanently altering the path it takes.
But can we predict this sort of thing before it happens?"
*** Xanni ***
Chief Scientist, Xanadu
Partner, Glass Wings
Manager, Serious Cybernetics