The Kekerengu Fault

Photo Tim Little @ VUW

Whilst there were many faults that ruptured during the recent M7.8 Kaikoura Earthquake, the Kekerengu Fault is perhaps the most awe inspiring in terms of its effect on the landscape and infrastructure. As it ripped through the countryside, it displaced the land to either side by an astonishing 8 to 10 metres sideways and about 2 metres vertically over many kilometres of its length.

Kekerengu Fault offset, J.Thomson @ GNS Science

In places this horizontal offset is even more - up to a whopping 12 m. This is impressive on a global scale. In the first two images here you can see what this looks like where farm tracks have been sliced through at a right angle.

Here is a drone's eye view from above:

Kekerengu Fault,   J.Thomson @ GNS Science

As the trace of the fault passes through different locations, it expresses itself in a number of ways.

Across the river from Bluff Station, it has opened up an enormous crevasse, not unlike the sort of thing that mountaineers often see on a glacier. This will be due to either a slight bend in the fault trace, and/or slumping of the downhill side of the fault where there is a slope.

Kekerengu Fault,   J.Thomson @ GNS Science

Slickensides is the name given to the scrape marks  on the surface of the wall of a fault. Here you can see that they are dipping down at about 28 degrees from the horizontal (towards the south-west). This is useful information to help understand the direction of movement of the rupture, and tells us that this fault moved obliquely (sideways and up).  When we looked across the fault we could see that the land on the far side had moved to the right. It is therefore a 'dextral' or 'right lateral' oblique slip fault.

Kekerengu Fault,   J.Thomson @ GNS Science

Fences are really useful markers to allow measurement of the fault offset, especially when they cross the fault at close to 90 degrees.as in this photo. Yes - those two lines of fencing used to join up!

Kekerengu Fault,   J.Thomson @ GNS Science

The hillside here appears scarred by a simple knife cut...

Kekerengu Fault,   J.Thomson @ GNS Science

...whereas in other places, the slip is distributed over a broad area of surface deformation. In this case it is likely that the groundshaking helped the hillside follow the call of gravity to spread the deformation over a large area.

Kekerengu Fault,   J.Thomson @ GNS Science

Near to the coast, the Kekerengu Fault tracks across this field towards the main state highway and the railway. Here the fault trace is a mound of huge clods of earth and ripped turf. We call this a "mole track", and it results from some compression rather than extension along this part of the fault trace.

Kekerengu Fault,   J.Thomson @ GNS Science

Not far away, State Highway 1 has been pushed sideways in several pieces...

Kekerengu Fault,   J.Thomson @ GNS Science

and the nearby railway has been pulled so hard that it snapped.

Kekerengu Fault,   J.Thomson @ GNS Science

The fault runs right under this small bridge which is totally destroyed.

Kekerengu Fault,   J.Thomson @ GNS Science

Lots of food for thought and plenty of work ahead for earthquake scientist Russ van Dissen and his colleagues.