New Zealand and its Faults

From Geonet

Kekerengu Fault has a Word to its Geologists

 
Written by Ursula Cochran

Last year Tim Little of Victoria University of Wellington and Russ Van Dissen of GNS Science thought it would be a good idea to find out more about the Kekerengu Fault in North Canterbury. Through previous work they had identified the Kekerengu Fault as likely to be the fastest slipping fault within 100 km of Wellington city apart from the Hikurangi subduction zone.

They knew this meant it posed a significant seismic hazard to the northeastern South Island and also to Wellington if linking faults in Cook Strait ruptured at the same time as the Kekerengu Fault.
In February of this year, with funding from the Natural Hazards Research Platform, Tim and Russ excavated three trenches across the Kekerengu Fault to look for evidence of past large earthquakes. The main aim of their project was to better constrain the seismic hazard posed by this major active fault.

In these trenches Tim and Russ found evidence that at least three past large earthquakes had occurred in the last 1250 years. These initial results confirmed that the Kekerengu Fault was capable of producing large earthquakes frequently (on average, about every 300 or 400 hundred years), and was likely to do so again in future.

Then, two weeks ago, as if to say, “Don’t underestimate me!” the fault ruptured right through those same trenches. Tim was awe-struck. As a geologist working on active faults he said, “I had often wondered what it would look like if a fault moved while we were working on a trench cut across it, but I had never expected this to happen to me.”

When the Kekerengu Fault moved as part of the M7.8 Kaikoura earthquake the impacts on the landscape were dramatic. One side of the fault has moved as much as eleven metres with respect to the other side. Tim did not expect quite this amount of slip on this fault during a single earthquake. Russ, though, was less surprised – he says it fits with the long-term slip rate calculated for the fault – but he is still amazed to see such fault movement in action.

So, we knew about this fault, we knew it posed a seismic hazard, we even thought it was possible that it would rupture jointly with other faults – New Zealand’s National Seismic Hazard Model specifically includes scenarios that involve joint rupture of the Jordon, Kekerengu, and Needles Faults. And this, now confirmed by NIWA’s offshore survey of the Needles Fault, is exactly what happened on Monday 14^th November. What we had not foreseen is that even more faults would be involved in a single earthquake sequence.

Currently we have evidence for seven faults rupturing in the M7.8 Kaikoura earthquake so work on the Kekerengu Fault is just a small part of the earthquake geology response. There are teams from University of Otago, University of Canterbury, University of Auckland, Victoria University, GNS Science, and NIWA, not to mention volunteers from overseas, currently mapping and measuring the faults that moved last week. We want to understand what happened in this event but, most importantly, what it means for future events.

The Kekerengu Fault has been speaking to geologists-in-the-making for generations because Victoria University’s third year structural field geology course is held near its northern end. I still clearly remember the Kekerengu Fault back in the early 1990s as a subtle, curious line in the landscape that our professor – Tim Little – stood astride inciting us to notice. Today, the fault has spoken and it is impossible not to notice.

One of Tim and Russ’s trenches (pink rectangles) was cut in half by movement on the Kekerengu Fault (red line) in the M7.8 Kaikoura earthquake. The two sides of the trench are now about nine metres apart. Drone photo by Julian Thomson

Rupture of the Kekerengu Fault can be seen especially clearly where it cuts across man-made features such as this farm track. Will Ries is measuring how far the road has been moved both horizontally and vertically across the fault. Photo by Tim Little

The green “sausages” on this map show the current understanding of where surface fault rupture occurred in the M7.8 Kaikoura earthquake.

Russ Van Dissen walking the Kekerengu Fault to observe the ground deformation that occurred in the Kaikoura earthquake. Photo by Kate Clark.