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Wednesday, 7 December 2016

Landslide Dam

Seaward Slide, J.Thomson @ GNS science
Rockfalls and landslides were one of the dramatic consequences of the M7.8 Kaikoura Quake. This first photo shows one that is actually so huge that you might not at first recognise it for what it is. The white cliff in the distance is the landslide scarp and the huge green capped pile of grey in the middle distance is the debris that fell away. This landslide was of course made famous on TV by the cows that became trapped on an isolated hummock in the debris pile.



SH1 and Railway, Steve Lawson @ GNS Science
A large number of coastal cliffs collapsed, causing spectacular damage to the coastal transport infrastructure. In this image you can see how the raiway line has been lifted up and dropped across the road and across the beach.




J.Thomson @ GNS Science

Another example of rockfall damage along the coast:






Hapuku Landslide, Steve Lawson @ GNS Science



In the Canterbury ranges, a short distance inland, a number of landslides have blocked river valleys and created landslide dammed lakes that are now filling up. This image shows the massive Hapuku landslide, which has buried the valley in over 150 metres of debris, weighing many millions of tonnes. The grey coloured lake in the centre of the image is a couple of hundred metres long



Hapuku landslide, J. Thomson @ GNS Science

This is a close up view of the lake taken a few days later. The lake is now near to the point of overflowing the dam. The problem with these dams is that they can fail catastrophically, sending a debris flow of water, mud and rock down the valley with potentially very destructive consequences.





Linton landslide survey, J.Thomson @ GNS Science


In this image you can see another landslide, this time in the Linton Valley. It has also dammed a small river. The team here are surveying the debris and the shape of the valley in order to calculate the possible downstream consequences of a breach of the dam.





Linton landslide, J.Thomson @ GNS Science




This photo shows the size of the landslide.  A large section of forest has slid down with it with many trees still standing. The debris has again blocked the valley to form a lake.







Linton landslide dam, J.Thomson @ GNS Science


The lake level is still about 10 metres below the rim of the dam:








Linton landslide dammed lake, J.Thomson @ GNS Science







In order to measure the lake's water level safely, Chris Massey took a GPS reading from the helicopter whilst it hovered just above the water surface.







Linton landslide, J.Thomson @ GNS Science

Meanwhile at the base of the dam, some water is percolating through the debris, although the flow in the stream bed is much less than usual:
Linton landslide, J.Thomson @ GNS Science








This photo shows the toe of the landslide - a mass of rock debris and damaged trees.







Linton landslide, J.Thomson @ GNS Science




By the end of a few hours, we had lots of data in the form of laser scans of the slip from different locations, as well as hundreds of drone and aerial photos, which are combined to make a 3D digital image that can be used to model the possible consequences of the dam breaching in different ways.




This video made by Steve Lawson is a virtual 'fly through' of the digital model:


video

And here is a short video about these landslide dams:



Finally, there is more information about landslides on the GeoNet website here

Thursday, 24 November 2016

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.

Monday, 21 November 2016

A Ruptured Landscape

J,.Thomson @ GNS Science
On the ground in the Kaikoura Quake aftermath:

Following the recent M7.8 Kaikoura Earthquake, a number of teams of scientists have been deployed to survey the geological impacts and assess the potential ongoing risks to people and infrastructure.

This gallery of images shows some of the numerous dramatic impacts of the quake in the coastal area to the north of Kaikoura.

 J.Thomson @ GNS Science
Accessing the area by road involves careful driving. The road surfaces next to many of the bridges have subsided, creating a crack at either end of the bridge:








 J.Thomson @ GNS Science
Slumping has occurred along parts of the highway:




 J.Thomson @ GNS Science





This photo shows the now famous house at Bluff Station that had the mis-fortune to be built directly on top of the Kekerengu Fault. The house was shunted about 7 metres sideways leaving some of its foundations behind.



J.Thomson @ GNS Science 




The house was pushed across its own driveway...



J.Thomson @ GNS Science






The coastal highway and railway have unfortunately been cut through in several places by fault ruptures. This view looking south at Waipapa Bay shows the northern branch of the Papatea Fault crossing SH1 and heading out to sea.



J.Thomson @ GNS Science






This is what the road now looks like on the ground. The fault scarp has been bulldozed to allow vehicle access.

J.Thomson @ GNS Science
A short distance away, the railway line was lifted up and dropped in the grass next to its original gravel bedding.



J.Thomson @ GNS Science






From the top of the fault rupture, you can see that the displaced railway tracks extend for about 300 metres into the distance.



Will Ries @ GNS Science




A few hundred metres further south, the southern branch of the Papatea Fault crosses the road and railway.




J.Thomson @ GNS Science






The earthquake ripped right through the concrete culvert that ran under the road, and again lifted the railway off its bed.




J.Thomson @ GNS Science






From the air, the scarp of the southern branch of the Papatea Fault is seen to extend like a knife-cut across the shore platform. In this image you can sea the uplifted coastline extending into the distance. The total uplift of the area left (east) of the fault is 5 to 6 metres, whilst the area to the right was uplifted by a smaller amount. Water has been ponded up against the new fault scarp.

J.Thomson @ GNS Science




A helicopter view showing the scarp of the Papatea Fault close up (across the top of image). The fault movement is thought to have been mostly horizontal with about 2 metres of vertical uplift in addition.






J.Thomson @ GNS Science

The Papatea Fault scarp is a sheer wall about 2 metres high.



J.Thomson @ GNS Science







Part of the task for scientists is to measure the uplift along the coast. The high and low water marks make a useful reference point that can be surveyed against the new sea level positions.



J.Thomson @ GNS Science




Sadly the raised shoreline stranded innumerable sea creatures that now litter the area amongst the seaweed.




J.Thomson @ GNS Science
Rockfalls have been numerous, and have caused a lot of damage where the road and railway are squeezed up close to the coastal cliffs.



J.Thomson @ GNS Science





The end of the road? The reason why you won't be travelling into Kaikoura from the north anytime soon. This rockfall is at the south end of Okiwi Bay, and there are more slips like this further south.

Wednesday, 28 September 2016

Tracking Dinosaurs in NW Nelson


Greg Browne. Image Julian Thomson @ GNS Science
In New Zealand there is only one area (with six individual locations not far from each other) in which dinosaur footprints have been identified. This is in NW Nelson in the South Island. They were discovered and researched by Greg Browne, a sedimentologist at GNS Science who has spent many years doing geological fieldwork in the area. The first announcement of their discovery was in 2009 as shown in this video.




Dinosaur footprints near Rovereto, Italy. Image J Thomson
When compared to the easily recognisable dinosaur trails that are found in other parts of the world, the structures that have been classified as footprints in New Zealand are not initially obvious.  The photo shows an example from near Rovereto in northern Italy where each footprint is about 30 cm across.






Image Julian Thomson @ GNS Science
In comparison, the New Zealand examples are irregular in shape and position. It took a lot of research and a process of elimination to be certain that these structures are indeed trace fossils of dinosaurs, rather than originating from another biological or mechanical cause.. 
In order to be able to point at a dinosaur origin for these impressions, there are several factors that have to be considered. As a starting point we can look at horses on a modern beach:

Image: Van der Lingen, G.J. & Andrews, P.B
This photo was taken by researchers who investigated horse hoof marks that were imprinted on a beach sand in New Zealand (from van der Lingen, G.J. & Andrews, P.B. 1979, Journal of Sedimentary Petrology). They carefully cut a vertical slice through the imprint to study the details of how the horizontal layers of sand were deformed by the weight of the passing animal. The hand lens shows the scale:







Base image: Van der Lingen, G.J. & Andrews, P.B
There are essentially three ways in which the original sediment has been affected:
(A) - Jumbled particles and blocks of sand have  fallen into the depression made by the footprint. (B) The footprint has a clear vertical margin on either side (C) The sediment underlying the footprint has been compressed downwards.

 




It is most likely that these horse footprints were soon eroded after their formation in the late seventies, due to tides, storms, wind or even the action of shore creatures such as crabs, worms or shellfish. On the other hand, there is a small possibility that they were  preserved quickly beneath a new layer of sand and are still intact beneath this protective covering.

Base image: Van der Lingen, G.J. & Andrews, P.B
Over geological time, sediments such as these can become buried deeply, compressed into solid rock and later revealed by uplift and erosion at the modern land surface. In the case of the horse footprint, its appearence on the surface (in 2 dimensions)  would then depend on the amount and angle of erosion. For example, if it is were eroded near to the top of the footprint (the level of line 1 in the photo) it would appear relatively large compared to if the erosion had removed most of the material, and only the lower part of the footprint were showing (line 2).

Similarly if a vertical section of the footprint were to  be exposed, its size and appearance would differ depending on whether the section that was revealed represented the centre of the footprint (3) or its edge (4).

Image Greg Browne @ GNS Science
Here is an example of one of the footprints that Greg identified in the Upper Cretaceous rocks of Nelson. It shows similar features in cross section to the horse footprint (at approximately the same scale)- the infilling (A), the distinct margin (B) and the compressed underlying layers (C).





Image Greg Browne @ GNS Science


Here is another example of a vertical slice through a footprint, with the dotted line highlighting the distinct margin of the structure:







Julian Thomson @ GNS Science
This photo shows a footprint eroded horizontally. The heel has cut a sharp edge into the sediment at the back end of the feature (lower left), while the front has been compressed into ridges as the foot tipped forwards during locomotion (near finger).




 







Having confirmed these features as footprints being preserved in sediment from an intertidal environment, the question then arises as to whether animals other than dinosaurs could have made them. Having tackled this question over many years, Greg Browne worked through the following possible examples and discounted them for the reasons given:

  1.  Fish feeding or resting traces: depth of penetration and lack of deformed strata below.
  2. Amphibian foot prints: unlikely to have an amphibian large enough.
  3. Bird foot prints: bird would have to be large and heavy.
  4. Mammals: the only pre-Pleistocene mammals known from New Zealand are Early Miocene mouse-like fossils. Evidence throughout the world indicates that Cretaceous mammals were small, and did not develop into large animals until after the end of the Cretaceous extinction event and the demise of the dinosaurs.
  5. Reptile foot prints: dinosaurs: only dinosaurs would be of sufficient size and weight to have generated these deformed point source compression structures.
Recently, with funding from the Unlocking Curious Minds Fund of the Ministry for Business, Innovation and Employment (MBIE), a team from GNS Science were assisted by teachers and students of Collingwood Area School, to clean up a large rock slab in the search for more dinosaur footprints.



With a lot of hard work, involving cleaning mud
off the rocks with buckets of water, brooms and shovels, some hitherto unseen dinosaur footprints were revealed for the first time since the Cretaceous Period, about 70 million years ago.

Here are some quotes from our assistants:

"It was a wonderful once-in-a-lifetime opportunity to work with a team of scientists and look at a real dinosaur footprints."






"It was an honor and very humbling knowing that we were the first people to see these footprints in 70,000,000 years."


"It was an incredible opportunity. We were able to work alongside the scientists and they taught us about how to identify the footprints and showed us how they took peels of them."


 This video tells the story of the expedition:



For more information about this Dinosaur Footprints project, including  newsletter updates, click here.