The Wairarapa Fault is one of New Zealand's large active faults running along the eastern edge of the Rimutaka range from Palliser Bay north into the Wairarapa. It was responsible for the massive magnitude 8.2 earthquake that violently shook the lower North Island in 1855 in New Zealand's largest historically recorded 'quake.
This Google Earth view shows the surface trace of the fault, with the Rimutaka Range to the west and the Tararuas in the distance. An interesting location called Pigeon Bush is indicated by the red circle. It is about 50 kilometres north-east of Wellington City.
|Photo Andrew Boyes / GNS Science|
A close up view shows some interesting features beside the fault scarp. Two stream channels (middle and foreground of image) appear out of the scarp, with no sign of any catchment gully above them. Meanwhile a bit further along (where the trees are) you can see that there is a deep cut gully in the scarp itself.
In this photo, Rob Langridge, an earthquake geologist from GNS Science, is standing between the first (most recently beheaded) stream channel on the left, and the vegetated gully that was originally connected with it on the right. Some idea of the amount of offset that occurred in the 1855 earthquake can be appreciated from the image. There would also have been some uplift during that earthquake of perhaps one or two metres at this location.
We also measured the offset of the older stream channel which was about 15 metres away from the first beheaded channel.This previous earthquake is thought to have occurred about 1000 years ago. The average repeat interval for ruptures of the Wairarapa Fault is thought to be about 1200 years.
|Offset stream channels at Pigeon Bush, A Boyes / GNS Science|
About 45 kilometres north of Pigeon Bush it is possible to see a view of the fault itself in a cutting of the Ruamahanga River near Masterton. In the photo you can see how older grey rock on the right (west) have been pushed up relative to the younger gravels on the left (east) in a reverse fault. The substantial horizontal movement may also have caused this juxtaposition of older rocks against younger ones.Phillip Robinson is inspecting the older shattered greywacke rocks that have been thrust over the gravels from the west (left), tilting the relatively young 50 000 year old gravel layers from a horizontal to a vertical orientation.
this previous post to learn about the amazing uplifted beaches at Turakirae Head.
Note that you can now find out how to visit Thrust Creek (and many other geology locations) on our GeoTrips website here: https://geotrips.org.nz/trip.html?id=255