Active Fault

Drilling into New Zealand’s most dangerous fault

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The Alpine Fault forms the plate boundary in New Zealand’s South Island, and is a very significant fault on a global scale. It last ruptured in 1717 AD and appears to produce large earthquakes on average every 330 years. Its next rupture has a high probability (28%)  of occurring in the next 50 years. Each time the Alpine Fault ruptures, there is roughly 8 metres of sideways movement and about 1 to 2 metres of vertical uplift on the eastern side. These magnitude 8 (M8) earthquakes can rip the fault along about 400 kilometres of its length. Slowly, over millions of years, this is what has created the Southern Alps, and offset rock formations on each side of the fault sideways by a phenomenal 480 kilometres. Massive and continual erosion of the Southern Alps keeps them relatively small (below 4000m) inspite of about 20 kilometres of uplift over the last 12 million years. For a lot more information about the Alpine Fault and its earthquakes, check the GNS Science website. Later this year, scientists plan to drill through the Alpine Fault at a depth of more than one kilometre  to sample the rocks and fluids of the fault at depth, and to make geophysical measurements down the borehole to better understand what a fault looks like as it evolves towards its next earthquake rupture. This is phase two of the Deep Fault Drilling Project (DFDP-2). The first phase of the project (DFDP-1) was successfully carried out in 2011 when two shallow boreholes were drilled through the fault to about 150m and the first observatory set up at Gaunt Creek.  DFDP-2 will involve drilling a short distance away in the Whataroa River valley, not far upstream from the road bridge on State Highway 6. This short video gives some background and information about the project:  You can also find out lots more detailed information about DFDP-2 at the GNS public wiki site here. The prospect of drilling through a massive fault could  sound alarming to some people. Is there a possibility that this project could cause a damaging earthquake? Check this next video to hear about the safety review:

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Stepping Over the Boundary

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This is a classic view of the Southern Alps from Lake Matheson on a still morning, showing the high peaks of Mount Tasman and Mount Cook.The Alpine Fault runs along the foot of the steep rangefront, extending right up the West Coast of the South Island. The mountains are therefore part of the Pacific Plate and all the flat land in front, made up of glacial outwash gravels, is on the Australian Plate. This graphic shows the Alpine Fault as a very distinct line dividing the high mountain topography to the East and from the coastal lowlands along the West Coast. Arrows show the horizontal directions of fault ruptures along the fault, but there is also a vertical component that is pushing up the Southern Alps. At Gaunt Creek near Whataroa, you can get right up close to a cliff exposure of the Alpine Fault.  The pale green rocks in the foreground have endured being crushed and uplifted along the  fault line. They have been altered into what is known as cataclasite, consisting of clay and lots of crushed rock fragments.You can visit this location by checking out our GeoTrips website here: www.geotrips.org.nz/trip.html?id=57 The low angled line of the Alpine Fault is very distinct on the right side of the photo, with the metamorphosed cataclastic rocks that have been uplifted from kilometres down in the crust being pushed over the much younger gravels to the West (right). You really can put your finger on New Zealand’s plate boundary here! The Pacific Plate is on the upper left, thrust over ice age gravels of the Australian Plate on the right hand side of the image. The photo gives a good impression of the nature of the crushed rocks. A more distant view of the cliff section from the creek shows how the uplifted rocks have over-ridden the gravels which are about 15 to 16 thousand years old. The two white arrows show the line of the fault. A short distance away is the Deep Fault Drilling Project (DFDP1) Observatory that was set up after two boreholes were drilled here in 2011. The fault is dipping at about a 40 degree angle, and the boreholes were positioned to intercept it at around 100m depth. Instruments down the boreholes include seismometers and other sensors that have been installed to better understand the physical conditions along the fault as it extends down below the surface. For a bit more background to the DFDP have a look at this previous post from 2011

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A dynamic landscape in Hawkes Bay

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Last week I was in Hawkes Bay with geologist Kyle Bland, who led a field trip for teachers, students and parents of Crownthorpe School. Hawkes Bay geology is a story of uplift along fault lines, combined with rapid erosion and deposition by rivers flowing from the inland mountain ranges. This story is etched into the geomorphology of the landscape. The Mohaka fault last ruptured between AD 1600 and 1850, and forms an amazingly straight scar across the landscape. Like many faults in New Zealand, it is an oblique strike slip fault, including both sideways and vertical movement.  If you click on the image to enlarge it you can see how streams crossing the fault have been offset by sideways movement from the last rupture. Combined sedimentation, uplift and erosion have produced stepped terraces alongside the Ngaruroro river flowing from the Ruahine range out towards the coast. There are many fossils to be found in the sedimentary rocks that have been uplifted and exposed. Fossil hunting Hawkes Bay style involves using a digger to get access to your specimens! Ancient greywacke sediments are exposed in the Ruahine Range, having been uplifted by tectonic movements of the North Island fault system (Mohaka and Ruahine faults). These rocks were deposited in a trough at the edge of Gondwanaland, long before New Zealand ever existed. In the video below, Kyle gives us a Hawkes Bay case study of landscape evolution.

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New Zealand’s Alpine Fault

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 For the latest on the Alpine fault drilling visit Rupert’s Blog This NASA photo of the South Island of New Zealand shows the green of lowland vegetation contrasting clearly with snow in the mountains of the Southern Alps. The straight edge of the mountains is the line of the famous Alpine Fault. This fault is the boundary between the Pacific and Australian tectonic plates which are sliding past each other in this region at an average rate of about 40 mm per year. The Alpine Fault is a globally significant feature and similar in character to the San Andreas Fault in America or the North Anatolian Fault in Turkey. Every 200 – 400 years the plate movement is accomodated by a violent earthquake of about magnitude 8, and dislocation along a segment of the fault of roughly 8 metres. The last big earthquake rupture occurred in 1717 AD. Most of the fault movement is sideways, but a portion of it is vertical, and has uplifted the mountains on the eastern side to reveal exposures of the rocks along the fault that have been buried and altered over millions of years. The second image shows a slice of this metamorphic rock with white streaks of quartz, black mica and a red garnet crystal that is about 5mm across.   Rupert Sutherland at GNS Science is one of the leaders of the Deep Fault Drilling Project (DFDP) which is about to drill into the Alpine Fault. Many other research organisations are involved in this very large, multi year project. To listen to a short radio interview of Rupert talking about the project go here. From next week, the  DFDP project will start by drilling two shallow (150m) boreholes through the fault near Whataroa on the West Coast. Rocks will be sampled and analysed and instruments will be left in the ground as part of a long term monitoring programme.  In future years this reasearch will be extended by drilling down several kilometres. In the photo of Rupert he is holding a specimen of fault breccia – a bit of rock that has been fragmented by rupturing of the Alpine Fault. In the close up image you can see how the rock is broken up. The dark patches are where some of the rock was melted and then solidified again in the spaces between the fragments.

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The Wellington Fault with LEARNZ

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LEARNZ is a unique kiwi organisation that runs ‘virtual’ field trips for primary and secondary schools in New Zealand. Using videos, audioconferences and internet based information, school kids are able to interact with scientists and other expert professionals in different parts of New Zealand. LEARNZ even runs virtual field trips to Scott Base in Antarctica. Last week, Shelley Hersey and Andrew Penny from LEARNZ were investigating the Wellington Fault. Russ van Dissen, Julia Becker and Hamish Campbell from GNS Science joined me to assist them. We looked at the way scientists work with planners and emergency services to understand the earthquake risk and prepare for the possibility of a ‘Big One’ striking Wellington. There is nothing like a detailed description of the potential impact of a natural disaster to remind you to re- check your personal Civil Defence emergency preparedness! In addition to the many other active fault lines in the region that could cause an earthquake, there is a small but real possibility of a Wellington Fault earthquake occurring. This would cause a rupture along the fault line with perhaps 5 metres of sideways movement and one or two metres of vertical dislocation. Houses built across the fault would be ripped apart, and the whole city shaken violently, resulting in severe building damage, streets full of glass and other debris, broken water, gas and electricity supplies, roads, railways and the ferry terminal out of action and communications largely cut off. Did you click on that emergency preparedness link yet? Over three days we visited a number of city viewpoints and structures, the emergency operations centre of the Wellington Regional Council, the fault line itself, and the water supply lakes at Te Marua. It was reassuring to see how much thought and effort has been put into planning for the earthquake risk by the authorities. In this photo of the place where the fault runs right below the Thorndon overbridge and the Wellington Railway, you can see the steel re-inforcement casings around the motorway support pillars and the large concrete slabs that are designed to prevent the motorway segments from collapsing. The take home message is very much that local government and other organisations are doing their bit, and it is up to us individuals to make sure that we have our personal survival plans in place as well. Over one hundred school classes participated in the event. The GNS Science website has a lot of information about the Wellington Fault, including a tour guide, a fault line field trip, a photo gallery , a Google Earth flyby video as well as our Wellington Fault short doco movie:

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