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The Dart Landslide

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Simon Cox   GNS Science M. McSaveney GNS Science Slip Stream is a tributary to the Dart River in the South Island of New Zealand. There has been an active landslide here for several thousand years, periodically sending down lobes of debris to gradually build up a large fan in the Dart Valley. There was vegetation established right across the fan, but over the last few years the widespread cover of trees has been largely buried and killed off by a very active phase of erosion and deposition. Debris volumes of the order of 100 000 cubic metres have been coming down during heavy rains in the spring and summer periods. Simon Cox  GNS Science The debris gets mobilised into a wet mix of mud and boulders.  The latest large event occurred early in this month (4th January 2014), and the flows continued to build up over several days. M. McSaveney GNS Science The debris flows crossed right over the valley, blocking the Dart River with a low angled, shallow pile of soft sediment. M. McSaveney GNS Science A lake formed in the valley above the slip, becoming about 4 kilometres long. The river is cutting down into the debris, and it is expected that the depth of the lake will fluctuate during landslide activity. The Department of Conservation is diverting the affected part of the Dart Valley track so that trampers can continue to visit the area. Photo DoC/Vladka Kennett This image gives a good overview of the affected area.  It shows the fan with the darker coloured triangle of recent debris, as well as the length of the lake. This is a graph from the Otago Regional Council website showing 7 days’ rainfall recorded from the 9th to 16th January at the Hillocks, about 24 kilometres down the Dart Valley from Slip Stream. The second graph shows how the river flow responded to the rain, with a sharp peak and a gradual tailing off after the rain stopped falling. The tail is not entirely smooth with a dip when the flow gets below 100 cubic metres per second. This suggests that when the river level drops, the continuing input of debris at the slip impedes the flow for a while, until the blockage is overcome and the flow rate increases again. Mark Rattenbury (left), Simon Cox (right) and Mauri McSaveney (behind the camera) visited the area to assess the impact and any possible downstream hazard. Note that a special DOC permit is required to visit Slip Stream as it is in the sacred Te Koroka topuni area of Mount Aspiring National Park.  The slip is in a state of continual instability and the area is hazardous. In this video Simon explains some of the interesting features of the slip, including some very strange bubbles that release dry dust when they burst:

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The growth of Tasman Glacier Lake

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The Tasman Glacier is the largest glacier in New Zealand. Its upper section is mostly white as you would expect of a river of ice. However, the lower half is covered with a layer of rock debris about a metre thick. This forms an insulating layer that slows down surface melting and allows the glacier to descend a long way down the valley to warmer elevations. This photo, taken from the location of the the top of the moraine wall near the old Ball Shelter in 2007, shows what the debris covered surface of the lower Tasman Glacier looks like.The moraine walls show how much the glacier surface has lowered in the last century. Before about 1912 the glacier surface was higher than the lateral moraine. New Zealand’s other large valley glaciers have all been suffering a similar loss of ice. Tasman Moraine Wall.      Julian Thomson GNS Science This is what the moraine wall of the Tasman Glacier looks like close up. The unstable terrain is very hard to travel over, especially when you are carrying heavy gear like this group of glaciologists. For more information about fascinating processes of glaciation check out this GNS Science web page. On our flight up to the Grand Plateau for the height survey of Mount Cook recently, it was interesting to see the state of the terminal lake of the Tasman Glacier. This has been expanding rapidly in recent years.  Once the lake became well established, the water could undermine and erode the ice much more quickly. This photo illustrates the process of melting of the ice. The surface water cuts away into the ice face to create a notch at water level. Once this notch is several metres deep, the overhanging ice collapses, leaving buoyant ice underwater that eventually breaks off in big pieces to float up to the surface as a new iceberg. The icebergs will continue to be eroded by the water in the same way. As they get lighter, they rise up in the water, lifting the ice notch up to give a mushroom like profile. The bergs often get top heavy by this process and can unexpectedly roll over. This video that we made several years ago gives a dramatic illustration of this process seen from a boat at close quarters: Here is some information on our GeoTrips website if you want to visit the lower reaches of the Glacier for a closer look: www.geotrips.org.nz/trip.html?id=147 I have flown up the Tasman Glacier several times on various glacier field expeditions in recent years. This is a photo of the lower section in 2002, looking down valley. The glacier itself is about 2 kilometres wide and the lake is already extending up the east side of the glacier by about 5 kilometres. Two years later (November 2004) you can see that the lake has continued to expand. The large ponds that can be seen near to the lake have grown and started to join together as more and more of the ice melts. November 2007, after a large break out of ice bergs, the lake has greatly increased in size. November 2013 from our recent flight up to the Grand Plateau on Mount Cook. It is inevitable that the lake will continue to expand. Due to the overdeepening effect of the glacier on its bed, the deepest point of the lake will be some distance up from the terminus, probably below the  area in the foreground of the image. After expanding past the deepest point, the lake will get shallower and shallower as it progresses up the valley, potentially to the point where the bed of the glacier meets the lake surface. It has a long way still to go. This image (added as an update in early March 2015) shows that the basic shape of the lake hasn’t changed substantially since the previous photo was taken over a year ago. However, if you look at the position of scree slopes on the right of the photo you can see that the glacier’s retreat is continuing. In this last photo you can see that as the lake erodes further up the glacier, the terminal ice cliff at the edge of the lake is getting higher due to the increasing surface elevation of the ice. There is a very good view of the lateral moraine wall in the background, that used to be below the level of the glacier surface back in the nineteenth century.  The glacier ice in this area has thinned vertically by roughly 200 metres since that time.

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The Changing Height of Mount Cook

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Mount Cook  rock avalanche 1991. Lloyd Homer, GNS Science On 14th  December 1991 a massive rock avalanche occurred from the East Face of Aoraki /Mount Cook, sending an estimated 14 million cubic metres of rock in a 1.5 kilometre wide cascade across the grand plateau and down onto the Tasman Glacier. It is thought that the avalanche travelled at speeds of 400 to 600 km per hour, and the resulting seismic recording at Twizel, 75 km away, lasted well over a minute, registering the equivalent of a magnitude 3.9 earthquake. Mt Cook Dec. 1991.  M. McSaveney, GNS Science Prior to the avalanche the surveyed height of New Zealand’s highest peak was 3764m. The Department of Survey and Land Information (now LINZ) calculated that this was reduced by 10 metres after the summit fell off with the rock fall. As you can see from the photo, the peak became extremely narrow and unstable. In the image taken by GNS geomorphologist Mauri McSaveney a few days after the event. A lone climber that can be seen as a tiny dot inside the red circle  gives some idea of the scale. The “new” summit was obviously highly unstable, and would be subject to quite rapid erosion following the rockfall. Since 1991, there has been no re-calculation of the revised elevation of 3754m until recently. At the end of November 2013, I flew up to Plateau Hut with a climbing team who planned to take direct GPS measurements of the summit ridge of the mountain, a short distance from and a few metres below the highest point. The measurement would then be used to validate a computer model made from recent aerial photos to give a precise calculation of the present height of the peak itself. The team was made up of (left to right): Geoff Wayatt, mountain guide; Nicolas Cullen from Otago University; Brian Weedon, mountain guide; Pascal Sirguey (project leader) from the National School of Surveying at the University of Otago; Jim Anderson from Survey Waitaki and myself. Geoff, Brian, Nicolas and Jim made up the climbing team. GNS Science provided support in terms of the helicopter flights.  I was able to accompany the team to Plateau Hut where I spent two days gathering a visual record whilst they were involved in their climb. Mount Cook East Face   Julian Thomson, GNS Science The plateau of Mount Cook is arguably the most spectacular alpine setting in New Zealand. This image shows the 1500m high East Face of Mount Cook in the early morning light seen from Plateau Hut. The normal route up the mountain follows the Linda Glacier, starting on the right hand side of the image and following into the shadow behind the long low angled rock ridge (Bowie Ridge) up to the summit rocks. As well as Mount Cook itself, the Grand Plateau has views of many other summits along the main divide, including Silberhorn, Tasman and Dixon. This image shows the top section of Syme Ridge on Mount Tasman. There are three climbers just visible on the ridge just above the centre of the photo, about 10 hours into their climb from the hut. This image shows the patterns of crevasses on the grand plateau, just above the Hochstetter Icefall. Plateau Hut at night.  Julian Thomson, GNS Science The climbing party left Plateau Hut just after midnight with clear, cold weather conditions that were perfect for the climb. Mt Cook Summit Rocks, Photo Geoff Wayatt Aoraki / Mount Cook is a challenging peak to climb, with very dynamic glaciers and steep rock and ice faces to negotiate. In this photo, the climbers are in the ice gullies that run through the summit rocks. Photo Nicolas Cullen View from the summit, with Mount Tasman in the background Photo Nicolas Cullen Looking along the summit ridge of Aoraki / Mount Cook, with the two GPS units in place. The very highest point is about 45 metres distant. The GPS units measured a height of 3719 metres at their position. This measurement was consistent with the height from the computer model which then allowed the height of the high peak to be calculated as 3724 metres above mean sea level. This means that Aoraki / Mount Cook is a full 30 metres lower than the 1991 estimate of its height, showing that the mountain peak has continued to erode significantly during the last 22 years. There is more information about the project at the Otago University School of Surveying website. Here is our video of the expedition : Mountaineers staying at Plateau Hut can get an incredible 360 view of the surroundings from nearby glacier dome. We have even created a GeoTrip for the location:  www.geotrips.org.nz/trip.html?id=450

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Global Catastophe in a thin rock layer

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K-Pg boundary layer – when the Earth changed forever The K-Pg Boundary (or Cretaceous Paleogene boundary, or K-T boundary as it is still sometimes called) is a layer in the Earth’s crust that marks a very dramatic moment in the history of life on earth about 65 million years ago. There is a huge change in the fossil communities of plants and animals across this boundary. Over half the species that are found in Cretaceous rocks are missing from the younger Paleogene rocks above them. Included amongst the creatures that vanished forever at this precise point in time are the ammonites, large marine reptiles (such as mosasaurs and plesiosaurs), large flying reptiles and of course the dinosaurs. New Zealand has a unique record of the K-Pg boundary. These eight localities in the northern South Island provide the only Southern Hemisphere record of how the catastrophe affected land plants (Moody Creek Mine) and marine life (Waipara River and six localities in near Blenheim).   An artists image of the impact by Don Davis of NASA The now well established explanation for this dramatic crisis in the history of life is that an asteroid, about 10 km across, struck the earth near Mexico, causing a huge tsunami and a global dust cloud that darkened the skies worldwide for months, thus killing plant and animal life. After a period of recovery that lasted several thousand years, the remaining plants and animals were able to diversify into the ecological niches made vacant by this mass extinction. Mammals were one of the groups that flourished and ultimately gave rise to humans.  The dark line of the K-Pg boundary at Chancet Rocks Recently I joined a group of scientists visiting several sites in Canterbury and Marlborough, where the K-Pg boundary is exposed. K-Pg boundary at Chancet Rocks centre left of photo At Chancet Rocks, just north of Ward Beach, the light coloured Cretaceous limestone contrasts with the darker grey Paleocene rocks on the right side of the photo. These rocks were laid down in several hundred metres of sea water, and the fossils found within them are mostly microscopic unicellular plants and animals. These have been studied in detail and are very different assemblages. This slab cut through a section of K-Pg boundary by John Simes and Chris Hollis was taken from the coast south of Chancet Rocks. If you click on the image to enlarge it you can see some of the features in more detail. You can see the thin layer of clay that precisely marks the boundary itself. This layer has been found at sites around the world, including drill cores from the ocean floor, and is remarkable for containing high levels of an element called iridium. Iridium is common in asteroids and its abundance at the boundary was a key part of the evidence that lead to the asteroid impact theory. We also visited Woodside Creek, the first K-Pg boundary locality in New Zealand that was found to be enriched in iridium. Here you can see that the river was quite high, making access a little bit difficult. This is the Woodside Creek section. It has been sampled a lot over the years so that quite a lot of the rock has been mined away. The drill holes you can see in the rock layers on either side of the boundary itself show where scientists took samples for the analyses that led to the discovery of iridium enrichment. The image at the top of this page was taken from here. This is a close up view of the very top surface of the Cretaceous at Woodside Creek, just beneath the iridium rich boundary clay. The masses of tiny pock marks in this surface are thought to have been caused by droplets of glassy impact ejecta raining down onto the sea floor from high in the atmosphere after the impact thousands of kilometres away. Chris Hollis at GNS Science has done very detailed studies of the Cretaceous and Paleogene rocks in New Zealand. He is a paleontologist who specialises in tiny microfossils called Radiolarians. Radiolarians are marine plankton that construct complex shells of glass (opaline silica); each species has a distinctly different shell. Radiolarians are one group of organism that didn’t go extinct at the K/Pg boundary. Instead, some species became very rare, while new species evolved and flourished.  These microfossil changes are clearly shown in rock samples from the K/Pg section at Flaxbourne River, where over the distance of a few millimetres one group of radiolarians (nassellarians) are almost completely replaced by another group (spumellarians). This change is thought be a consequence of rapid cooling of the ocean waters around New Zealand. In this video Chris tells us about the Woodside Creek K-Pg boundary section:

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Weather Station at Lake Ohau

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To quantify the linkage between weather events and sedimentation in Lake Ohau, a weather station has been set up in the valley at the head of the lake. This is only possible due to kind assistance from the Inkersell family at Lake Ohau Station. I accompanied Heidi Roop to the weather station as it needed a bit of maintenance. We had a few visitors join us while we were there. No doubt they have an interest in weather data. In fact, some of the maintenance we were doing was because the cattle had chewed through the wiring to the weather station! The weather station measures air temperature, relative humidity, solar radiation, wind speed, wind direction, and precipitation . Data is collected every 10 minutes and is recorded in the data logger below the mast.   Precipitation events that  produce surges of sediment transport into the lake are recorded and linked to data collected by other instruments in the lake and up in the Hopkins River valley. This is helping to build up a detailed understanding of erosion, transport and sedimentation processes  in action in Lake Ohau.

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Lake Ohau Sediments

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Ever since its creation by the retreat of a huge glacier at the end of the last ice age, Lake Ohau has been gradually filling up with sediments washed down from the nearby mountain ranges. This is the view looking north from the lake, up towards the Dobson Valley. The valley profile has the classic ‘U’ shape created by glacial erosion, and the flat valley floor is blanketed by sediments brought down by rivers, especially during floods when the water flows with high energy. Here you can see the lake inlet. You can see the delta created as the sediments fill the lake. Lake Ohau has been receiving a high level of scientific interest over the last few years, by scientists from GNS Science in collaboration with others from Victoria and Otago Universities. Gavin Dunbar of VUW preparing equipment They aim to understand the processes of sedimentation in the lake, and work out how these processes relate to weather patterns affecting the catchment. With that information, a study of the lake floor sediments will potentially give a detailed record of how the climate has changed in the area over the last 18 000 years, since the lake’s formation. A number of limnological (lake) measurements are being made to help understand the way water currents, water temperatures and water clarity vary seasonally in different parts of the lake. This is important because it allows for understanding of the factors that influence the deposition of mud on the lake floor. In this photo Heidi Roop (GNS Science PhD student) is helping pull a sediment trap out of the water at the end of the lake nearest to the outflow. At the bottom of the trap there is a bottle of sediments that have accumulated over the last 4 months. The 1 litre bottle is removed and replaced with an empty one. The bottle is quite full because it contains concentrated sediment that has fallen into the wide mouth of the trap. Careful recording is one of the most important parts of any scientific data collection. Marcus Vandergoes and Heidi Roop prepare to lower a gravity corer into the lake to sample a small core of the top layers of sediment. As the corer penetrates about 25cm into the lake floor, the mud enters the plastic tube. A cap then seals the top end of the tube so that the mud is held in place by a vacuum as the corer is pulled back up to the boat. Once at the surface, the lower end of the tube is sealed to prevent loss of the core which is then prepared for transport back to the lab for close study of the different layers, including thicknesses of the different layers, grain size and density. Heidi and Marcus pulled up a second core to show me what can be seen when it is sliced through to show a flat surface. Darker and lighter layers are visible, which have been shown to correlate with summer and winter deposition. The thickness of each layer is thought to be related to the number and size of storms and flood events. This core includes sediment accumulated over the last 25 to 30 years. Heidi has devised a way of comparing the sedimentation of particles from different depths in the lake water at each end of the lake. She has a line with several upside down cut plastic bottles that act as mini sediment traps attached at different levels in the water column. This shows whether the currents that deliver sediments to the lake are flowing at the surface, the bottom, or at intermediate depths. It turns out that this varies between summer and winter. In summer, the warm water entering the lake carries the sediment load at a high level, whilst in winter, the particles travel along with cold bottom currents. This is why the summer and winter layers of sediment have different physical characteristics. Clear as mud – a successful day’s sample collecting from Lake Ohau,

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Beryllium-10 dating of moraines around Lake Ohau

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Lake Ohau is one of several very large lakes in the Southern Alps that fill valleys once carved out by huge glaciers during the Ice Age. As the ice retreated, it left spectacular and classic landforms in its wake, including concentric lines of moraines, erratic boulders, ‘U’ shaped valleys and extensive outwash plains. The rapid tectonic uplift of the Southern Alps, and extreme climatic conditions, have created the landscape we see today. The rock debris left behind as the ice retreated, has been mapped by geologists, and a lot of work has gone into dating the ages of the various moraines to gain a better understanding of how the landforms relate to specific changes in the climate as it gradually warmed up after the coldest phase (last glacial maximum or LGM) of the last ice age. This photo shows how Lake Ohau is dammed by a rim  of 18 000 year old moraines around its southern margin. They are the low lying hummocks you can see near to the lake, as well as in the foreground. The dark red lines on this map  show the extent of these moraine ridges, extending around the south end of the lake. The brown colours are river sediment (glacial outwash) that was spread across low lying areas by braided rivers. The lines crossing this mountainside above Lake Ohau are lateral moraines left behind as the glacier gradually lowered, and finally vanished at the end of the ice age. Richard Jones and Kevin Norton of Victoria University, Kevin Norton measuring the tilt on the surface of a boulder One of the best methods of dating these moraines is by measuring the concentration of the isotope beryllium-10 in the top surface of large boulders situated on them. The technique depends on the fact that the atoms in quartz (SiO2) in the rock are constantly being bombarded by cosmic ray neutrons. When such a neutron collides with the nucleus of a silicon or oxygen atom it splits the nucleus into fragments which will be smaller, different nuclides such as beryllium-10.  (Since they are produced by a cosmic ray interaction, all these products are known as cosmogenic nuclides). With time, a freshly exposed rock surface will gradually accumulate more and more beryllium-10 so that by careful measuring of its concentration in a boulder, the length of time that it has been exposed can be calculated. The accuracy of this method hinges on good callibration, and selection of a rock that hasn’t moved or been buried since it became exposed Richard Jones cutting out a rock sample as the glacier retreated. Lots of factors have to be taken in to consideration when sampling, including the angle of the surface of the boulder, the presence of nearby mountains that block some of the sky from view, the exact altitude and also the latitude of the sampled boulder. These photos show samples being collected around Lake Ohau this week. The boulders being sampled have already been dated. The purpose of re-sampling them is to test calibration between laboratories in New Zealand and the US. Richard Jones is cutting small 2cm thick pieces off the surface of a boulder with a rock saw. Albert Zondervan and David Barrell (GNS Science)  Once the sample has been labelled, bagged and transported to the laboratory, it needs a lot of physical and chemical processing. An accelerator mass spectrometer is used to make supersensitive measurements  of the the very small concentrations of beryllium-10 that allow the age to be calculated. This video gives a very good introduction to the use of this surface exposure dating method for dating glacial moraines in New Zealand, featuring David Barrell from GNS Science, along with colleagues from the US.

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Liquefaction Effects from the Cook Strait / Lake Grassmere Quakes

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The recent magnitude 6.5 Cook Strait and  6.6 Lake Grassmere earthquakes were comparable in size to the ‘quakes that rattled Canterbury in 2010 and 2011.  This map of peak ground accelerations for the Lake Grassmere Earthquake shows recordings of up to about 0.75g.(or 0.75  the acceleration due to gravity). One of the notorious and extremely damaging effects of groundshaking in Christchurch was the widespread liquefaction and flooding that affected large areas in the eastern suburbs. This occurred in areas where the land was made up of soft low lying sediments by the river or near the coast such as Bexley and Avonside. (photo by Dick Beetham, GNS Science)The most significant damage in Wellington was long the edge of the port where a large section of the road collapsed into the sea. This photo by Graham Hancox of GNS Science, shows the scene after the combined effects of the two recent earthquakes. So what were the effects on the ground near the earthquake epicentres? Are there any areas of soft, waterlogged sediments beside an estuary or river in Marlborough that might be expected to compare with those severely damaged parts of Christchurch? A team from GNS Science went to look at the ground damage alongside the Opawa River, near its confluence with the Wairau River. Dougal Townsend took these photos during their visit: Near to the river, there were several long cracks created by lateral spreading. Sand boils and sand volcanoes had left their mark over the paddocks beside the river. With the help of a spade, one of the sand volcanoes was sliced vertically to show a clean cross section through it. You can see the thin crack in the soil that was opened up during the earthquake, that allowed the sand loaded water to ‘erupt’ at the surface. Compared to Christchurch, these liquefaction effects from the recent Cook Strait and Lake Grassmere Earthquakes are relatively minor. This is an important finding as it shows that the extreme level of liquefaction in Christchurch was not necessarily a typical example of what to expect from future earthquakes in the rest of the country. Scientists now have some more useful data to help to differentiate between the impacts of ‘quakes in apparently similar environments.

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Earthquake impacts in Marlborough seen from the air

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Dougal Townsend of GNS Science was part of a team that flew over Marlborough to assess the impact of the recent earthquakes on the landscape and infrastructure. Although relatively minor compared to those that impacted the Christchurch area in 2010 and 2011, there were nonetheless some isolated, but significant effects. All these photos were taken by Dougal: Here you can see damage (cracking) to State Highway 1 between Seddon and Ward (near Caseys Road turnoff) following the Lake Grassmere Earthquake. Large landslide in the Flaxbourne River catchment (about 8.5 km west of Ward). Another Large landslide. This is  in Miocene mudstone, just south of Cape Campbell. A whole section of the hillside has slipped Bell’s dam near Seddon. Damage (cracking) was sustained during the Cook Strait Earthquake and was exacerbated during subsequent aftershocks and also during the Lake Grassmere Earthquake. The channel was dug to partially drain the dam, to lessen potential flood risk to the town of Seddon, which is 10 km downstream to the NE. A closer view of the cracks along the top of Bell’s dam. alongside the vehicle track This image shows rock fall on a farm track about 2 km southeast of Ward (track goes up to Weld Cone). The rock is Late Cretaceous sandstone and siltstone.  Ground damage in Needles Creek, west of Ward. Cracking of the farm track (centre left) is from the Lake Grassmere Earthquake, whereas the minor landsliding of the terrace gravels on the right may be from a combination of storm (rainfall) and earthquake (ground shaking) damage. 

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Lake Grassmere Quakes

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Following the earthquakes in southern cook straight, the GeoNet rapid response team left immediately to place seismometers around the area, to allow more detailed monitoring and get better information with which to model the fault ruptures. This meant that when the Mag 6.6 occurred, the enhanced array of seismometers was already in place. Here is a screen shot of the Mag 6.6 Grassmere Earthquake and immediate aftershocks over the following hours: This GeoNet video gives an idea of the number and locations of aftershocks from the 16th to19th August

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