The White Terraces Reappear after 125 years

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On 10th June 1886, exactly 125 years ago today, Mount Tarawera erupted briefly and violently, resulting in the disappearance of the Pink and White Terraces of Rotomahana, and devastation of the landscape. The former lake disappeared and was slowly replaced by the much larger and deeper lake which remains to this day. This 1880 Charles Spencer image is  courtesy of Te Papa Museum Last January, in a GNS Science led international expedition, Cornel de Ronde and his team rediscovered the Pink Terraces at the bottom of the  modern lake, which had been so drastically altered and deepened by the eruption. The Pink Terraces were first spotted in images from a side-scan sonar that was mounted in an autonomous underwater vehicle (AUV) used to survey the lake. Today Cornel de Ronde announced that the White Terraces have also been found using data retrieved on the last day of the expedition, that had not been analysed until recently. When the Pink Terrace side-scans were first seen, they were nothing like anything that had been observed by the team before. An underwater camera was used to confirm that they did indeed represent the Pink Terraces. (For details of the Pink Terrace discovery watch this video). Similar looking side scan images have now been found in the location where the White Terraces are expected to have once existed. A horizontal segment of the formations over 150 metres across may be the remains of the silica terraces along the former shoreline of the lake, now tens of metres below water level. It is not yet known whether more of the terraces lie hidden beneath volcanic mud, or whether the rest of them were forever destroyed in 1886. Future exploration may settle this question. Ron Keam of Auckland University is an expert on the history of the Tarawera Eruption and the Rotomahana landscape. He compiled this map of the former Lake Rotomahana as accurately as possible by detailed study of  pre 1886 photographs. The Pink Terraces can be seen on the left (west) side of the lake, with the White Terraces at the top (northern) end, about a kilometre northeast of the Pinks. The image to the right is the compiled side scan of the part of the modern lake under which the remains of the terraces lie.  The long straight lines show the path of the AUV as it progressed up and down the lake area.  The red circles show the locations of the two sets of terraces, about 1 kilometre apart. Lower left are the Pinks and upper right are the newly refound parts of White Terraces. This close up of the side scan image  shows the curved overlapping terrace formation on the lower half below the blank, unscanned area. These features are very similar in general appearance to the photographically verified scans of the Pink Terraces found last summer. (All sidescan images courtesy of our US project partners at the Woods Hole Oceanographic Institution) For more details have a look at our media release, and watch the video of Cornel de Ronde describing how the discovery unfolded step by step, including the crucial hook shaped landform that first led to the location of the Pink Terraces, followed now by the Whites:

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Shooting the SAHKE Seismic Survey across the North Island

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The blasts from the seismic survey were detectable by GeoNet as very small local ground tremors along the seismic line last week:                    PRELIMINARY EARTHQUAKE REPORT                             GNS SCIENCE                         GeoNet Data Centre                      Lower Hutt, New Zealand                      http://www.geonet.org.nz        The following earthquake has been recorded by GNS Science:        Reference number:        3511346/G       Universal Time:          12 May 2011 at 10:24       NZ Standard Time:        Thursday, 12 May 2011 at 10:24 pm       Latitude, Longitude:     41.15°S, 175.38°E       Location:                10 km south-east of Featherston       Focal depth:             0 km       Richter magnitude:       2.3        Web page: http://www.geonet.org.nz/earthquake/quakes/3511346g.html Man-made explosion as part of a science experiment in lower North Island                                                                            *         *         *  Here is a video in which Stuart Henrys explains the seismic survey :

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SAHKE – Seismic Array Hikurangi Experiment

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About a dozen field teams have been out over the weekend  deploying geophones along the 90 kilometre transect of the SAHKE seismic survey. The first photo shows some of the Orica contractors  loading and priming one of the transect shot holes. 500 kilogrammes of explosive emulsion is being pumped down a 50m bore hole. The pile of gravel in the foreground is used to back fill the hole on top of the explosives. The idea is that the shock wave is directed downwards into the earth rather than up into the air.  (Photo by Stuart Henrys) It is very important that every geophone is in perfect working order and set up in exactly the right way, as  there will be no possibility of repeating the survey if anything goes wrong. The second photo shows Stuart Henrys, project co-ordinator, with some of the equipment being prepared at GNS Science, Lower Hutt. Getting all the equipment set up and deployed is a huge organisational feat. Stuart is holding one of the many hundreds of geophones that will be embedded in the ground along the survey line. Apart from the New Zealand participants (Victoia University and GNS Science), a large amount of equipment and expertise is being contributed by the Earthquake Research Institute at Tokyo University, Japan, and the University of Southern California The map shows the actual location of the seismic survey line, with the positions of the shot holes indicated as stars. Depending on the time required for putting all the geophones in place, the detonations will be set off overnight during this week. The explosions are detontated at night to avoid too much interference from vibrations caused by traffic on the roads. When the geophones are deployed they have to be pushed into the ground so that they are well embedded. This ensures a solid contact. In this photo by Margaret Low (Photo Librarian at GNS Science) Vaughan Stagpoole is burying one of the 900 geophones alongside a road in the Wairarapapa. Check out our time-lapse of the busy science teams preparing the equipment for the SAHKE survey. Two days compressed into just over a minute to the music of Lykke Li!

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Wellington’s Stuck Plate Boundary

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Ever since 1855, when New Zealand’s largest ever recorded earthquake (magnitude 8.1) shook the Wellington Region, a lot of effort has gone into understanding the earthquake risk in and around New Zealand’s capital city. There are several large fault lines in the area, including the Wellington Fault. This is the most active fault of the system, and stands out clearly, passing along the Hutt Valley and right through Wellington City itself. For more background information check here or watch this video. However, the largest fault of all, the interface between the Pacific and Australian Plates, underlies the whole region. The two dimensional map shows  the line of the boundary between the plates east of the North Island. In three dimensions, it is a sloping boundary (known as a subduction zone), with the Pacific Plate dipping under the Australian Plate. Plate collision is occurring at an oblique angle rather than head on, which is why there is such a large component of strike slip (sideways) motion in the North Island Fault System. The hidden, subsurface plate boundary has been mapped over the years using evidence from thousands of small or medium sized earthquakes generated on or nearby to  it. Seismometers are used to locate these earthquakes, and the seismic waves give information about the geological structures and rock types that make up the two interacting plates. Under Wellington the boundary dips gently down to the North-West at an angle of about 9 degrees, and is about 25 kilometers deep under the city. Over New Zealand there is a widespread array of GPS stations continually monitoring their location with great precision. This station is set up in the Tararuas, not far north of Wellington and the Hutt Valley. Scientists also carry out GPS campaigns to make repeated measurements at a large number of locations when they want more detailed coverage. Over time these recordings show that the surface of the landscape is being deformed by tectonic movements. These measurements indicate that a large segment of the crust of the Australian Plate in the Lower North Island is stuck to the underlying slab of Pacific Plate, and is being dragged along to the west faster than the Hawkes Bay or East Cape areas. There have been different reasons for this proposed by scientists, but it is believed to be caused mainly by friction on the interface between the two plates. It is very important for us to develop our understanding of the nature of this plate interface and the earthquakes that it produces, as subduction zone ruptures potentially create the most destructive earthquakes and tsunamis worldwide. The recent earthquake in Japan is one such example. In this coloured image, the red colour indicates a high “slip rate deficit” or high degree of coupling between the subducting and overriding plates in the Lower North Island. This segment of stuck plate boundary is about 70 km wide and 140 km long.  If it ruptured it would produce an earthquake of magnitude 8 or above. It is even possible for larger sections (eg the length of the North Island) to rupture occasionally in a single massive earthquake. For more information about the locked plates under the North Island, check out our website here. In order to improve our knowledge of the plate boundary, a major GNS Science co-ordinated project is being carried out next week. This involves a 90 km seismic survey crossing the lower North Island from one side to the other. Instead of listening out for natural earthquakes, the survey will use explosives, detonated down boreholes, to produce the seismic waves. Hundreds of geophones, spaced 100 metres apart, will pick up reflected sound waves to map the plate interface, faults and other features in the crust. Scientists from GNS Science, Victoria University, Tokyo and California are collaborating in this project. For some more background to this project, have a look at our media release, or listen to Tim Stern of Victoria University in this radio interview.

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Canterbury Gravity Survey

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There are a number of urgent scientific studies being carried out around Christchurch to help inform decision makers involved in the repair and recovery process following the recent earthquakes. These projects are being co-ordinated under the Natural Hazards Research Platform which is a collaboration of many of New Zealand’s research institutions (universities and Crown Research Institutes). One of these thirty ‘recovery projects’ is aiming to gain a more detailed understanding of the subsurface geological structure of the area using geophysical methods such as seismic reflection, magnetism and gravity measurements. Although we usually think of the gravitational force of attraction at the Earth’s surface as being something uniform wherever our location, there are actually subtle variations in different places. These depend on our distance from the equator (latitude), our altitude above or below sea level, the nearby landscape topography, and also the density of underlying rock masses in the crust below us. Last week I joined a small GNS Science team who have been making a gravity survey over a wide area around Christchurch City and Canterbury. In the second photo, Vaughan Stagpoole, Jiashun Yu and Dan Barker are setting up the GPS base station at a survey mark, to calibrate the GPS location measurements of the gravity survey. Measurements are made using a gravity meter that contains a very precise spring scale and weight. Minute changes in the force of gravity on the weight result in changes in the extension of the spring and gives a measure of the gravity at a particular location. This is read off on an electronic gauge and verified on a tiny scale in the meter that is observed using a magnifying lens. When readings are taken over a wide area, and latitude, and altitude, as well as local topography are factored in, areas of anomalous gravity can be mapped and interpreted in terms of geological structure. For example, faults completely hidden beneath the sedimentary strata of the Canterbury plains, that have offset underlying high density rocks, will have a distinctive gravity characteristic that is different to areas where the underlying rocks are uniformly flat. The mapped gravity is used in conjunction with other geophysical observations to get a 3D picture of the subsurface. Data from different geophysical surveys or other sources (such as aftershock locations) are then overlaid on top of the gravity map to help distinguish significant features. We can look at some earlier surveys to illustrate this:. This is the present geological map of the Christchurch area, with different colours denoting the different rock types that occur immediately below the surface soil. The pink colours show volcanic rocks such as old lava flows that make up the Banks Peninsula, whilst the yellow and buff colours are sediments such as gravels that have been eroded off the mountains and laid down by rivers across the Canterbury Plains. Red lines are surface rupture faults, including the Greendale Fault in centre left, that ruptured during the September 4th earthquake. (The fault under the Port Hills that moved on February 22nd is not shown here as it is a ‘blind’ fault that did not extend to the surface). This diagram is a gravity map of the same area. It was compiled recently from data collected some years ago. The colours show gradients of gravity intensity. You can see that quite a number of features become visible that are not seen on the geological map. Several of the linear structures are caused by fault lines criss-crossing through the basement rocks underneath the superficial rock deposits. If you click on the image to enlarge it, you will see many little red dots. These are the measurement stations where the actual gravity readings were taken.  You will notice that there are significant gaps in some places where data from adjacent stations is extrapolated to fill in the map, rather than actual readings.These are the places where the present gravity survey is being carried out in order to add to this pre-existing data and fill out the missing details. The last image shows the distribution of aftershocks superimposed on the previous gravity map. (The aftershock data is derived from the GeoNet website Quake Search facility). This helps us to find relationships between basement rock types, their distribution and structure, and the fault ruptures that have been causing the recent earthquakes. These diagrams were compiled by Bryan Davy who is a geophysicist at GNS Science, specialising in the use of gravity and magnetic data and the use of interactive mapping software. When the present gravity survey is completed, along with the seismic and magnetic surveys, the added information will further our knowledge of the distribution, length and alignment of fault lines in Canterbury. This information will be included in models that will help evaluate the potential size and frequency of future earthquakes.

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Rockfall impacts from the Christchurch ‘Quake

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Today I have been in the Port Hills of Christchurch with Chris Massey, an engineering geologist in the GNS Science Active Landscapes team. Chris is part of a team undertaking a detailed study of the rockfalls that have been triggered in and around Christchurch as a result of the recent earthquakes. The aim of his research is to gather data from the recent rockfalls to map out the potential danger zones and quantify rockfall risk around the city. His research will help planners decide which areas will require mitigation of rockfall risk before rebuilding can occur. We visited a few key localities which had suffered damage from rockfall during the February 22nd ‘quake. This property in Heathcote  suffered severe damage from a large boulder that travelled roughly 500 metres from the slopes above, bouncing tens of metres at a time, and clearing a 2 metre fence before entering the house through the garage roof. (Photo 1). Later in the day we joined Marty and Mike  of  Solutions 2 Access who are removing unstable boulders from Castle Rock, a popular climbers venue in the Port Hills. This outcrop was shaken and destabilised by the recent earthquakes, sending many boulders flying down slope towards the Lyttelton Road Tunnel entrance. Many other boulders were left precariously perched on the rock face, posing a risk to motorists on the road below, hikers and climbers. Marty and Mike have been contracted by Opus Consultants to remove these rocks which they do by levering them off with a crowbar or by other mechanical methods.(This is known as ‘rock popping’). In the second photo, Marty has just sent large boulder off the edge of the cliff. Chris is interested in analysing the rock fall trajectories as part of the hazard mapping.work. My job was to record video footage of the falling boulders as they were released from the cliff and hurtled down the slopes. This will improve our understanding of how the boulders travel, including  their bounce heights, velocities and angular rotation, which is important when analysing their destructive potential. Here you can see the dent in the hillside caused by a flying boulder impacting on the surface. At the end of the day we went further along Summit Road to the area below the Gondola cable car. The road was covered with rockfall debris that had been shaken off the cliffs just above. The rocks had broken through the metal safety barrier, and some of them had travelled all the way down as far as the lower cable car station about 1 kilometre away. It was interesting to observe the impact craters in the road. The last photo shows a rock deeply embedded in the asphalt – a sobering impression of the power of a falling rock. Check out the video of Chris’s rockfall research in Christchurch:

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Rockfalls and slips in Christchurch

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This week I have been with Garth Archibald, surveying areas in Christchurch that have been affected by rockfalls and slips. These surveys provide data which is used to calculate the stability of cliffs and slopes, and this provides useful information to planners and geotechnical engineers. At Redcliffs, Garth set up his laser scanner to make a 3D scan of the rock face. Houses in this area suffered catastrophic damage from rockfall during the February 22nd quake. Click here to listen to Radio NZ’s Morning Report interview with Garth at work at Redcliffs. The laser scanner sends out about 11000 laser pulses per second. The time it takes for the light to be reflected back to the scanner, gives a very precise measurement of the distance to each point, allowing Garth to make high resolution scan images. He will compare the results with those of a previous survey to see if any areas of the cliff are bulging or tipping over, if cracks are opening up, or if there have been any further rock falls. Another area we worked in was part of Hillsborough where a large area of hillside slipped during the earthquake. This time we used a GPS (Global Positioning System) unit to precisely locate several points. These are being re-surveyed regularly to better understand the nature of the slip. In this photo Garth is setting up the GPS base station at a survey point well clear of the slipped area. In the final photo, Garth is taking a GPS reading at the lower end of the slip. Here the ground has been compressed, and you can see how it has ridged up along the driveway. The fence has also buckled by the compression. .

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Kermadec Arc Videos

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Our expedtion to explore the hydrothermal activity and mineralisation of the Kermadec arc volcanoes is now over. We arrived back in Auckland yesterday, after a successful three week research cruise. Amongst the discoveries that were made were areas of present day and ancient hydrothermal activity, relatively fresh lava flows from previously uninvestigated volcanic craters, and possibly some new species of deep sea life, yet to be verified. Hundreds of geological and biological samples were collected, along with thousands of images of the sea floor, and innumerable sonar, magnetic and gravity measurements. The volcanoes surveyed included Clark, Rumble III, Rumble II West, Healy and Brothers.  Rob Stewart of NIWA took the image of a squid that was pulled up by one of the sled tows. It is only a few centimetres long. These videos will give you some idea of the methods used and the findings of our Kermadec Expedition 2011: As a final image, here is a photo of our last sunset of the voyage as we steamed towards Auckland. It had many of us captivated as we stood on the deck admiring the changing colours:

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