GeoPhysics

The White Terraces Reappear after 125 years

Leave a Comment / Earth Science, Julian's Blog / / , , ,

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:

The White Terraces Reappear after 125 years Read More »

Shooting the SAHKE Seismic Survey across the North Island

Leave a Comment / Earth Science, Julian's Blog / / , ,

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 :

Shooting the SAHKE Seismic Survey across the North Island Read More »

SAHKE – Seismic Array Hikurangi Experiment

Leave a Comment / Earth Science, Julian's Blog / / , ,

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!

SAHKE – Seismic Array Hikurangi Experiment Read More »

Wellington’s Stuck Plate Boundary

Leave a Comment / Earth Science, Julian's Blog / / , , ,

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.

Wellington’s Stuck Plate Boundary Read More »

Canterbury Gravity Survey

Leave a Comment / Earth Science, Julian's Blog / / , ,

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.

Canterbury Gravity Survey Read More »

Kermadec Arc Videos

Leave a Comment / Earth Science, Julian's Blog / / , , ,

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:

Kermadec Arc Videos Read More »

Adventures of the Sentry

Leave a Comment / Earth Science, Julian's Blog / / , ,

Sentry is the showpiece of our 2011 expedition to the Kermadec Arc Volcanoes. It is an autonomous underwater vehicle (AUV) that has been developed by the team at the Wood’s Hole Oceanographic Institution who are world leaders in submarine technology. Unlike the other devices that are put over the side of the ship and lowered towards the seafloor, Sentry travels independently and therefore has the capacity to make long journeys over the volcanoes covering a wide area.It has enough battery power to last for up to about 20 hours per mission.   All its missions are pre-programmed according to the bathymetry map that has been created from the ships multibeam sonar scanner. It moves at a constant height above the slopes of the volcanic cones, recording a range of measurements. Here you can see the track lines of a mission over Brothers Volcano yesterday, overlaid on the red contour lines. For the duration of each Sentry mission, a couple of transponders are sunk down to the sea floor nearby to provide extremely precise location reference points that greatly increase the spatial accuracy of the resulting maps and records.  At the end of the mission, these float back up to the surface for collection. Because it is able to travel so close to the sea floor and can move in any desired grid or spiral pattern, Sentry enables incredibly high resolution maps to be made with previously unachievable detail. With up to 10 different sensors including side scan sonar, temperature, pH and magnetics, Sentry is able to detect and measure widely distributed hydrothermal hot spots. Whilst Sentry is operating far below, the ship can move away and perform its other operations such as magnetic surveys, CTDO (water chemistry and cloudiness), TOWCAM and sled sampling, which all help to add layers of useful information to the total picture of these volcanic seamounts.The Sentry team monitor the progress of the AUV via short acoustic messages that are updated every few minutes. If necessary they can send commands back to it to redirect it or get it out of trouble. In this photo, Dana Yoerger, leader of the Sentry team, is at his workspace in the Sentry control centre. At the end of its mission, Sentry floats up to the surface, and the ship pulls alongside so that it can be winched back on board. Because it has a broad flat profile, it catches the wind, and these deployment and retrievals with the winch can be exciting to watch!  During this voyage, the team were faced with some major technical challenges, requiring new parts to be sent from the US to New Zealand and then dispatched to the ship via helicopter and boat. Al Duester and Andy Billings spent many hours involved in complex problem solving to allow the show to go on.

Adventures of the Sentry Read More »

The Magnetic Charms of the Sea Floor

Leave a Comment / Earth Science, Julian's Blog / / , ,

Fabio Caratori Tontini is interested in measuring the magnetic properties of the rocks on the sea floor. Because most of them are volcanic lavas that contain a lot of iron, they have become magnetised as they cooled and solidified in the presence of the Earth’s magnetic field. When the hot geothermal liquids pass through them, the rocks  become progressively demagnetised because the hot fluid dissolves and carries away the metal (iron) ions. This is of course why the hydrothermal fluids become enriched in these ions, and bring them up to be precipitated when they contact cold sea water. In the second photo you can see that there is a lot of red iron in this rock. Rocks that have been affected by hydrothermal activity will remain demagnetised even after the activity stops.  By mapping the magnetic intensity across a volcano, it is possible to locate areas of present or past hydrothermal activity (low magnetism). This adds a time dimension to the other surveys that focus on present day hydrothermal activity only, and potentially reveals other areas rich in hydrothermal deposits.   Fabio uses a magnetometer that is towed behind the ship in a grid pattern above the volcanoes. This measures the variations in magnetism which are then plotted on a map. His results can be compared to maps of present day hydrothermal activity, to tell us something about how the activity has changed over time.  There is also a magnetometer on board our yellow submarine SENTRY that is run much closer to the sea floor, and picks up a lot more close-up detail. Here is a high resolution image of the magnetic anomalies on Clark volcano that were recorded by SENTRY a few days ago, and shown graphically by Fabio. The blue lowly magnetised areas are the ‘ burn holes’ that will generally be centres of rich hydrothermal mineralization because the minerals that have been leached from the deeper rocks are now spread out in deposits at or near the surface. The orange and red areas retain their more of their original magnetism and will not have been strongly altered by hydrothermal fluids. In the second graphic, Fabio has added to the picture by overlaying the magnetic data onto a 3D image of the cone of Clark Volcano.   On a previous expedition, Fabio got some strange readings on his magnetometer, and noticed that there was extra tension on the cable. After pulling the device back on board, he found that it had been severely mauled by a shark, with nasty bite marks on two sides. In the photo you can see that there is even a small piece of white shark’s tooth left behind in one of the gashes. I guess that the magnetometer now has a lower level of attraction for the shark who will think twice before attacking a large fast moving goldfish again…

The Magnetic Charms of the Sea Floor Read More »

Help from above

Leave a Comment / Earth Science, Julian's Blog / / ,

We have now been traversing above Clark Volcano for several days, and a variety of surveys have been operating, including dredging for mineral and biological specimens, photography of the sea floor, magnetic surveys and water chemistry sampling.  The technological centrepiece of our expedition to the Kermadec Arc is a strange object known as ‘Sentry’. Sentry is a bright yellow un-manned submarine that can be programmed to dive to the sea floor on journeys of up to 19 hours long, manoeuvre up and down over obstacles, and take a whole variety of close up readings with its many sensors. On Sentry’s initial dive a few days ago, its multibeam scanner stopped working. This feature is a very important part of the Sentry’s armoury of equipment. It is used to make very high resolution maps of the sea floor, which are extremely detailed because Sentry is travelling so close by. So while the other scientists have been very active with their own projects, the Sentry team have been working on solving this key problem, finally organising to get a replacement scanner. This was flown to New Zealand from the US, cleared through customs, and immediately flown to us yesterday by helicopter out of Auckland. The delivery was lowered down from the helicopter in a large container, whilst a crowd of us enjoyed the spectacle from the front of the ship. Shortly afterwards, these beautiful fish called mahimahi, each about one and a half to two metres long, paid us a visit from below. Gently cruising around the ship for about half an hour, their offering to us was just the simple appreciation of their presence. And as from last night, Sentry is on duty, now deep below us.

Help from above Read More »