There has been a lot of ongoing scientific work recenlty to assess Christchurch’s future geological risks. In our latest video Vaughan Stagpoole explains the esoteric arts of Geophysics and what the surveys can help us to find out.
Christchurch Geophysical Surveys Read More »
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 »
In this video Garth Archibald explains the use of a laser to monitor rockfall hazards in built up areas around Christchurch City: In the second video, Chris Massey analyses rock fall trajectories in the Port Hills to calculate future risks to housing:
Videos of Christchurch Rockfall Research Read More »
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 »
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:
Rockfall impacts from the Christchurch ‘Quake Read More »
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. .
Rockfalls and slips in Christchurch Read More »
I asked Kevin Fenaughty, the GeoNet data centre manager at GNS Science, to explain something about the different types of seismic waves affecting Christchurch during the present aftershocks:
Earthquake shock waves explained Read More »
If you are in Christchurch and need basic information and updates about the emergency response go to the Environment Canterbury Earthquake Website. Whilst many scientists are busy at GNS Science with seismic data and interpretation of the physical and social impacts of the earthquake in Christchurch, everyone here is feeling sympathy for those still struggling to cope with the aftermath and continued aftershocks. The image shows the ground shaking intensity across New Zealandas measured by the GeoNet network of seismometers during the magnistude 6.3 quake yesterday. (Click on the image to enlarge.) For an updated time lapse map of the aftershocks have a look here.
Christchurch Earthquake Feb 22 Read More »
Julia Becker is one of the Social Scientists at GNS Science. Her role is to understand the human response to natural hazards, and she works with other scientists and organisations to help plan for future disasters. I asked her to write a diary of her experiences of working in Christchurch in the week following the Darfield ‘Quake: 06/09/2010We arrive in Christchurch, two days after the earthquake. Driving into the city from the airport, much of the city looks quite normal. There are a few chimneys down and damaged brick fences. As we approach the city you start to see more damage, mostly to older brick buildings. Some are just a bit damaged with a few bricks fallen off while others’ facades have completely collapsed. There is quite a wide area around the city that is cordoned off and the cordons are blocked by police officers and military personnel. We have to take a big detour all the way around the city to reach our hotel. Many shops and supermarkets we pass seem to be operating. We drive past a clock tower and it has a big crack in it. At our hotel we drop our gear and then head to visit the city and regional emergency operation centres. They are very busy with many people coming and going, working away hard to respond to the earthquake. After having a few meetings with people to let them know we are here and providing offers of science and research support, we head back to our hotel for dinner and bed. 07/09/2010Small aftershocks in the night occasionally wake me from sleep. I wake at about 6.00 am. While the water is working there is uncertainty over potential contamination so I have to remember to brush my teeth with boiled water. All drinking water needs to be boiled as well. In the morning we reconnect with the regional emergency operations centre and begin to set up an office there from which the scientific research response can be coordinated. I speak briefly to Red Cross volunteers at the regional emergency centre and they say the welfare centres are being well used, with more people turning up after the aftershocks in the night. A little later two of us from GNS obtain official passes to enter the cordoned-off city centre and inspect the buildings from the outside. Many new buildings look like they have stood up well to the earthquake. The Christchurch Cathedral seems pretty sturdy too. There are a number of buildings scattered throughout the centre that have red placards indicating that they cannot be entered due to structural damage. In the afternoon we head out to the suburbs and see the impacts of liquefaction. A few suburbs have experienced significant liquefaction, and sand volcanoes have popped up out of cracks in ground. Residents have already begun to clear away the sand by shovelling it up and placing on the grass verge in front of their houses, and the council is driving by and collecting it in trucks. Port-a-loo toilets have been set up for residents who don’t have working sewerage systems.08/09/2010Sleep is again disrupted by aftershocks throughout the night. This morning before breakfast we are also rattled by a large 5.1 aftershock (scary!), followed by several smaller ones. After breakfast we visit the town centre to observe more of the building damage. We see a whole row of shops made of bricks where the front of the building has simply fallen away on to the pavement in front. Luckily the earthquake happened at approximately 4.30 am so there were few people on the streets at that time and people were not hit by the falling masonry. A restaurant is housed in one of these buildings, and as you look through the gaping hole left by the fallen bricks you can still see the tables set up and ready for customers. In the afternoon we visit the newly ruptured fault near Darfield. You can see where the fault has torn through several roads and fields. The rupture has offset the roads sideways and vertically. Initial repairs to the roads have been made to make them drivable again. You can also see where the fault has offset power pylons, trees and fences. At the very western end of the fault, an electricity substation is perched directly over the rupture and the surrounding fences and gates are twisted, but because the fault is quite distributed at this location the substation appears to still be intact and in working order. At the eastern end of the fault a house-owner is not quite so lucky, with the fault causing building damage to a resident’s home. 09/09/2010 Today we visit a couple of land use planners in Christchurch and discuss issues related to recovery. We discuss having a strategic vision for Christchurch when going into the rebuilding and recovery phase. Following our meeting we take a drive out to Pegasus township, a new subdivision located approximately 30 minutes drive north of Christchurch. Previous studies have shown the site has liquefaction potential. There are a number of houses built there already, and also some empty lots. Pegasus seems to have survived remarkably well and our untrained eyes could not see any significant cracking or liquefaction. It is suggested by the planners we speak to that geotechnical engineering works were undertaken when the subdivision was set up to try and avoid such problems. In the evening I attend a com munity meeting in Darfield. The meeting was set up with the aim of to giving residents information about the earthquake, providing discussion on how they might feel after the earthquake, and providing advice on practical issues that might need addressing (e.g. water pumps, insurance, finance, government support). At the end of the meeting two guest speakers tell a couple of funny tales to provide some light humour to the meeting. The meeting is well attended with nearly 400 people coming along. On our way ho me, driving down Riccarton Road we
Julia Becker’s Observations following the Darfield Earthquake Read More »