Videos of Christchurch Rockfall Research

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:

Canterbury Gravity Survey

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.

Rockfall impacts from the Christchurch 'Quake

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:

Rockfalls and slips in Christchurch

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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