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The Port Hills rockfall problem
The Port Hills area on the edge of Christchurch was very seriously affected by the Christchurch earthquake sequence. The Canterbury Earthquake Recovery Authority will today start the process of demolishing the rockfall affected houses in the Port Hills area. Yesterday they released a statement describing the challenges of this work; this statement is well-covered in an article in The Press, which also includes a nice video taken by a drone of some of the sites. A couple of years ago I visited many of these sites, and I have an old post that presents some of the images that I collected. As a reminder, this is typical of the state of some of the buildings at the top of the slopes on the Port Hills:
New Zealand is a country with a high rate of tectonic activity, such that earthquakes are an ever-present hazard across much of the country. In South Island the most important geological structure is the Alpine Fault, which runs for about 500 km along the west side of the mountain chain before splitting into a series of faults (known as the Marlborough fault system) that cross the northern part of the land mass (image from the University of Otago):
I have spent the last week collecting samples from the rock slopes affected by the Christchurch earthquake sequence, and am now travelling back up to Wellington, where I will spend the coming week. En route I have stopped off for the weekend in Nelson so as to be able to take a look at the landslides at Golden Bay. These landslides were triggered in an exceptional rainstorm in mid December – I covered it briefly at the time – and have been quite devastating for the local people.
In Golden Bay the most seriously affected area that I saw was the margin of the Abel-Tasman National Park, as shown in the Google Earth image below, especially between Pohara and Tata Beach, although there is almost certainly further problems to the east that I could not access. The road shown on the map across the area was closed at Tata Beach.
A good overview of the main impacts of this storm is shown below – this is the small community of Ligar Bay, which has been an area of beachside houses, with new subdivisions on the slopes behind. If you look carefully you will see:
- Slips below the road affecting the carriageway;
- Slips on the slopes above the road, which deposited debris that blocked the highway;
- Multiple slips in the slopes behind the village; and
- Debris released from those landslides that has then damaged the houses
The recent 7.1 earthquake in Canterbury, New Zealand has reportedly caused damage to 100,000 homes amongst a range of other problems in the South Island. Dave Petley, a researcher with IHRR and Durham University is currently in New Zealand attending a conference and is reporting on the size and magnitude of the earthquake from his blog. More than 80 aftershocks have been recorded, according to the BBC. There is concern from the public and scientists alike that they could experience another earthquake in the very near future. Earthquakes are generally not new to New Zealand, but it is unusual for the country to experience an earthquake of this magnitude. GeoNet, a geological hazard monitoring system in New Zealand, provides a list of the largest earthquakes that have occurred historically in New Zealand that gives some idea of when and where they occur: http://www.geonet.org.nz/earthquake/historic-earthquakes/ Read more