Not so lovely - one year on from a rude shock

Published: Tue Feb 14 2017 2:52 PM
Updated: Tue Feb 14 2017 5:45 PM

Today marks one year since the magnitude 5.7 Valentine’s Day earthquake hit Christchurch at 1.13pm on a sunny summer Sunday.

The earthquake, which happened more than four years after the previous large aftershock in the Christchurch earthquake sequence (the magnitude 5.9 23 December 2011 earthquake), struck under the sea floor, 2 km off the New Brighton coast at a depth of 8 km. As well as causing more liquefaction in eastern suburbs, and rattling more than a few nerves, one of the biggest impacts of the earthquake on the environment was more cliff collapses and boulder rolls. Our landslide scientists headed to Christchurch two days after the earthquake to survey the landslides in the Port Hills with the help of local geotechnical engineers and Christchurch City Council staff.

Most of the landslides generated by the earthquake were cliff collapses (technically called cliff-crest recession and debris avalanches) and boulder rolls (individual boulders bouncing down hills) on the steep cliffs between Redcliffs and Taylors Mistake. For those of us who like maps, the one below shows where the most rock falls and cliff collapses occurred compared to where the strongest ground shaking was felt.

Wakefield Ave in Sumner, showing the area where rock fell from and where it landed

Wakefield Ave in Sumner, showing the area where rock fell from and where it landed

Thousands of cubic metres of rock fell into the sea from Whitewash Head Photo RNZ Sally Murphy

Thousands of cubic metres of rock fell into the sea from Whitewash Head Photo RNZ Sally Murphy

Earthquake shaking in the Valentine’s Day earthquake (units are maximum peak ground acceleration as a % of gravity or, in normal people speak, red = strong, blue = not so strong) and the main areas of cliff collapse

Earthquake shaking in the Valentine’s Day earthquake (units are maximum peak ground acceleration as a % of gravity or, in normal people speak, red = strong, blue = not so strong) and the main areas of cliff collapse

Drone photo of Whitewash Head showing new cracks in the cliff face - the rest of the road on the right fell into the sea during the 13 June 2011 earthquake

Drone photo of Whitewash Head showing new cracks in the cliff face - the rest of the road on the right fell into the sea during the 13 June 2011 earthquake

By using UAVs (unmanned aerial vehicles or drones), laser scanners, and walking around the main sites affected (where it was safe enough) the scientists and engineers were able to measure how much rock had fallen and where new cracks had opened in the rock, or old cracks had grown wider.

By far the largest amount of rock fell from Whitewash Head near Scarborough. Around 40,000 m3 of rock – 16 Olympic swimming pools’ worth – has fallen into the sea off Whitewash Head since it was last surveyed in August 2011, most of which happened in the December 2011 earthquake and the Valentine’s Day earthquake. Our scientists also mapped many new cracks at the top of the cliff here.

Far behind, the runners up were Richmond Hill Road cliff above Wakefield Ave in Sumner (about 500 m3, as much as you could tip into five double garages), Shag Rock Reserve (about 70 m3, a couple of shipping containers’ worth) and the cliff behind Redcliffs (about 50 m3 or seven of Paeroa’s giant L&P bottles).

These amounts were less than amounts that fell from these same sites during the 23 December 2011 earthquake, even though the amount of shaking was similar. This is probably because there were less unstable rocks that were ready to fall as many of them had already fallen before the Valentine’s Day earthquake.

There was no movement recorded on any of the landslides on the Port Hills being monitored by GeoNet and Christchurch City Council. The earthquake shaking was below the level that we would expect these landslides to move, and Christchurch City Council had also worked over the previous five years to stabilise some of these slides.

We were fortunate that no one was injured in the earthquake given that it happened when many people were out enjoying the beaches and coastal walkways in the most affected areas. More good management than good luck though was the fact that all the cliff collapses and new cracking happened on red zoned land, where houses had been purchased by the Government and demolished because of the high risk of further cliff collapse and bolder rolls.

For all the technical details you can download the Valentine’s Day earthquake landslide report (SR_2016-035.pdf).