GeoNet landslide response to the Mw5.7 14 February 2016 (Valentine's Day) Earthquake, Christchurch, New Zealand

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Massey, C.I.; Ries, W.F.; Archibald, G.C.; Thomson, J.; Lukovic, B.; Jones, K.E.; Yetton, M.; Mangos, C.; Horspool, N.A. 2016 GeoNet landslide response to the Mw5.7 14 February 2016 (Valentine's Day) Earthquake, Christchurch, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/35 71 p.; doi: 10.21420/G2S881

Abstract: A MW 5.7 earthquake occurred on Sunday 14 February 2016 (the Valentine’s Day Earthquake), at 1:13 pm New Zealand time (00:13:43 universal time), and is reported by GeoNet to have been located 10 km east of central Christchurch (epicentre: 43.50°S, 172.75°E) at a focal depth of 8 km. The largest horizontal Peak Ground Acceleration (PGA, single component) recorded by the GeoNet strong motion network in the Port Hills was 0.28 g (28% g) at station RCBS located at the cliff crest near Redcliffs in Sumner. The largest recorded vertical PGA in the Port Hills was 0.11 g (11 %g) at station HVSC in Heathcote Valley. A landslide response was initiated by GeoNet on 14 February 2016. The initial response was to document accounts of landslide damage reported in the news media and social media as well as from discussions with Christchurch-based geotechnical engineers and those from Christchurch City Council. This was done to determine whether a field response was needed. Based on these accounts of the damage it was decided by GeoNet to send two response teams to Christchurch, one focused on landslides and the other on liquefaction. The objective of the landslide response team was to quantify the landslides generated by this earthquake in the Port Hills. This report contains the observations and survey results made by the landslide response team. The team was on the ground in the Port Hills 16 – 20 February 2016, and again on 3 March 2016. The main observed landslide hazards that occurred in response to the 14 February 2016 earthquake were cliff-crest recession and debris avalanches (locally referred to as cliff collapses) and the fall of individual rocks (boulder rolls). The main cliffs affected were in the following areas (in order of the decreasing volume of debris that fell from them): 1. The coastal cliff at Whitewash Head (about 40,000 m3 since the last survey in August 2011); 2. The cliff below Richmond Hill Road (about 500 m3); 3. Shag Rock Reserve (about 70 m3); and 4. Redcliffs (about 50 m3). At Whitewash Head, new cracks appeared at the cliff crest, which passed through areas where red-zoned dwellings had been demolished. No permanent ground movement was observed or reported for the Quarry Road, Maffeys Road, Defender Lane and Clifton Terrace landslides, or for the toe slumps, which were first identified following the MW 6.2 22 February 2011 Christchurch Earthquake. The cliff collapses, bolder rolls and cracking observed after the 14 February 2016 earthquake were contained within the government’s red-zoned area and within the Christchurch City Councils Cliff Collapse Hazard Management areas 1 and 2. The landslides initiated by this earthquake are similar (in terms of the volumes that fell and the amounts of landslide displacement) to those triggered by the MW 5.9 23 December 2011 earthquake. (auth)