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Seismic slope stability assessments based on two-dimensional (2D) analyses limits the interpretation of in-plane motion as the effects on ground motions due to internal wave reflections by lateral heterogeneities are neglected. In this study, we assess the extent to which a 2D model of a simple topographic crest (spur) is valid, using equivalent 3D simulations. We show that seismic analyses based on only 2D analysis of irregular topography can misrepresent the response of the slope at the surface. Regular slope geometries representing a laterally infinite slope can be well represented by 2D analyses, however as the slope becomes laterally irregular 3D simulations are required.

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.

The mineralogical and geomechanical properties of a distinctive early Quaternary marine clay unit that underlies the eastern and southern flanks of Mt Etna, which may play a role in gov-erning the style of instability on the edifice, have been determined. Preliminary clay mineral identification revealed the presence of interlayered illite-smectite and disordered kaolinite. The clay is heavily over consolidated (OCR≥32) and is a very stiff, extremely high undrained strength closely fissured, dark yellowish grey silty calcareous CLAY. Direct shear and ring shear tests measured peak shear strength parameters of c΄ = 51 kN/m2 and φ΄ = 20˚ and re-sidual shear strength parameters c΄r = 0 to 3.0 kN/m2 and φ΄r = 6.4 to 9.7˚. The mass strength of the clay will be influenced by the polished fissures, which are close to φ΄r, and may be a controlling factor in the style and rate of movement of the observed flank instability depend-ing on fissure orientation.