Survey of Port Hills red zone resident's experience following the 22 February 2011 Earthquake

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Taig, T.; Massey, C.I.; Taig, M.; Becker, J.S.; Heron, D.W. 2015 Survey of Port Hills red zone resident's experience following the 22 February 2011 Earthquake. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/09 32 p.

Abstract: Around 100 Christchurch Port Hills homes were severely impacted by rockfall and cliff collapse (debris avalanches) during the 22 February 2011 earthquake. Five people were killed by debris avalanches but none by boulder rolls, despite more homes being affected by the latter than the former. A survey questionnaire was undertaken to explore the discrepancy between the extensive damage to property and minimal harm to people, and to learn any lessons from those directly affected relevant to avoiding future harm from rockfall. It has also proved useful in validating the GNS Science boulder roll risk assessment model developed for Christchurch City Council (CCC) and the Canterbury Earthquake Recovery Authority (CERA), and the assumptions used in those models for the purpose of helping with the zoning of Port Hills properties. The questionnaire survey involved locating and carrying out telephone interviews with former residents of red-zoned Port Hills properties. It asked questions on: i) the number of people normally resident at the property; ii) the number of people present at the time of the main 22 February 2011, earthquake; iii) if present, where were they on the property; and iv) their personal experience during the earthquake. The survey was carried out by Marian Taig of GNS Science over the period April to July 2013. The survey was used to interview the householders of 70 red zoned Port Hills properties from across the Port Hills suburbs. 196 persons were normally resident in those 70 properties, which represent about 14% of the properties red-zoned because of slope instability in the Port Hills. Boulder rolls or cliff collapses directly impacted 27 of the 70 respondent properties either on 22 February 2011, or afterwards. None of the residents had been killed or seriously injured, though some had suffered considerable cuts and bruises and others thought they had been lucky to escape unharmed. The risk assessment method used by GNS Science for CCC and CERA in support of zoning decisions for boulder rolls was used to estimate the probabilities of death for each person present in properties impacted by boulder roll: a. under the assumptions used in the CCC/CERA assessments – which were intentionally pessimistic as the intention of this work was to inform future planning decisions, b. under alternative assumptions based on the actual locations and travel distances of boulder impacts, and c. under alternative assumptions as to the likely locations of residents by day and by night, and alternative assumptions as to the numbers of residents at home at the time of the 22nd February 2011 earthquake. (auth)