Assessment of active fault and fold hazards in the Twizel area, Mackenzie District, South Canterbury Report No. R10/25 ISBN 978-1-877542-91-6 Prepared for Environment Canterbury by DJA Barrell GNS Science Consultancy May 2010 Report R10/25 ISBN 978-1-877542-91-6 58 Kilmore Street PO Box 345 Christchurch 8140 Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636 Assessment of active fault and fold hazards in the Twizel area, Mackenzie District, South Canterbury D.J.A. Barrell GNS Science Consultancy Report 2010/040 Environment Canterbury Report No. R10/25 May 2010 DISCLAIMER This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to Environment Canterbury (ECan). Unless otherwise agreed in writing by GNS Science, GNS Science accepts no responsibility for any use of, or reliance on, any contents of this report by any person other than ECan and shall not be liable to any person other than ECan, on any ground, for any loss, damage or expense arising from such use or reliance. The data presented in this report are available to GNS Science for other use from May 2010 BIBLIOGRAPHIC REFERENCE Barrell, D.J.A. 2010. Assessment of active fault and fold hazards in the Twizel area, Mackenzie District, South Canterbury. GNS Science Consultancy Report 2010/040. 22 p. © Environment Canterbury Report No. R10/25 ISBN 978-1-877542-91-6 Project Number: 440W1435 2010 CONTENTS EXECUTIVE SUMMARY .........................................................................................................II 1. INTRODUCTION ..........................................................................................................1 2. INVESTIGATION METHODS.......................................................................................1 3. GEOLOGICAL SETTING.............................................................................................2 4. RECOGNITION OF ACTIVE FAULTS AND FOLDS...................................................6 5. FAULT AND FOLD HAZARD ASSESSMENT ............................................................8 5.1 Data collection............................................................................................................... 8 5.2 Summary description of fault deformation features....................................................... 9 5.3 Do the fault-related features in the Ostler Fault Zone represent hazards?................. 11 5.4 Types of fault and fold hazards ................................................................................... 12 5.5 Fault avoidance zones ................................................................................................ 13 5.6 Implications for planning and land-use........................................................................ 14 5.7 Hazard assessment..................................................................................................... 16 6. CONCLUSIONS .........................................................................................................19 ACKNOWLEDGEMENTS......................................................................................................20 REFERENCES.......................................................................................................................20 FIGURES APPENDIX A – DEFORMATION ANALYSES APPENDIX B – GIS LAYERS & REPORT PDF TABLES (in text) Table 1: Ground classification Table 2: Building Importance Categories and representative examples Table 3: Relationships between recurrence interval and Building Importance Category Table 4: An example of resource consent categories in relation to fault complexity classes for the Ostler Fault Zone PHOTOS (in text) Photo 1: Panorama of the Ruataniwha and Y faults, looking southwest from Mt Ostler Photo 2: Aerial view looking northeast across the investigation area. Photo 3: Detail of the surface expression of the Ruataniwha and Y faults Photo 4: Fault scarp formed on the Chelungpu Fault during the magnitude 7.6 Chi-Chi Earthquake, Taiwan, 1999 Photo 5: The Haybarn Fault Photo 6: Tilted deposits in the cliff beside Fraser Stream FIGURES (following text) Figure 1: Location and geological setting Figure 2: Ground classification Figure 3: Fault complexity classification Figure 4: Fault avoidance zonation GNS Science Consultancy Report 2010/040 i 2010 EXECUTIVE SUMMARY This report presents an assessment of active geological fault and fold hazards in the vicinity of Twizel, South Canterbury. The feature of interest is the Ostler Fault Zone, which has offset or buckled a series of geologically-young landforms in a zone stretching from the west side of Lake Pukaki southwest to the Omarama area. Geological evidence from radiocarbon dating indicates that on average, the Ostler Fault Zone experiences an earthquake large enough to cause rupture (i.e. offset) and buckling of the ground surface every 3,000 ± 1,000 years. The most recent known surface-rupture earthquake was about 3,600 years ago. While this does not mean that an earthquake is imminent, it leaves no room for complacency. It should be assumed that significant and abrupt ground breakage, perhaps involving up to several metres of movement, and buckling of the ground, will occur within the Ostler Fault Zone during the next rupture. We do not know when that may be, but the geological evidence implies that this is a credible hazard that could happen at any time in the future. Minimising or avoiding the adverse effects of a ground-rupturing earthquake should therefore be a priority issue in land-use planning and development in the Twizel area. The approach to active fault and fold hazard assessment described in this report involves three principal components: • mapping and description of physical landform features in the vicinity of the fault zone • interpretation and classification of the landform features that relate to past ground deformation events • relating this information to standard hazard planning guidelines, in order to produce a hazard zonation map that can be used to assist in land-use zonation and for assessing the suitability of different areas for development, such as rural residential subdivision. This work has highlighted areas that, according to guidelines and interpretations of legislation, are sufficiently hazardous as to warrant avoidance of certain types of buildings. Various uncertainties in the landform evidence of past movements are accommodated by placing hazard avoidance set-back distances from the most hazardous areas. Further, more detailed site investigations may be able to locate more precisely the extents of hazardous areas, and may provide a basis for narrowing the width of hazard avoidance zones. More detailed assessments may be able to show that some areas subject to less severe ground deformation hazards are suitable for certain categories of buildings. The approaches described in this report should be applicable elsewhere in the general region, on the Ostler Fault Zone or on other active fault or fold systems. GNS Science Consultancy Report 2010/040 ii 2010 1. INTRODUCTION One of the South Island’s better known active geological faults, the Ostler Fault Zone, lies, in part, just to the west of Twizel township in the Mackenzie basin of South Canterbury. The spectacular open landscape surrounded by mountains and proximity to sporting and recreational opportunities have created demand for residential development in Twizel, in particular to the west of the town. Mackenzie District Council (MDC) is considering a change to land-use zoning in an area surrounding and west of Twizel. Part of that area (proposed plan change area) lies within the Ostler Fault Zone (Fig. 1). The main potential hazard associated with active faults is the occurrence of a sudden slip event (‘rupture’) on the fault, causing a large earthquake. Ruptures commonly begin deep in the earth’s crust and, if of large size, may extend up to the ground and cause ‘surface rupture’, which involves a sudden offset (faulting) or buckling (folding) of the ground surface by up to several metres. Buildings situated within a zone of sudden offset and/or buckling are likely to suffer serious damage or even destruction. This poses a significant threat of injury or death to occupants of any such buildings. Guidelines for planning to mitigate the threat to life safety due to active fault ruptures are presented in a Ministry for the Environment report on “Planning for development on or close to active faults” (Kerr et al. 2003). To aid in the implementation of these guidelines, a number of active fault hazard assessments have been undertaken elsewhere in New Zealand, such as Wellington City (Kerr et al. 2003) and Kapiti Coast District (Van Dissen and Heron 2003, Van Dissen et al. 2004, 2006). In consultation with MDC, Environment Canterbury engaged the Institute of Geological and Nuclear Sciences Limited (GNS Science) to provide an assessment of active fault and fold hazards in relation to the proposed plan change area. This report presents the results of that hazard assessment. Information in this report is intended to provide a technical basis for the avoidance or mitigation of active fault and fold hazards in the proposed plan change area. 2. INVESTIGATION METHODS This assessment of active geological fault hazard is based on: • examination of existing publicly-available information on the Ostler