DISCLAIMER This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to Earthquake Commission & Greater Wellington. 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 Earthquake Commission & Greater Wellington and shall not be liable to any person other than Earthquake Commission & Greater Wellington, 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 February 2010. BIBLIOGRAPHIC REFERENCE Litchfield, N.; Van Dissen, R.; Hemphill-Haley, M.; Townsend, D.; Heron, D. 2010. It’s Our Fault – Ohariu Fault Paleoearthquake Investigations: Final Report, GNS Science Consultancy Report 2010/17. 32p. Project Number: 430W3130 Confidential 2010 CONTENTS EXECUTIVE SUMMARY ........................................................................................................III 1.0 INTRODUCTION ..........................................................................................................4 2.0 OHARIU FAULT – PREVIOUS WORK........................................................................4 3.0 PALEOSEISMOLOGY SITES AND RADIOCARBON DATING..................................5 4.0 TRENCH AND SITE DESCRIPTIONS.........................................................................6 4.1 Boom Rock Road Trench (T06/1) ................................................................................. 6 4.2 Bryant Trench (T08/1) ................................................................................................... 7 4.3 Horokiri Valley site......................................................................................................... 8 5.0 DISCUSSION ...............................................................................................................8 5.1 Ohariu Valley trenches .................................................................................................. 8 5.2 Post-300 year event ...................................................................................................... 9 5.3 Triggered rupture?....................................................................................................... 11 5.4 Implications for seismic hazard ................................................................................... 12 6.0 CONCLUSIONS .........................................................................................................12 7.0 ACKNOWLEDGMENTS ............................................................................................13 8.0 REFERENCES ...........................................................................................................13 APPENDIX 1 – TRENCH UNIT DESCRIPTIONS..................................................................27 APPENDIX 2 – PROFESSIONAL SERVICES PROJECT BRIEF.........................................29 FIGURES Figure 1 Location map showing the Ohariu Fault (bold) in relation to other active faults in the Wellington region. The inset shows the major features of the New Zealand plate boundary and the relative Pacific-Australian plate motion vectors along the Hikurangi Trough (labelled in mm/yr), after Nicol & Wallace (2007). Active faults (on-land only; black lines) are from the New Zealand Active Faults Database (http://data.gns.cri.nz/af/index.jsp). AkF - Akatarawa Fault, AwaF – Awatere Fault, NOF – Northern Ohariu Fault, PkF - Pukerua Fault, SGF - Shepherds Gully Fault, VF – Vernon Fault, WairF – Wairarapa Fault, WF – Wairau Fault, WgtnF – Wellington Fault, W - Wellington City. B, Detailed map of the Ohariu Fault (black line) and nearby active faults of the Wellington region (white lines), from the same data source as A. The three sites described in this study are labelled in bold, and the remainder are sites where radiocarbon ages constraining Ohariu Fault surface rupture events have been obtained (Heron et al. 1998; Litchfield et al. 2004, 2006; Cochran et al. 2007). Black open circles are the locations of major population centres. Grid references are in New Zealand Map Grid 1949 projection....................................................19 Figure 2 A, Oblique aerial view of the Boom Rock Road Trench site in the Ohariu Valley (view to the southeast). Yellow arrows denote the surface trace of the Ohariu Fault. (Photo: Lloyd Homer, GNS Science, CN 17409a) B, Photograph of the northeast wall of the Boom Rock Road Trench. Major faults (bold red lines) and the top of the greywacke bedrock (thin black line) are delineated. C, Summary trench log of the northeast wall of the Boom Rock Road Trench (T06/1). Unit numbers are for descriptive purposes only, and field descriptions are given in Appendix 1. Details of radiocarbon ages are contained in Table 1. Brown coloured units are interpreted to be correlatives across the fault and to be fluvial terrace deposits (see text for further discussion). Grid numbers on the left hand side and along the base are spaced 1 m apart. ...........................20 Figure 3 A, B & C A, The Bryant Trench site in the Ohariu Valley (view to the southeast). Yellow arrows denote the surface trace of the Ohariu Fault. B, Photograph of the southwest wall of the Bryant Trench, showing the major faults (bold red lines, dashed across the bench). C, Summary trench log of the southwest wall of the Bryant Trench (T08/1). Unit numbers, colours, radiocarbon ages, and grid numbers are as described in the caption for Fig. 2C. ...............................................................................................................21 Figure 3 D, & E D, Photograph of the northeast wall of the Bryant Trench. Major faults (bold red lines) and key contacts (thin black lines) are shown. E, Summary trench log of the GNS Science Consultancy Report 2010/17 i Confidential 2010 northeast wall of the Bryant Trench (T08/1). Unit numbers, colours, radiocarbon ages, and grid numbers are as described in the caption for Fig. 2C.....................................22 Figure 4 Oblique aerial view of Horokiri Valley (locally called Transmission Gully) showing the location of the Horokiri Valley site described here. Yellow arrows denote the surface trace of the Ohariu Fault. (Photo: Lloyd Homer, GNS Science, CN 14101/11).....................23 Figure 5 Radiocarbon ages which constrain the timing of recent (post-2500 cal. yr BP) events on the Ohariu Fault. The ages presented in this study are shown in bold (BRR = Boom Rock Road, HV = Horokiri Valley). Those from the Ohariu 1a, 1b, 2, Kakaho, Horokiri, and Waikanae sites are from Heron et al. (1998), and were recalibrated by Litchfield et al. (2004). Those from the Muaupoko site are from Litchfield et al. (2004). The age from the Elsdon site is from Grant-Taylor 1974), calibrated by Rafter Radiocarbon Laboratory, as described in the text. The dashed line and arrows indicate whether the sample is a minimum or maximum age constraint. The shaded grey boxes are (top to bottom) the best estimate for the timing of the small, possibly triggered, surface rupture event identified in the Bryant Trench (310-150 cal. yr BP), the most recent major surface rupturing event along the Ohariu Fault (1050-1000 cal. yr BP), and the possible event recorded by the buried forest in the Ohariu 2 Trench (post-2470 cal. yr BP). ..........................................................................................................24 Figure 6 Comparison of the timing of Ohariu Fault surface rupture events with those on other major central New Zealand active faults. Modified from Van Dissen et al. (2009), compiling data from Little et al. (2009), McSaveney et al. (2006), Cochran et al. (2007) (Wairarapa Fault), Langridge et al. (2009) (Wellington Fault), Barnes et al. (2008), Pondard et al. (in prep.) (Wairau Fault), and Benson et al. (2001), Mason et al. (2006) (Awatere Fault). Trapezoidal shapes are schematic interpretations of the best estimate (2σ) probability distribution for the timing of events which are constrained by few radiocarbon dates. Note that the post-300 yr Ohariu Fault event identified in this study coincides with the timing of events on the Wairarapa, Wellington, and Awatere Faults. ...........................................................................................25 TABLE Table 1 Radiocarbon ages.................................................................................................................18 APPENDICES Appendix 1 – Trench unit descriptions ..................................................................................................................27 Appendix 2 – Professional services project brief...................................................................................................29 GNS Science Consultancy Report 2010/17 ii Confidential 2010 EXECUTIVE SUMMARY An important component of quantifying seismic hazard and risk in regions such as Wellington is to characterise both the long term rate of occurrence of the region’s major earthquake- generating active faults, as well as potential interactions between faults (e.g., the potential for earthquake triggering). This report describes paleoseismic data from two trenches (Ohariu Valley) and a natural stream bank exposure (Horokiri Valley) c. 24 km apart, which constrain the timing and size of recent surface rupture events on the northeast striking Ohariu Fault