Understanding Local Source Tsunami: 1820S Southland Tsunami
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EQC Project 03/490 - Understanding local Client Report 2005/153 source tsunami: 1820s Southland tsunami November 2005 by Downes, G., Cochran, U., Wallace, L., Reyners, M., Berryman, K. (GNS) Walters, R., Callaghan, F., Barnes, P., Bell, R. (NIWA) EQC Project 03/490 - Understanding local source tsunami: 1820s Southland tsunami by Downes, G., Cochran, U., Wallace, L., Reyners, M., Berryman, K. (GNS) Walters, R., Callaghan, F., Barnes, P., Bell, R. (NIWA) Prepared for EQC Research Foundation Institute of Geological & Nuclear Sciences client report 2005/153 Project Number: 410W1034 National Institute for Water and Atmospheric Research Consultancy report HAM2005-135 Project Number: EQC04201 The data presented in this Report are available to GNS and NIWA for other use from November 2005 COMMERCIAL – IN – CONFIDENCE This report has been prepared by the Institute of Geological & Nuclear Sciences Limited and the National Institute of Water and Atmospheric Research exclusively for and under contract to EQC Research Foundation. Unless otherwise agreed in writing, all liability of the Institutes to any other party other than EQC Research Foundation in respect of the report is expressly excluded. CONTENTS EXECUTIVE SUMMARY...................................................................................................................................V 1.0 INTRODUCTION ..................................................................................................................................... 1 2.0 TECTONIC SETTING OF THE FIORDLAND REGION ................................................................... 3 2.1 The Plate Boundary ............................................................................................................ 3 2.2 Previous Earthquakes.......................................................................................................... 5 2.2.1 Modern (post-1960) Earthquakes........................................................................ 5 2.2.2 Historical Earthquakes ........................................................................................ 5 2.2.3 Pre-historical Earthquakes................................................................................... 6 2.3 Previous Tsunami ............................................................................................................... 7 3.0 HISTORICAL RECORD.......................................................................................................................... 8 3.1 1820s Southland tsunami.................................................................................................... 8 3.1.1 Timing ................................................................................................................. 9 3.2 1826 Fiordland earthquakes and tsunami ......................................................................... 10 3.2.1 Uplift ................................................................................................................. 12 3.2.2 Landslides and rockfalls.................................................................................... 12 3.2.3 Tsunami and seiching........................................................................................ 13 3.2.4 Timing ............................................................................................................... 13 3.2.5 One mainshock or several large shocks?........................................................... 14 4.0 PALEOTSUNAMI RECONNAISSANCE ............................................................................................ 15 4.1 Introduction....................................................................................................................... 15 4.2 Methods ............................................................................................................................ 15 4.3 Field Observations and Discussion................................................................................... 17 4.3.1 Martins Bay ....................................................................................................... 17 4.3.2 Cascada Bay ...................................................................................................... 23 4.3.3 Goose Cove ....................................................................................................... 30 4.4 Summary........................................................................................................................... 37 5.0 EARTHQUAKE DISLOCATION AND TSUNAMI PROPAGATION MODELS........................... 38 5.1 Introduction....................................................................................................................... 38 5.2 Initial conditions: dislocation models ............................................................................... 39 5.2.1 Alpine Fault....................................................................................................... 39 5.2.2 Fiordland subduction zone ................................................................................ 40 5.2.3 Puysegur subduction zone................................................................................. 42 5.3 Numerical tsunami propagation model............................................................................. 44 5.3.1 Model grid and bathymetry ............................................................................... 45 5.4 Results............................................................................................................................... 46 5.4.1 Alpine Fault....................................................................................................... 47 5.4.2 Fiordland subduction zone events ..................................................................... 50 5.4.3 Puysegur subduction zone events...................................................................... 55 5.5 Summary........................................................................................................................... 60 6.0 DISCUSSION........................................................................................................................................... 61 6.1 1820s tsunami ...................................................................................................................61 6.2 1826 earthquake and tsunami ........................................................................................... 63 6.3 Earthquake shaking hazard ............................................................................................... 64 EQC Project 03/490—Understanding local tsunami: 1820s Southland Tsunami i 7.0 CONCLUSIONS...................................................................................................................................... 68 7.1 Historical........................................................................................................................... 68 7.2 Paleotsunami reconnaissance............................................................................................ 68 7.3 Numerical Modelling........................................................................................................ 69 8.0 ACKNOWLEDGEMENTS .................................................................................................................... 70 9.0 REFERENCES ........................................................................................................................................ 70 10.0 APPENDICES............................................................................................................................... 75 Appendix 1 Source Material: 1820s Southland tsunami....................................................... 75 Appendix 2 Source material: 1826 Fiordland earthquake and tsunami ................................ 81 Appendix 3 Site selection notes: Fiordland paleotsunami reconnaissance ........................... 85 Appendix 4 Model description............................................................................................. 91 LIST OF TABLES Table 1 Molluscan assemblage from a pebbly, sandy silt unit 90-120 cm below river level near the mouth of Lake McKerrow. Re-collected from Wellman’s locality with identification and ecological notes by Alan Beu.......................................................................22 Table 2 Radiocarbon age results for Martins Bay samples. .................................................................23 Table 3 Radiocarbon age results for Cascada Bay samples. ..............................................................28 Table 4 Radiocarbon age results for Goose Cove samples.................................................................37 LIST OF FIGURES Figure 1 Location map showing important sites mentioned in the text. 1:250,000 topographic map courtesy of Land Information New Zealand (Crown Copyright Reserved)....................2 Figure 2 A: Tectonic setting of the South Island on the boundary between the Pacific and Australian tectonic plates. Macquarie Ridge (not labelled) lies west of the plate boundary south of 50ºS. B: Simplified tectonic setting of Fiordland illustrating three main components of the plate boundary in this region (modified from Barnes et al., 2005).................................................................................................................................................4 Figure 3 View of the Orepuki coastline looking south from Gemstone