It's Our Fault : Active Faults and Earthquake Sources in Cook Strait NIWA Client Report: WLG2008-56 June 2008 NIWA Project: IOF07301 It's Our Fault : Active Faults and Earthquake Sources in Cook Strait Philip M. Barnes 1 Nicolas Pondard 1 Geoffroy Lamarche 1 Joshu Mountjoy 1,2 Russell Van Dissen 3 Nicola Litchfield 3 1National Institute of Water & Atmospheric Research Ltd 2Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. 3 GNS Science, PO. Box 30368, Lower Hutt, New Zealand. NIWA contact/Corresponding author Philip M. Barnes Prepared for GNS Science NIWA Client Report: WLG2008-56 June 2008 NIWA Project: IOF07301 National Institute of Water & Atmospheric Research Ltd 301 Evans Bay Parade, Greta Point, Wellington Private Bag 14901, Kilbirnie, Wellington, New Zealand Phone +64-4-386 0300, Fax +64-4-386 0574 www.niwa.co.nz All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. DISLCAIMER In preparing this report and accompanying electronic data, NIWA have used the best available information, and interpreted such information exercising reasonable skill and care. Nevertheless, neither NIWA nor GNS Science accept any liability, whether direct, indirect or consequential, arising out of the provision of information contained in this report, or provided otherwise by NIWA to GNS Science. This report was provided for the sole purpose of supporting the Its Our Fault Programme, and the National Seismic Hazard Model, and may not be used for any other purposes. All digital GIS data provided by NIWA to GNS Science is to be used for internal purposes only and is not to be distributed to a third party in any form without the prior written approval of NIWA . The report must be used in whole and can not be materially modified without NIWA’s consent . All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. Contents Executive Summary iv 1. Introduction 1 1.1 It’s Our Fault programme 1 1.2 Cook Strait project objectives and methodology 1 2. Data Sources 3 2.1 Seismic reflection data 3 2.2 Multibeam bathymetry and side-scan sonar data 3 2.3 Onshore fault data 4 3. Geological and Tectonic Background 4 3.1 Geometry and kinematics of the Australian-Pacific plate boundary 4 3.2 Cook Strait morphology and sedimentary basins 5 3.3 Previous studies of active faulting and earthquake sources 6 4. Late Quaternary Sediments and Stratigraphic Markers 6 5. Active Submarine Faulting in Cook Strait 7 5.1 Focus of new mapping 7 5.2 Results: Tectonic structures 8 5.2.1 Mana – Narrows Basin 8 5.2.2 Central Cook Strait 8 Wairau Fault 9 Awatere, Vernon, and Cloudy faults 9 Wellington, Ohariu, and Shepards Gully faults 10 Wairarapa, Nicholson Bank, and Wharekauhau faults 10 5.2.3 Eastern Marlborough – Southern Cook Strait 11 Kekerengu, Needles, Chancet and Campbell Bank faults 11 Boo Boo Fault 12 Hope and Te Rapa faults 12 5.2.4 Southern Hikurangi Margin 12 6. Earthquake Sources in Cook Strait 13 7. Conclusions 14 8. Acknowledgments 15 9. References 16 10. Figure Captions 26 Reviewed by: Approved for release by: Neville Ching Andrew Laing Executive Summary This study of active faulting and earthquake sources in Cook Strait is one of several simultaneous studies in the first two years of the Its Our Fault programme , aimed at identifying and constraining the location, size, and history of large earthquakes on major faults in the Wellington region. The objectives of this study of active faulting and earthquake sources in Cook Strait are to: (1) determine the location, geometry, segmentation, and rate of activity of major faults in Cook Strait; and (2) interpret these structures in terms of their potential as earthquake sources, with consideration to onshore-to-offshore fault relationships. These results are required for input into models of earthquake recurrence, seismic hazard, and plate boundary deformation. In this study, high-quality multibeam bathymetric data together with new and archived seismic reflection profiles are used to develop a new interpretation of active faults in Cook Strait. The findings are integrated with the onshore active fault database at GNS Science to reveal a complete deformation picture of the region. Submarine fault displacements and slip rates are determined where possible using seismic reflection markers, interpreted within the framework of glacio- eustatic sea-level fluctuations, and displaced seafloor features. Net slip rates quoted in this report are best estimates based on a combination of: (1) dextral slip rates on the onshore parts of faults which cross the coast; (2) estimated fault slip rates from submarine data; and (3) a geologically reasonable interpretation of predicted slip distribution within the plate boundary zone. Uncertainties in rates are typically large, of the order of 30-50%. The structural results of this study indicate the following: • In Cook Strait there is a general discontinuity between the major faults of North and South islands. However, despite the discontinuous nature of the faults, there is a first order alignment between the Wairau and Awatere faults in South Island and the Kapiti-Manawatu, Ohariu and Wellington Faults in North Island, and between the Kekerengu Fault in the South Island and the Boo Boo Fault and the Wairarapa Fault in North Island. • Many of the major faults are seaward extensions of faults onshore, however, numerous large structures are entirely submarine. • Whereas the predominant structural trend in South and North islands is SW-NE, faults in central and southern Cook Strait are predominantly E-W trending, dextral strike-slip faults with moderate to high slip rates. Submarine fault traces are typically 10 to 90 km long. Its Our Fault : Active Faults and Earthquake Sources in Cook Strait iv • The structure between Wellington and Blenheim indicates the faults accommodate a combination of strike-slip and extension, which is consistent with the Australian–Pacific plate motion vectors. In contrast, faults in southern Cook Strait accommodate a combination of strike-slip and compression, and local areas of active uplift are evident. • Off the western Wellington (Mana) coast, predominantly NE-SW striking reverse faults with relatively low slip rates are continuous with the southern components of the offshore Kapiti – Manawatu Fault System. Based on their shallow structure and tectonic geomorphology we infer the faults off the western coast of Wellington to potentially have a larger component of dextral displacement than those in the Kapiti – Manawatu Fault System further north. • The continental slope of southern Wairarapa and eastern Marlborough is dominated by NW- SE striking thrust faults. In Cloudy Bay, paleo-earthquake records have been derived for the offshore Wairau, Cloudy, and Vernon faults. Six paleo-earthquake ruptures are inferred on the offshore Wairau Fault since 12 ka. The last two events have timing and coseismic slip (vertical ~1-2 m) in good agreement with onshore data. From our longer record, we observe an average recurrence interval of ~2000 yrs for the Wairau Fault (similar to what has been inferred from onshore data based on single event displacement and slip rate considerations) and, importantly, variable duration of time between individual events (ranging from 700-3000 yrs). Four to five earthquakes are recognised on the Vernon Fault since 18 ka. These have spatially variable vertical slip per event (1.0-2.5 m) and variable time between individual events (ranging from 2000 to 6000 yrs) (average recurrence interval ~ 4000 yrs). The Cloudy Fault exhibits five events since 17 ka, with slip per event of 1.0- 4.0 m (mean ~3 m) and inter-event times that range from 1500 to 4000 yrs (with an average recurrence interval of ~3500 yrs). Interpreted earthquake sources in the upper crust, above the Hikurangi subduction megathrust, include some onshore to offshore fault ruptures, and some entirely marine sources. Estimates are presented for the moment magnitude (M w), coseismic displacement, and average recurrence interval for the earthquake sources. The results indicate potential earthquakes with magnitudes ranging from M w 6.6 to 7.9, and with recurrence intervals ranging from about 500 years to >20,000 years. Its Our Fault : Active Faults and Earthquake Sources in Cook Strait v 1. Introduction 1.1 It’s Our Fault programme The It’s Our Fault programme was established in 2006 to help Wellington become a more resilient city through a comprehensive study of the likelihood, size, and location of large earthquakes, and assessment of their impacts on the city and its region. The programme is funded by the Earthquake Commission (EQC), Accident Compensation Corporation (ACC), and Wellington Regional Council (WRC), is managed by GNS Science, and directionally overseen by a Steering Committee comprising members from each organisation. Its Our Fault will take about seven years, and includes four main components: Likelihood, Size, Effects and Impacts. In a national context, seismic risk is concentrated in the Wellington region and is predominantly the result of large earthquakes on the Wellington Fault. These risk estimates, though based on international best-practice, assume that earthquakes occur randomly in time and that large earthquakes on a given fault do not affect other nearby faults. Preliminary analysis indicates, however, that the 1855 M~8 Wairarapa earthquake may have delayed the next Wellington Fault earthquake, perhaps by as much as several hundred years.