Active Faults and Earthquake Sources in Cook Strait

It's Our Fault :

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 .

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. This result, if true, would have a profound effect on estimation of seismic risk in Wellington. Accordingly, this result needs rigorous testing through the development of more realistic long term synthetic seismicity catalogues, and validation through comparisons with actual fault activity and earth deformation data. This, in essence, is the focus of the Likelihood component of It’s Our Fault.

There are three main aspects to the Likelihood component: (1) Geological investigations to extend and further constrain the location of faults, their slip rate, and earthquake history; (2) Geodetic, Global Positioning System studies of the Wellington region to constrain the extent of the currently locked portion of subduction thrust under Wellington; and to possibly further constrain slip rate uncertainties for the major upper plate faults in the region; and (3) Synthetic seismicity modelling of the Wellington region to investigate the stress interactions of the major faults, and to specifically assess the rupture statistics and interactions of the Wellington-Wairarapa fault-pair. Results from (1) and (2) will provide critical input and validation for (3).

The study of active faulting and earthquake sources in Cook Strait, documented in this report is, therefore, one of several simultaneous studies in the first two years of Its Our Fault . The others, being undertaken by GNS Science and Victoria University of Wellington, concern onshore faulting and paleo-earthquake studies, geodetic monitoring, and development of the synthetic seismicity catalogue. All of these projects are focused on the Likelihood component of the programme.

1.2 Cook Strait project objectives and methodology The specific objectives of this study are to: (1) determine the location, dip (where possible), sense of slip, segmentation, potential structural linkages, and displacement rates of the active offshore faults in Cook Strait; and (2) interpret these structures in terms of their potential as earthquake sources, with consideration to onshore-to-

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 1

offshore structural relationships. These results are required for the synthetic seismicity model of the Wellington region, and to improve estimates of earthquake potential in close proximity to the city. Whilst the geographic scope of this study incorporates the Cook Strait region (Fig. 1), there has been a concentrated effort on the offshore Wellington, Ohariu, Wairau, and Awatere faults in Cloudy Bay, which were previously very poorly constrained.

To undertake this work, high-quality bathymetric data together with archived and new seismic reflection profiles are integrated to determine fault structure. New interpretations of active faulting were imported into NIWA’s marine GIS database, and maps of fault structure and bathymetry derived as outputs. Fault displacements are determined using seismic stratigraphy as time markers, and knowledge of sedimentary responses to glacio-eustatic sea-level fluctuations in the region. One of the critical questions of our work is to assess whether or not the plate boundary deformation zone in Cook Strait is composed of through-going faults directly connecting the Marlborough and southern North Island fault systems, or if the faults are discontinuous. Implications for earthquake rupture propagation along faults in the area are important, as various studies indicate that earthquake ruptures may be arrested at geometric fault complexities, such as jogs and fault bends, commonly named asperities and barriers (e.g. Das and Aki, 1977; Kanamori, 1978; Aki, 1979; King, 1983; King and Nabelek, 1985; Sibson, 1985; Wesnousky, 2006).

Identification of the post-glacial transgressive erosion surface (c. 20-7 ka) and post glacial sediment cover on the shelf of Cook Strait, enable the rate of vertical separation on active faults to be determine from seismic profiles. In general these represent minimum net slip rates, given the dominant structural style in the region is strike-slip. The slip rates quoted in this report are considered best estimates of net slip rate based on a combination of: (1) dextral slip rates determined elsewhere on the onshore parts of faults which cross the coast; (2) direct measurement of submarine fault slip rates (e.g., Cloudy and Boo Boo faults); and (3) a geologically reasonable interpretation of expected slip distribution within the plate boundary zone. Uncertainties in rates are typically large, of the order of 30-50%.

This report presents a brief overview of the major active fault structures in Cook Strait, but not a discussion of their significance in terms of plate boundary deformation and kinematics. Earthquake sources are interpreted by considering the fault geometry, segmentation, onshore–offshore structural relationships, and Late Pleistocene – Holocene (< 20 kyrs) faulting activity (e.g., Lamarche and Barnes, 2005). Estimates of the earthquake magnitude and recurrence are made following the methodology of Stirling et al. (2002, 2007). For breavity, we do not discuss in this report the derivation of the uncertainties associated with these sources, which are functions of the many input parameters, including displacement measurements, stratigraphic age, seismic velocity, fault dip and depth, source length and area, and slip rate. Such uncertainties will be developed fully in peer reviewed journal articles that we plan to follow this work.

Electronic GIS outputs of the active faulting have been provided to GNS Science. These data are currenly being used as input to the It’s Our Fault synthetic seismicity modelling of the Wellington region, and earthquake source parameters will be input to the GNS–NIWA National Seismic Hazard earthquake source model. Both these derivative outputs – modelling of synthetic seismicity and of earthquake sources (e.g.,

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 2

Stirling et al., 2002, 2007) – will lead to improved understanding of seismic hazard in the Wellington region, Kapiti coast, and Marlborough.

2. Data Sources

2.1 Seismic reflection data Extensive marine seismic reflection data with various depths of penetration and resolution are used in this study (Fig. 2). In 2005 about 400 km of high-resolution Boomer seismic profiles (penetration ~150 m, resolution 30 cm) were acquired from the Wairau region (Cloudy Bay) of central Cook Strait, with the aim of mapping the major faults in this poorly constrained area (Pondard et al., in prep.). These profiles provide exceptional imaging of major active faults, including the offshore Awatere and Wairau Faults (e.g., Figs. 3 and 4). In addition, a substantial set of new 3.5 kHz profiles were acquired throughout the strait, largely from water depths of >100m, during multibeam bathymetric surveys by NIWA between 2002 and 2005 (Mountjoy et al., submitted).

In addition to these new high resolution data, particularly in the northern and southern Cook Strait regions, extensive archived multichannel (MCS), single-channel, and 3.5 kHz seismic data sets recorded by NIWA and the former New Zealand Oceanographic Institute (DSIR) are also used (Carter et al., 1988; Lewis et al., 1994; Barnes and Audru, 1999a, b; Barnes et al., 1998). The single-channel airgun profiles typically reveal sub-bottom penetration of 400-600 ms TWT (~300-500 m).

Through data sharing agreements, an archived deep-penetration seismic line across the Marlborough shelf, acquired by GNS Science in the mid 1990s (Field et al., 1997), and three new high-fold deep-penetration (>4 km) sections acquired in 2005 by Discovery Geo Corporation for exploration purposes, were also accessed. These latter data are the best quality MCS sections available in Cook Strait (e.g., Fig. 5). A reconnaissance network of archived marine seismic reflection profiles acquired by oil companies between 1969 and 1973 were also reinterpreted. These archived oil industry profiles are 3-12 fold multi-channel data, are sufficient to image strata at a subsurface depth of up to 4 s TWT, and reveal the location and structure of numerous faults within and bounding the sedimentary basins (Uruski, 1992; Barnes and Audru, 1999a, b).

2.2 Multibeam bathymetry and side-scan sonar data Between 2002 and 2005 NIWA acquired high-resolution (10 m grid) SIMRAD EM300 multibeam bathymetric data from Cook Strait on board RV Tangaroa (Fig. 2). In addition, multibeam data were also acquired from selected areas on the northeast Marlborough shelf near Cape Campbell. These data provide unprecedented details of the geomorphology associated with active faulting, submarine canyons, landslides, sediment transport bedforms, and erosional scour (e.g., Fig. 6). The grid data were illuminated from various grazing angles and azimuths to reveal the active fault traces (e.g., Barnes, 2005a, b).

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 3

2.3 Onshore fault data In this study interpretations of offshore faulting are integrated with GIS coverage of the onshore active faults database at GNS Science. The integrated offshore-onshore active fault structure provides the basis for interpretation of earthquake sources.

3. Geological and Tectonic Background

3.1 Geometry and kinematics of the Australian-Pacific plate boundary The Australian-Pacific plate motion is characterized by oblique convergence in New Zealand (DeMets et al., 1994; Beavan et al., 2002) (Fig. 1). The relative plate motion rate is ~45 mm/yr in northern New Zealand, ~40 mm/yr in Cook Strait, and ~35 mm/yr in southern New Zealand. Whereas the rate decreases southward, the obliquity of convergence increases progressively south. Along the North Island, the plate motion vector of the Pacific Plate relative to Australian Plate forms an angle of ~50° to the subduction margin. New results presented here indicate that in the Cook Strait area, the plate boundary structures become nearly parallel to the plate motion vector, increasing the cumulative rate of strike-slip displacement (Pondard et al., in prep.). As the plate boundary extends into South Island, the angle between the boundary and the plate vector increases progressively to reach ~20°.

In the North Island, the oblique convergence is mainly partitioned across the North Island Dextral Fault Belt (NIDFB) and the Hikurangi subduction margin (Beanland, 1995; Barnes and Mercier de Lépinay, 1997; Barnes et al., 1998; Beanland and Haines, 1998; Nicol and Wallace, 2007, Nicol et al., 2007), and some motion is accounted for by rotation of crustal blocks (Wallace et al., 2004). The converging component of plate motion is mainly accommodated on the subduction thrust, while most of the right-lateral motion is accommodated in the upper plate. The most prominent active faults of the NIDFB are the Wairarapa, Wellington and Ohariu faults (Fig. 7). The Wairarapa fault accommodates 8-12 mm/yr of dextral slip (Van Dissen and Berryman, 1996), while the Wellington and Ohariu faults are associated with slip- rates of 6-7 mm/yr and 1-2 mm/yr respectively (Heron et al., 1998; Langridge et al., 2005).

The Hikurangi subduction zone intersects the seafloor in 2500-3000 m water depth in the Hikurangi Trough east of North Island (e.g., Barnes et al., 1998), and dips gently to the northwest (Robinson, 1986; Reyners, 1998). Cook Strait and Marlborough straddle a transition between the Hikurangi subduction zone and very oblique continental collision along the Southern Alps. The Pacific plate has been subducted beneath Marlborough to depths of more than 150 km, but geological studies indicate that most of the total relative plate motion is currently being accommodated by strike- slip faults in the upper crust (Fig. 1) (e.g., Holt and Haines, 1995). These northeast- trending faults pass through the Kaikoura Ranges and extend offshore beneath southern Cook Strait and the eastern Marlborough continental margin (Carter et al., 1988; Barnes and Audru, 1999a, b).

In the South Island, the oblique collision is mainly accommodated by dextral-reverse slip on the Alpine Fault (Berryman et al., 1992; Norris and Cooper, 2001). The strike- slip component of displacement along the Alpine Fault is relatively constant (27 ±5 mm/yr) while the dip-slip component varies from zero to ~10 mm/yr. In northern South Island, the Alpine Fault continues as the Wairau Fault to the coast in Cook

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 4

Strait; whereas, several other major strike slip faults branch from it to form the Marlborough Fault System (MFS). The MFS includes the Wairau, Awatere, Clarence, Kekerengu, and Hope faults. Smaller faults are also present in the region such as the Vernon Fault. The Hope Fault accommodates most of the slip-rate transferred from the Alpine Fault (23 ±4 mm/yr) (Langridge et al., 2003; Langridge and Berryman, 2005). Towards the east coast, most of the slip-rate accommodated by the Hope Fault is transferred to the Kekerengu Fault (~19 mm/yr) (Van Dissen and Yeats, 1991; Van Dissen et al., 2005). The Clarence, Awatere, and Wairau faults have slip-rates of ~4 mm/yr, ~6 mm/yr, and ~4 mm/yr , respectively (Knuepfer, 1988; 1992; Nicol and Van Dissen, 2002; Mason et al., 2006).

Two major earthquakes have affected the Cook Strait area in historic times. These are the 1848 M7.5 Marlborough earthquake generating Modified Mercalli intensities of MMVIII in Cook Strait, and the 1855 M8+ Wairarapa Earthquake generating MMIX+. No significant earthquakes have occurred in the last 150 years (Downes, 1995).

3.2 Cook Strait morphology and sedimentary basins Cook Strait links the Tasman Sea with the Pacific Ocean through central New Zealand, pinching to just 22 km in width at “the Narrows” and widening to over 80 km in the south. Much of the seabed is at water depths of less than 150 m except through the Narrows Basin where depths reach >300 m, and to the south east in the Cook Strait Canyons (Fig. 7). The canyon system dominates the seafloor of the south eastern part of Cook Strait, incising the shelf with a three branched canyon head to water depths at the canyon rim as shallow as 50 m, and plunging to a depth of over 2500 m at the Hikurangi Trough. The shelf that separates the canyon rim from the present day coastline varies in width from as little as 1.3 km to over 30 km; however, during glacial maximum periods when sea-level was 120 m lower, the shelf would have been all but non-existent with the coastline at or near to the canyon rim. The seaway is dominated by strong semi-diurnal tidal flows, and regular storms given its exposure to both the southern ocean and the predominant northwesterly winds (Heath, 1978; Lewis, 1979).

The shelf geology in Cook Strait principally comprises three major sedimentary basins: the onshore-offshore Wairarapa Basin, the entirely offshore Flaxbourne Basin; and the Wairau Basin. The smaller bathymetrically expressed Narrows Basin lies to the north. The Wairarapa Basin underlies the upper reaches of the canyon system, is about 15-20 km wide, and contains about 3 km of gently dipping Miocene to Pliocene mudstone and siltstone (Barnes and Audru, 1999a; Carter et al., 1988; Uruski, 1992). The Flaxbourne Basin underlies the southern shelf, is about 15-20 km wide, and contains >4.5 km of sedimentary deposits at the Cook Strait Canyons. Flaxbourne Basin sedimentary sequences are correlated to Motunau and Awatere Groups comprising sandstone, siltstone and mudstone units (Audru, 1996). The Wairau Basin contains in excess of 4 km of sediment, including a thick section of inferred Miocene to Recent age (Uruski, 1992). Underlying the upper continental slope the geology is characterised by deformed slope basins and a foundation of actively deforming older rocks landward of accreted trench sediments (Collot et al., 1996; Barnes and Mercier de Lépinay, 1997). Beyond the continental slope to the south east the Hikurangi Trough is dominated by turbidite deposits >5 km thick (Lewis et al., 1998).

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3.3 Previous studies of active faulting and earthquake sources Prior to this study active offshore faulting along the coasts of Kapiti (Lamarche et al. 2005; Nodder et al., 2007), eastern Marlborough (Barnes and Audru, 1999a, b), and southeast Wairarapa (Barnes et al., 1998; Mountjoy et al., submitted) were relatively well constrained. Until now, however, the tectonic structures have been poorly known where the MFS and NIDFB approach each other in central Cook Strait. Early studies proposed various geometries of the submarine fault system there, including suggestions of continuous faults (Lensen, 1958; Kingma, 1974; Stevens, 1974), and discontinuous faults (Ghani, 1974; Katz and Wood, 1980; Carter et al., 1988; Lewis et al., 1994). These early studies suffered from a lack of well-resolved data.

Barnes and Audru (1999a, b) discovered a close co-alignment of the Wairarapa Fault with a major fault off Cape Campbell named the Needles Fault (Fig. 7), and suggested that a relatively youthful strike-slip fault zone is developing across the southern strait. Active fault traces on Nicholson Bank, possibly part of the offshore Wairarapa Fault, were recognised by Barnes (2005a) using the new EM300 multibeam data. The results of Barnes and Audru (1999a, b), Barnes (2005a) and unpublished work on the Boo Boo Fault (Barnes, 2005b), were considered in recent interpretations of earthquake sources and seismic hazard in the Canterbury region (Stirling et al., 2007, in press).

4. Late Quaternary Sediments and Stratigraphic Markers The continental shelf of Cook Strait is characterised by a ubiquitous erosion surface that is also recognised widely elsewhere on the New Zealand continental shelf (e.g. Figs. 3 and 4) (e.g., Lewis, 1973; Herzer, 1981; Carter et al., 1986; Nodder, 1993; Barnes 1995, 1996; Foster and Carter, 1997; Barnes and Audru, 1999a,b; Barnes et al., 2002; Lewis et al., 2004; Barnes and Nicol, 2004; Lamarche et al., 2005, 2006; Nodder et al., 2007). This surface has a diachronous age between the present shelf break and the coast. It formed in very shallow water (<~20 m) within the littoral zone of wave-base marine abrasion. During the last glacial maximum about 20,000 yrs ago it was forming at its outer edge near the present shelf break, when sea-level was some 120 m below present. As post-glacial sea level rose, and the shoreline migrated landward across the shelf, the underlying substrate, formerly part of the exposed coastal plain, was progressively submerged, eroded and abandoned. This surface is commonly referred to as the post-last glacial marine transgressive ravinement surface. Its age is typically about 6500-7000 yrs old near the present coast. Since ~7300 yrs ago, absolute sea level has been relatively stable (Gibb, 1986), and any changes in the position of the late Holocene shoreline result from coastal erosion, coastal aggradation, tectonic uplift, or subsidence. The modern, highstand equivalent is commonly forming now as a shallow water platform near rocky coasts.

The post-glacial erosion surface (PGS) can be identified in high-resolution seismic profiles of the shelf in Cook Strait, including where it has been covered by substantial post-glacial sediments (Figs. 3 and 4). The overlying sediment cover has variable thickness and is developed in several connected depocentres. It is thickest (up to about 45 m) off the eastern Marlborough coast (Barnes and Audru, 1999b), and in Cloudy Bay (Fig. 5). It is relatively thin in Palliser Bay, and is essentially absent from the rocky, current-swept coast of Wellington.

Because of its pervasive occurrence in the region, characteristic seismic appearance, and well understood origin, the PGS is an extremely valuable marker for tectonic studies on the shelf. For our study it was critical that the diachronous age of its

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formation be determined, as it directly impacts on the analysis of fault displacement history, and estimation of vertical displacement rate at any one location. In this study the method of Lamarche et al. (2006) is used to estimate the diachronous age of the PGS in Cloudy Bay, by considering the depth of the present erosion surface relative to a calendar calibrated sea-level curve, and taking into account estimated rates of spatially varying tectonic uplift and/or subsidence derived from longer term deformation of older sequences and erosion surfaces (e.g., Fig. 3). The latter were considered within a sea-level cycle sequence stratigraphic framework, and correlated to marine oxygen isotope stages over the last 150 kyrs (Imbrie et al., 1984). In Cloudy Bay, we confirmed the younger part of our sea-level cycle stratigraphic interpretation by geometrically projecting the nearshore last glacial fluvial deposits, PGS, and overlying muddy highstand sequence to an 80 m deep borehole (P28/w1773) at the coast (Ota et al., 1995). The age of the post glacial sediments above the PGS can be estimated at any location on the shelf by simple interpolation of the average post- glacial sedimentation rate at that site. On the Marlborough and southern Wairarapa continental slope such sea-level cycle deposits are not easily recognised, and the youngest sediments in the vicinity of major thrust faults are not dated.

Because Cook Strait is characterised by extremely strong tidal flows and regular storms, there is locally substantial seafloor erosion, particularly in the Narrows region and along the south coast of Wellington (Carter, 1992; Gorman et al., 2003). Post glacial sediments are absent from any such region in which the current or wave regime is too vigorous to allow fine sediment to settle to the seafloor. In areas of widespread scour and active sediment transport, active fault traces can still be identified crossing the current-scour fabric in the high-resolution multibeam data. In some others areas, including Campbell Bank and southern Palliser Bay, the PGS has not been covered by mud, and relict lowstand and transgressive shoreface sand and gravel sediments have been reworked into large sediment waves. We infer these bedforms largely developed in a few tens of metres of water depth during the early sea-level transgression (~17-14 ka) (L. Carter, pers. comm. 2005). The dextral displacement of such bedforms at Campbell Bank has been used to estimate the slip rate on the Boo Boo Fault (Barnes, 2005b).

5. Active Submarine Faulting in Cook Strait

5.1 Focus of new mapping In this study our new mapping focussed on the weakest areas of previous knowledge, particularly between the south Wellington coast and Blenheim, and west of Porirua. We identified active traces of the Wairau, Awatere, Veron, Ohariu, and Wellington faults, and confirm that these North and South island faults are not directly connected at or near the seabed (Fig. 7) (Pondard et al., 2007, in prep). Numerous faults are interpreted along the Mana coast, in the northern Narrows Basin. In addition we made revisions to previous interpretations of the structure of thrust faults on the SE Wairarapa margin (Mountjoy et al., submitted), the Needles Fault, the offshore extensions of the Wairarapa Fault on Nicholson Bank, and the western end of the Boo Boo Fault.

Structures mapped from EM300 multibeam data (Fig. 2), with seismic profile support, were interpreted at a nominal scale of 1:25,000. Faults on the shelf in Cloudy Bay were mapped at 1:50,000, whereas mapping on the eastern Marlborough shelf south of

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the Boo Boo Fault was undertaken at a scale of 1:100,000 (Barnes and Audru, 1999a, b).

Because our aim is to characterize the faults capable of producing large, surface- rupturing earthquakes, we focussed on those faults with seabed traces evident in multibeam bathymetric data or high-resolution seismic sections, and/or displaying clear evidence of activity during the last ~20 kyrs (e.g., Figs. 3, 4 and 6). The faults shown on Fig. 7 are therefore considered part of the present day deformation, and have been used to underpin our interpretations of earthquake sources in section 6.

5.2 Results: Tectonic structures New mapping reveals submarine fault traces in Cook Strait are typically 10 to 90 km long. Most of the major faults in the central part of the strait are seaward extensions of faults on land, whereas several large structures, particularly in southern Cook Strait, are entirely submarine. In North and South islands the predominant structural trend is SW-NE to WSW-ENE. In central and southern Cook Strait the dominant structures are E-W striking, dextral strike-slip faults, with the SW-NE striking Needles and Campbell Bank faults being notable exceptions to this. Predominantly NE-SW striking faults occur along the western Wellington (Mana) coast, whereas NW-SE striking thrust faults characterise the deeper, southern Hikurangi Margin.

In the following subsections the tectonic structures in Cook Strait are described briefly from north to south, with special emphasis on the major new, unpublished findings of this study.

5.2.1 Mana – Narrows Basin A 10-20 km wide array of faults is interpreted along the eastern and northern margin of the Narrows Basin, from the southwestern Wellington coast to north of Mana Island (Fig. 7) (Chalaron et al., 2000). These include the southern parts of the Waiorua, Fisherman, Rangatira, Otaheke, and Wairaka faults, which are southern components of the Kapiti – Manawatu fault system (Fig. 7) (Lamarche et al., 2005; Nodder et al., 2007). They also include offshore extensions of the Terawhiti Fault, and possibly the Shepards Gully Fault. The faults are recognised in multibeam bathymetry data as seabed lineaments, and in seismic reflection profiles as discontinuities in reflections. Despite strong seafloor erosion of fault traces, it is clear the faults converge towards the southwestern tip of the North Island, and that there is an absence of active traces in the centre and on the western margin of the Narrows Basin.

Between 3 and 10 faults are interpreted across the coastal inner shelf. They appear to have lengths of typically 5-15 km, and up to 25 km, and there are numerous minor traces that may be secondary splays. Overall, the shallow structural style of these faults appears to be more characteristic of strike-slip faulting than the reverse faults that characterise the Kapiti – Manawatu fault system further north. Due to a relative scarcity of post glacial muddy sediment in this area, their slip rates are not well constrained. However, their rates are inferred to be relatively low, ranging from about 0.2-1.0 mm/yr.

5.2.2 Central Cook Strait In central Cook Strait, there are no through-going fault traces that connect North and South islands (Fig. 7). The major faults include the offshore extensions of the Wairau,

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Vernon, Awatere, Ohariu, Wellington, Wairarapa, and Wharekauhau faults, together with the newly identified Cloudy Fault (Pondard et al., 2007, in prep.). In Cloudy Bay the structures are interpreted to accommodate a combination of strike-slip and extension, although we have no actual observations of dextral displacements (Figs. 3 and 5). Such deformation is consistent with the Australian-Pacific oblique convergence in the area, derived from kinematic models based on GPS vectors (Beavan et al., 2002).

Wairau Fault The Wairau Fault extends offshore for about 40 km. The northern part of the fault trace curves E-W, has a clear seafloor scarp where its vertical (normal) component of deformation increases, and it breaks up into several splays at the southern end of the Narrows Basin. The fault approaches the offshore traces of the Ohariu Fault, but there is a right step-over of about 5-10 km width associated with extension and subsidence between the major traces. The fault geometry, extension and subsidence between the Wairau and Cloudy faults is characteristic of pull-apart basins associated with dilatational jogs and releasing bends in strike slip fault systems (Mann et al., 1983; Adyin and Nur, 1985; Christie-Blick and Biddle, 1985).

The slip rate of the fault onshore is considered to be ~3-5 mm/yr (e.g. Berryman et al., 1992; Knuepfer, 1992). Offshore, its strike-slip rate is not determined, but its vertical slip rate is about 0.8 mm/yr in the middle of Cloudy Bay (Fig. 3), increasing somewhat to the north-east. The fault has an exceptionally well imaged incremental growth history recorded in the post-glacial sediments in Cloudy Bay (Fig. 4c), and from this data a paleo-earthquake history for the last 12ka has been extracted (Pondard et al., 2007, in prep.). Six rapid growth events, inferred to be large earthquake ruputres, are recognised since 12 ka. The last two events have timing and coseismic slip in good agreement with the onshore data of Zachariasen et al. (2006). Additionally, 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 between individual events (ranging from 700-3000 yrs). Given that the fault is a strike-slip structure, vertical slip per event in the middle of Cloudy Bay, of the order of 1-2 m constrains only the minimum net slip per event. Considering that the horizontal to vertical ratio of long term displacement is about 5 (i.e., ~4/0.8), these coseismic vertical displacements could be associated with dextral displacements of the order of 5-10 m. This is consistent with a coseismic displacement of about 6 m observed on the fault onshore (Zachariasen et al. (2006).

Awatere, Vernon, and Cloudy faults The Vernon Fault branches off the Awatere Fault about 15 km in land of the coast, southeast of Blenheim, and based on elevated topography between the faults it appears to have a reverse component. Both the Vernon and Awatere faults extend offshore between Cloudy and Clifford bays, where they converge in association with a bathymetric reef. The Vernon Fault extends offshore for 25 km, bringing the total fault length to 40 km, and it terminates on the outer shelf (Fig. 7). The fault is steeply dipping to the south (Fig. 5), but near the seabed is downthrown to the north (Fig. 3). The fault trace swings E-W and is interpreted to become increasingly strike-slip towards the northeast (Pondard et al., 2007, in prep.). Its strike-slip rate is

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 9

unconstrained but is inferred to be of the order of 4-5 mm/yr based on an assumed offshore partitioning of the 6 mm/yr rate of the Awatere Fault on land (Mason et al., 2006).

Like the Wairau Fault, the Vernon Fault has an exceptionally well imaged incremental growth (inferred earthquake) history recorded in the post-glacial sediments. Four to five paleo-earthquakes are recognised on the Vernon Fault since 18 ka. These have variable inter-event times, ranging from 2000 to 6000 yrs, and variable vertical slip per event (1.0-2.5 m). The average recurrence interval over 18 ka is about 4000 yrs.

The newly mapped Cloudy Fault is extensional, about 23 km in length, and joins the Vernon Fault in map view (Fig. 7) and probably in cross section (Fig. 5). The fault has a strongly curved trace, and a vertical slip rate of up to 1.5 mm/yr. The northern end of the Cloudy Fault approaches the southern end the Wellington Fault at a high angle in central Cook Strait, but the tips are separated by 7 km and profiles between them reveal no active deformation. The displacement history of the Cloudy Fault reveals five paleo-earthquakes since 17 ka, with slip per event of 1.0-4.0 m (mean ~3 m), and an average recurrence interval of ~3500 yrs with individual inter-event times ranging from 1500 to 4000 yrs. Because the fault is largely extensional the displacements per event will be closer to the net slip.

Wellington, Ohariu, and Shepards Gully faults The Ohariu Fault extends offshore for up to 20 km, as a series of discontinuous traces in multibeam bathymetry data. These traces swing NE-SW, and terminate between the northeastern end of the Wairau and Cloudy faults (Fig. 7). The Ohariu Fault has an onshore slip rate of about 1.5 mm/yr (Heron et al., 1998), but the rate is unconstrained offshore due to an absence of post-glacial sediments in this current scoured region.

The Wellington-Hutt Valley segment of the Wellington Fault has also been traced offshore for about 20 km, and curves gently to the southwest, parallel to the Ohariu Fault. Whilst the slip rate on land is of the order of 6-7 mm/yr, its submarine slip rate remains undetermined. The Shepards Gully and Terawhiti faults are recognised offshore as reef escarpments only within 1-2 km of shore.

Wairarapa, Nicholson Bank, and Wharekauhau faults The southern part of the Wairarapa Fault extends into mountainous terrain north of Turakirae Head, and active traces of it that appear to have ruptured in 1855 are recognised in close vicinity to the Wharekauhau Thrust, which extends offshore into Palliser Bay (Schermer et al., in prep.). The 1855 rupture has been inferred to have extended into the middle of Cook Strait (e.g. Beavan and Darby, 2005). Offshore, south of Turakirae Head, two fault traces are identified on the eastern part of Nicholson Bank. These faults are aligned approximately with Turakirae Head, and referred to here as Nicholson Bank West Fault and Nicholson Bank East Fault, respectively. The former fault is newly interpreted here. The latter was documented by Barnes (2005a), and appears to align with the northern trace of the Needles Fault northeast of Cape Campbell. The precise structural relationships between the Wairarapa, Nicholson Bank and Needles faults, however, are uncertain due to a lack of seismic data coverage in that crucial area. It is entirely possible that the Nicholson Bank faults continue westward beneath the northern shelf off Clifford Bay, between

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 10

the Needles and Vernon faults (Fig. 7). There are no offshore constraints on the slip rates of the Nicholson Bank faults. Based on the slip rate of the southern Wairarapa Fault onshore (Little and Rodgers, 2005; Rodgers and Little, 2007), and the inferred rate on the Needles Fault, we infer the cumulative rate of slip of the Nicholson Bank West Fault and Nicholson Bank East Fault could be of the order of 11 mm/yr, with at least 3-4 mm/yr uncertainty.

The offshore extension of the Wharekauhau Thrust crosses the eastern end of Nicholson Bank, implying a total onshore – offshore length of about 50 km (Barnes and Audru, 1999b; Barnes, 2005a; Mountjoy et al., submitted). The fault is expressed as a scarp in multibeam bathymetric data, and is a major structure in seismic reflection profiles (Barnes and Audru, 1999b). The onshore part of the Wharekauhau Thrust appears to have been inactive during the Holocene, as its tip is overlain by an un- faulted Holocene unconformity, however, steeper-dipping strike-slip faults in close proximity to it are active, as are possible contractional structures in its footwall east of the fault (Schermer et al., in prep.). Considering active uplift at the coast near Turakirae Head (Begg and McSaveney, 2005), we conservatively infer a slip rate on this zone of active faulting to be of the order of 2.5 mm/yr.

5.2.3 Eastern Marlborough – Southern Cook Strait In contrast to the Wairau Basin region, faults in eastern Marlborough and southern Cook Strait accommodate a combination of strike-slip and compressional deformation (Barnes and Audru, 1999b). This is consistent with their geometry with respect to the Australian-Pacific oblique convergence in the area (Beavan et al., 2002). Two major strike slip faults, namely the Hope and Kekerengu faults, extend off the eastern Marlborough coast. Offshore the major faults in this area include the Needles, Boo Boo, Chancet, Campbell Bank, Te Rapa and Hope faults (Fig. 7).

Kekerengu, Needles, Chancet and Campbell Bank faults The Kekerengu Fault extends offshore in the vicinity of the southwestern traces of both the Needles and Chancet faults, which converge towards the coast (Barnes and Audru, 1999a, b) (Fig. 7). The slip rate on the Kekerengu Fault (c. 19 mm/yr, Van Dissen et al., 2005) is interpreted to be partitioned onto these offshore structures, which are about 50 km and 30 km long respectively. The Needles Fault extends up the coast towards the western end of the Boo Boo Fault and southern ends of the Nicholson Bank faults. Its structure has been mapped with both seismic sections and EM300 multibeam data (see Fig. 2). The Chancet Fault strikes more easterly, and connects with the southern end of the Campbell Bank Fault (Barnes and Audru, 1999a, b).

Whilst there is clear structural and multibeam bathymetric evidence for strike slip deformation on the Needles and Chancet faults, their dextral slip rates are not constrained. The larger component (~75%) of the Kekerengu Fault slip rate was inferred by Stirling et al. (2007, in press) to be transferred onto the Chancet Fault, with only about 4 mm/yr carried on the Needles Fault. However, considering the slip rate determined on the Boo Boo Fault, and inferred at Nicholson Bank, it is inferred here that the slip rates on the Needles and Chancet faults are of the order 16 and 3 mm/yr, respectively. These estimates have large uncertainties, potentially of the order of (+2, - 8) for the Needles Fault, and (+8, -2) for the Chancet Fault.

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Boo Boo Fault The largest fault beneath the southern shelf is the dextral strike-slip Boo Boo Fault, at about 90 km length. This fault extends from east of Cape Campbell into the southern Hikurangi Margin east of Cape Palliser (Fig. 7). The eastern end of the fault projects into the hanging wall of the Opouawe–Uruti Fault. The Boo Boo Fault has two significant releasing bends on Campbell Bank (e.g., Fig. 6), clear dextral displacements along its length, and is estimated from the displacement of Campbell Bank and Cook Strait Canyon to have total displacement of about 3 km (Barnes, 2005b). Whilst dextral slip rates are inferred on the other submarine faults in the region, the Boo Boo Fault is the only one that a dextral slip rate can be constrained from offshore data. The rate is estimated to be about 7 mm/yr at Campbell Bank, where gravel/sand waves inferred to be 14-17 ka are displaced (Fig. 6), possibly increasing to ~10-11 mm/yr in the east near Cape Palliser, where ridge and gullies on the upper slope are displaced (Barnes, 2005b). Based on the total displacement and late Quaternary slip rate, the fault is estimated to be not more than 300-500 kyrs old.

Hope and Te Rapa faults The seaward segment of Hope Fault projects offshore south of the Clarence River, and extends for about 50 km along a complex structural high beneath the outer shelf (Fig. 7) (Barnes and Audru, 1999a, b). The northern end of this high merges with the southern end of the reverse, probably oblique slip, Te Rapa Fault. The slip rate of the seaward section of the Hope Fault, both onshore and offshore, is unconstrained. However, based on the majority of Hope Fault slip transferred to the Kekerengu Fault via the Jordan Thrust (Van Dissen and Yeats, 1991; Van Dissen et al., 2005), the dextral rate on the offshore Hope Fault is inferred to be of the order of 3-5 mm/yr.

5.2.4 Southern Hikurangi Margin The continental slope northeast and southwest of Cook Strait Canyon is dominated by discontinuous, mainly westward-dipping thrust faults that strike about 065° (Fig. 7) (Barnes and Mercier de Lépinay, 1997; Barnes et al., 1998; Mountjoy et al., submitted). Two major thrust faults dominate the Marlborough slope. These include the Upper Slope Fault Zone and Kekerengu Bank thrust, which have slip rates estimated to be of the order of 1.0-1.5 mm/yr (Stirling et al., 2007). North of Cook Strait Canyon, the westward-dipping Palliser–Kaiwhata Fault extends along the southern Wairarapa shelf less than 10 km from shore, and has a prominent seabed scarp up to 65 m high. The southern end of this fault converges with the Boo Boo Fault, and is inferred to be an oblique-slip dextral reverse fault (see Electronic Data Supplement in Barnes et al., 1998). Its net slip rate is not constrained, but considering a required balancing of plate motion across the southern North Island, the rate is inferred to be of the order of 5 mm/yr (Fig. 7).

On the slope to the southeast, thrust faults have formed well developed elongate ridge and basin relief associated with active anticlinal folds. These faults are well expressed in the EM300 multibeam bathymetry data, both as ridges and as discontinuous surface traces where the imbricate thrusts break out on the seaward fore-limbs of folds. The two principal mid-slope thrust faults are the Opouawe–Uruti and Pahaua faults, which are westward dipping thrusts rooted in the foundation rocks of the inner margin. These faults have a clear growth sequence on their hanging wall, but displacement rates have not been determined. Based on knowledge of similar tectonic geomorphology

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 12

elsewhere along the Hikurangi Margin (e.g., Barnes et al., 2002), it is inferred the faults have slip rates of at least 1-2 mm/yr.

6. Earthquake Sources in Cook Strait Given the absence of creep identified on New Zealand upper crustal faults, and considering that all faults mapped on Fig. 7 are identified at or near the surface or seabed, it is highly likely that they developed during large-magnitude, ground rupturing earthquakes (Stirling et al., 2002, 2007). On Fig. 8 and in Table 1, we have interpreted potential earthquake sources in the upper crust, above the Hikurangi subduction megathrust (not shown). Minor faults in the vicinity of the major traces are inferred to be secondary ground ruptures associated with coseismic displacements on the major faults. Whilst these interpretations are not the only potential earthquake scenarios, we consider them likely and indicative of the earthquake hazard.

Many of the earthquake sources involve onshore-to-offshore ruptures, others are entirely submarine. We took a conservative approach in defining just three earthquake sources beneath the interpreted fault array off the Mana coast west of Wellington. These include the Fishermans, Okupe, and Mana–Otaheke sources. We find no evidence to segment the Wairau Fault, and consider this a single earthquake source beneath the Wairau valley and Cloudy Bay. The Cloudy and Vernon faults may be individual earthquake sources, but it is also feasible that, at least on occasions of large observed coseismic slip on the Cloudy Fault (~4 m), that these faults have ruptured together. It is also possible that the Vernon Fault could rupture with the Awatere Fault. Modelling of the 1855 Wairarapa Earthquake indicates that, at least on occasions, earthquakes on the Wairarapa Fault extend offshore across Nicholson Bank (Beavan and Darby, 2005).

Earthquakes on the Kekerengu Fault may also rupture the offshore Chancet Fault (± Campbell Bank Fault) (Stirling et al., 2007, in press). Considering the fault geometry and inferred slip transfer, it is also feasible to interpret that a rupture on the Kekerengu Fault may continue onto the Needles Fault. Following Stirling et al. (2007, in press) a number of alternative scenarios are considered in Table 1, which cumulatively account for the inferred total slip rates on the faults. An earthquake on the Seaward segment of the Hope Fault is thought likely to extend offshore beneath the outer shelf structural high off the Clarence River mouth for at least 50 km (Fig. 7), and possibly involve the Te Rapa Fault.

Following the method of Stirling et al. (2002, 2007), we derive estimates for the moment magnitude (Mw), formula-derived single event (coseismic) displacement (SED), and formula-derived average recurrence interval (Recint) for the inferred earthquake sources (Table 1; the formulae employed to estimate some of the above parameters are listed in the notes of Table 1). These estimates derived from empirical relationships are considered only as indicative of the earthquake potential of the faults, particularly for structures in which there are no records of actual paleo-earthquakes. 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. We note that the formula-derived, preferred recurrence intervals on the Vernon and Cloudy faults in Table 1 are shorter than the average recurrence intervals determined from our marine observations (see section 5.2.2), indicating that these faults may rupture together, and potentially with the Awatere Fault. We anticipate some further

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 13

refinement of these source scenarios before their inclusion into revisions of the National Seismic Hazard Model.

7. Conclusions A new interpretation of active submarine faulting based on seismic reflection and multibeam bathymetric data in Cook Strait indicates there is a general discontinuity between the North Island Dextral Fault Belt and the Marlborough Fault System. Furthermore, whereas the predominant structural trend is SW-NE to WSW-ENE in the South and North islands, faults in central and southern Cook Strait are predominantly E-W striking, dextral strike-slip faults, with the SW-NE striking Needles and Campbell Bank faults being notable exceptions to this. The deformation associated with these faults is consistent with predicted Australian–Pacific plate motion vectors. Off the western Wellington (Mana) coast, predominantly NE-SW striking faults are continuous with the southern components of the Kapiti – Manawatu Fault System, and may be reverse dextral structures. The continental slope of southern Wairarapa and Marlborough is dominated by NW-SE striking thrust faults.

Whilst most of the major faults in the central part of the strait are seaward extensions of on land faults, several large structures, particularly in southern Cook Strait, are entirely submarine. There are no through-going fault traces connecting North and South island faults. 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. The submarine fault traces are typically 10 to 90 km long.

Right step-overs between major faults in between Wellington and Blenheim are associated with long-term subsidence in Cook Strait. The structure indicates the faults accommodate a combination of strike-slip and extension, which is consistent with the Australian–Pacific plate motions derived from GPS velocities. In contrast, faults in southern Cook Strait accommodate a combination of strike-slip and compression, and local areas of current uplift are evident.

Identification of the post-glacial transgressive erosion surface (~20-7 ka) and post glacial sediment cover on the shelf of Cook Strait, enable us to determine the rate of vertical separation on active faults from seismic profiles. In general these represent minimum net slip rates, given the dominant structural style in the region is strike-slip. The slip rates quoted in this report are best estimates of net slip rate 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 expected slip distribution within the plate boundary zone.

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 to 3000 yrs). Considering that the horizontal to vertical ratio of long term displacement is about 5 (i.e., ~4/0.8), these coseismic vertical displacements

Its Our Fault : Active Faults and Earthquake Sources in Cook Strait 14

could be associated with dextral displacements of the order of 5-10 m. This is consistent with a coseismic displacement of about 6 m on the fault onshore reported previously. 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 extensional 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 onshore to offshore faults, and some entirely marine structures. The results, derived from application of formulae, 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.

8. Acknowledgments This study was partly funded by Earthquake Commission (EQC), Accident Compensation Corporation (ACC), and Wellington Regional Council (WRC), as part of the It’s Our Fault programme managed by GNS Science. It was also partly funded by the Foundation for Research Science and Technology (FRST). Most of the data and much of the marine science knowledge drawn on in this study have been acquired by NIWA, as part of research contracts funded by FRST. These include, in particular, contracts CO1X0203, and a current contract CO1X0702. These contracts funded data acquisition, operational costs and also scientific labour (PB, GL and NIWA technical staff). In addition, NP and JM were funded during the two years of this study by NIWA Capability Fund post-doctoral and research contracts, and JM was partly funded by a Tertiary Education Top Achiever Award. We are indebted to John Mitchell for his leadership of R.V. Tangaroa multibeam surveys in Cook Strait, and for assistance in particular from Richard Garlick, Miles Dunkin, and Steve Wilcox. We thank Discovery Geo Corporation for allowing us access to private seismic reflection data, and Stuart Henrys for providing access to the OGS Explora line acquired by GNS Science.

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10. Figure Captions

Figure 1. Tectonic framework: oblique convergence in the New Zealand region. The vectors represent the NUVEL-1 velocity (mm/yr) referenced to a fixed Australian plate (DeMets et al., 1994). AF, Alpine Fault; MFS, Marlborough Fault System; NIDFB, North Island Dextral Fault Belt. The yellow box locates Cook Strait and is enlarged in Fig. 2.

Figure 2. Seismic reflection data used in this study of active faulting in Cook Strait. Coloured (pale yellow) area is the extent of SIMRAD EM300 multibeam bathymetric data. Background bathymetry is 50 m contour interval on the slope and 10 m on the shelf. The coloured lines represent different survey data sets and different types of seismic data. Noteably, the three black lines are high quality multichannel seismic sections provided by Discovery Geo Corporation.

Figure 3. High-resolution boomer seismic reflection profile (penetration ~150 m, resolution 30 cm) almost perpendicular to the major faults across the Wairau Basin. Vertical exaggeration is 48×. The profile position is shown on Fig. 7. Upper (a) and lower (b) panels present un-interpreted and interpreted processed data, respectively. Upper Pleistocene (red, yellow and blue) and Holocene (green) sedimentary units are identified. PGS, diachronous post glacial transgressive surface. Age and vertical displacements of the PGS are indicated. Vertical slip-rates associated with the major faults across the Wairau Basin are also shown.

Figure 4. High-resolution boomer seismic reflection profile showing the post-glacial (mainly Holocene) sequence across the Wairau Fault. The profile position is shown on Fig. 7. (a) Un-interpreted processed data with vertical exaggeration 31X. (b) Interpreted section. Lines represent post-glacial sedimentary layers used as markers to document the paleo-earthquake activity of the fault (Pondard et al. 2007, in prep). Coloured sedimentary units with thin black lines include layers associated with a similar vertical displacement across the fault (interseismic). Thick black lines bracket intervals with rapid displacement events (probable earthquake ruptures). Corresponding stratigraphic ages (yrs) are indicated. (c) Age – Displacement plot revealing timing of probable coseismic displacement increments (rapid vertical steps

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in the curve) and main interseismic periods (flat or gently sloping sections of the curve).

Figure 5. Mutlichannel seismic section (Discovery Geo corporation) across the Wairau Basin. The profile position is shown on Fig. 7. (a) The upper panel presents the un-interpreted migrated data. For comparison, the red box indicates the penetration of Boomer profiles presented in Figs. 3 and 4. (b) The lower panel shows the interpreted fault structure. Bold curves indicate the main active faults across the basin. Thinner curves show secondary active faults and buried inactive faults. Twt, two way travel time. Vertical exaggeration is about 2×.

Figure 6. EM300 multibeam (10 m grid) data over Campbell Bank (Fig. 7), illustrating the structure and morphology associated with a releasing bend on the Boo Boo Fault, and the dextral displacement of large gravel/sand waves inferred to be of the order of 14-17 kyrs old.

Figure 7. Active submarine faults in Cook Strait derived from multibeam bathymetry, high-resolution and multichannel seismic reflection data. Red numbers next to major faults are inferred best estimate slip rates in mm/yr (see text for explanation). Yellow lines and box indicate location of Figs 3, 5, and 6. Fine red line indicates Fig. 4. The red vectors represent the plate motion velocity referenced to a fixed Australia, based on GPS data (Beavan et al., 2002). The curved white dashed line is the general extent of seabed scour by strong currents.

Figure 8 . Likely earthquake sources interpreted from fault segmentation and other data in the Cook Strait region. Sources are summarised in Table 1.

Table 1. Summary of likely earthquake sources in the Cook Strait region. Refer to Stirling et al. (2002, 2007) for details of methodology and formulae.

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Type Fault Type Index Length (km) DipDip dir Depth Sr Width Area Mw Sed RecInt Min. Pref. Max. Min. Pref. Max. Min. Pref. Max. Min. Pref. Max. Min. Pref. Max. Min. Pref. Max. Min. Pref. Max. Min. Pref. Max. Min. Pref. Max. Boo Boo ss 3 81 90 99 90 90 90 13 15 178.0010.0012.00 13 15 17 1053 1350 1683 7.5 7.6 7.7 4.1 5.5 6.9 338 545 862 Ohariu Sth ss 3 47.7 53 58.3 65 75 85NW 13 15 17 1.00 1.50 2.00 13 16 19 622 823 1094 7.2 7.3 7.4 2.4 3.2 4.1 1196 2141 4060 Shepards Gully ss 3 45.9 51 56.1 90 90 90 13 15 17 0.30 0.50 0.70 13 15 17 597 765 954 7.1 7.2 7.3 2.3 3.1 3.9 3288 6180 13023 WellingtonHV ss 3 66.6 74 81.4 70 80 90SE 13 15 17 6.00 6.60 7.60 13 15 18 866 1127 1473 7.4 7.5 7.6 3.3 4.5 5.7 439 679 945 Wairarapa–Nicholson ss 3 125.1 139 152.9 60 70 80NW 13 15 17 8.00 11.00 14.00 13 16 20 1651 2219 3001 7.7 7.8 8.0 6.3 8.4 10.6 448 766 1331 Wairau ss 3131.4 146 160.6 70 75 80SE 13 15 173.00 4.00 5.00 13 16 18 1735 2267 2905 7.8 7.9 8.0 6.6 8.8 11.2 1318 2212 3728 Awatere ss 3 90.9 101 111.1 65 75 85NW 13 15 174.00 6.00 8.00 13 16 19 1186 1568 2084 7.5 7.6 7.8 4.6 6.1 7.7 570 1020 1934 Vernon ss 3 36.9 41 45.1 60 70 80SE 13 15 173.00 4.50 6.00 13 16 20 487 654 885 7.0 7.1 7.2 1.9 2.5 3.1 308 552 1047 Needles ss 3 43.2 48 52.8 70 80 90NW 13 15 170.50 1.00 1.50 13 15 18 562 731 955 7.1 7.2 7.3 2.2 2.9 3.7 1444 2908 7354 Kekerengu-Needles ss 3 75.6 84 92.4 70 80 90NW 13 15 17 1.00 2.00 3.00 13 15 18 983 1279 1672 7.4 7.5 7.6 3.8 5.1 6.4 1264 2545 6435 Jordan-Kekerengu-Needles ss 3 99.9 111 122.1 50 60 70NW 13 15 1711.00 13.00 15.00 14 17 22 1382 1923 2710 7.6 7.7 7.9 5.0 6.7 8.5 334 517 773 Jordan-KekerenguChancet ss 3 78.3 87 95.7 50 60 70NW 13 15 17 1.00 2.00 3.00 14 17 22 1083 1507 2124 7.5 7.6 7.7 3.9 5.3 6.7 1309 2636 6665 Kekerengu-Chancet ss 3 54 60 66 90 90 90 13 15 17 0.01 0.50 1.00 13 15 17 702 900 1122 7.2 7.3 7.4 2.7 3.6 4.6 2708 7271 459645 Kekerengu-Campbell ss 3 80.1 89 97.9 90 90 90 13 15 17 0.01 0.50 3.00 13 15 17 1041 1335 1664 7.5 7.6 7.7 4.0 5.4 6.8 1339 10785 681807 Chancet-Campbell Bank ss 3 47.7 53 58.3 70 80 90SE 13 15 17 0.01 0.05 0.50 13 15 18 620 807 1055 7.2 7.3 7.4 2.4 3.2 4.1 4784 64225 406020 SeawardHope-TeRapa ss 3 100.8 112 123.2 65 75 85NW 13 15 17 3.00 4.00 5.00 13 16 19 1315 1739 2311 7.6 7.7 7.8 5.1 6.8 8.6 1011 1697 2860 Cloudy ns 3 22.5 25 27.5 50 60 70SE 13 15 171.00 1.50 2.00 14 17 22 311 433 610 6.7 6.9 7.0 1.1 1.5 1.9 564 1010 1915 Honeycomb rv 3 32.4 36 39.6 30 40 50NW 13 15 170.50 1.00 1.50 17 23 34 550 840 1346 7.0 7.2 7.3 1.6 2.2 2.8 1083 2181 5516 Opouawe - Uruti rv 3128.7 143 157.3 30 40 50NW 8 10 12 0.50 1.50 2.50 10 16 24 1344 2225 3775 7.7 7.8 8.0 6.5 8.7 11.0 2581 5776 21910 Pahaua rv 3131.4 146 160.6 30 40 50NW 8 10 120.50 1.50 2.50 10 16 24 1372 2271 3854 7.7 7.9 8.0 6.6 8.8 11.2 2636 5897 22369 PalliserKaiw rs 3 58.5 65 71.5 30 40 50NW 13 15 17 3.00 5.00 7.00 17 23 34 993 1517 2431 7.4 7.5 7.7 2.9 3.9 5.0 419 788 1660 Wharekauhau rv 3 46.8 52 57.2 35 45 55NW 13 15 17 2.00 2.50 3.00 16 21 30 743 1103 1695 7.2 7.4 7.5 2.3 3.2 4.0 782 1260 1992 Ngapotiki rv 3 19.8 22 24.2 35 45 55NW 13 15 171.30 2.00 2.70 16 21 30 314 467 717 6.7 6.9 7.0 1.0 1.3 1.7 368 666 1296 Otaraia rv 3 31.5 35 38.5 50 60 70NW 13 15 170.10 0.20 0.30 14 17 22 436 606 854 6.9 7.1 7.2 1.6 2.1 2.7 5265 10603 26813 Dry River rv 3 56.7 63 69.3 50 65 80NW 13 15 170.40 0.70 1.00 13 17 22 748 1043 1538 7.3 7.4 7.5 2.8 3.8 4.8 2843 5453 12066 Fisherman rv 3 69.3 77 84.7 65 75 85NW 13 15 170.50 1.00 1.50 13 16 19 904 1196 1589 7.4 7.5 7.6 3.5 4.7 5.9 2317 4665 11798 Okupe rv 3 56.7 63 69.3 65 75 85NW 13 15 170.40 0.80 1.20 13 16 19 740 978 1300 7.3 7.4 7.5 2.8 3.8 4.8 2369 4771 12066 Mana-Otaheke rv 3 81.9 91 100.1 55 65 75NW 13 15 17 0.10 0.25 0.35 13 17 21 1102 1506 2077 7.5 7.6 7.7 4.1 5.5 7.0 11733 22055 69713 Wharanui-Campbell rv 3 39.6 44 48.4 55 65 75SE 13 15 17 0.15 2.95 3.00 13 17 21 533 728 1004 7.1 7.2 7.3 2.0 2.7 3.4 662 904 22472 Kekerengu Bank rv 3 79.2 88 96.8 35 45 55NW 13 15 17 0.5 1.0 1.5 16 21 30 1257 1867 2869 7.5 7.7 7.8 4.0 5.3 6.7 2648 5332 13483 Upper slope rv 3 37.8 42 46.2 45 55 65NW 13 15 17 0.5 1.0 1.5 14 18 24 542 769 1111 7.1 7.2 7.3 1.9 2.5 3.2 1264 2545 6435

TABLE 1. Earthquake Sources Notes: Type: Predominant sense of slip: ss, strike-slip; n, normal; rv, reverse Type index: Empirical relationships used to determine Moment magnitude (Stirling et al., 2007): 3, M w = 4.18 + 2/3logWidth + 4/3logLength. Length uncertainties ± 10% of pref. Depths. All assigned nominal depths of 15 km , except southern Hikurangi margin thrust. Sed, single event displacement (coseismic) Recint, average recurrence interval

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