Arrival Time Estimates for Local Source Tsunami for Wellington Suburbs

Xiaoming Wang Christof Mueller William Power Biljana Lukovic

GNS Science Report 2016/03 February 2017

DISCLAIMER

The Institute of Geological and Nuclear Sciences Limited (GNS Science) and its funders give no warranties of any kind concerning the accuracy, completeness, timeliness or fitness for purpose of the contents of this report. GNS Science accepts no

responsibility for any actions taken based on, or reliance placed on the contents of this report and GNS Science and its funders exclude to the full extent permitted by law liability for any loss, damage or expense, direct or indirect, and however caused, whether through negligence or otherwise, resulting from any person’s or organisation’s use of, or reliance on, the contents of this report.

BIBLIOGRAPHIC REFERENCE

Wang, X.; Mueller, C.; Power, W.L.; Lukovic, B. 2016. Arrival time estimates for local source tsunami for Wellington suburbs, GNS Science Report 2016/03. 53 p.

X. Wang, GNS Science, 1 Fairway Drive, Avalon, Lower Hutt, New Zealand C. Mueller, GNS Science, 1 Fairway Drive, Avalon, Lower Hutt, New Zealand W. L. Power, GNS Science, 1 Fairway Drive, Avalon, Lower Hutt, New Zealand B. Lukovic, GNS Science, 1 Fairway Drive, Avalon, Lower Hutt, New Zealand

© Institute of Geological and Nuclear Sciences Limited, 2017 www.gns.cri.nz

ISSN 1177-2425 (Print) ISSN 2350-3424 (Online) ISBN 978-0-908349-79-1 (Print) ISBN 978-0-908349-80-7 (Online)

CONTENTS ABSTRACT ...... IV KEYWORDS ...... IV 1.0 INTRODUCTION ...... 1 2.0 TSUNAMI SOURCE MODELLING ...... 3

2.1 LOCAL SOURCES ...... 3 2.1.1 Hikurangi Subduction Interface ...... 3 2.1.2 Wellington Fault ...... 4 2.1.3 Wairarapa Fault and Wharekauhau Thrust ...... 4 2.2 TSUNAMI SOURCE MODELLING ...... 5 2.2.1 Source Scenarios ...... 5 2.2.2 Fault Geometry, Non-uniform Slip and Slip Rate Deficit Weighting ...... 7 3.0 TSUNAMI MODELLING ...... 11

3.1 TSUNAMI MODEL ...... 11 3.2 MODELLING GRIDS ...... 11 3.3 SEAFLOOR AND GROUND DISPLACEMENT CALCULATION ...... 14 3.4 MODELLED DURATION AND ROUGHNESS CONSIDERATION ...... 15 3.5 AUTOMATION OF TSUNAMI SIMULATIONS WITH TSUNAMI-API ...... 16 4.0 TSUNAMI ARRIVAL TIME ESTIMATE ...... 17

4.1 TIME HISTORY DATA MODELLING ...... 17 4.2 ARRIVAL TIME IDENTIFICATION ...... 19 4.3 ARRIVAL TIME ESTIMATES AND DISCUSSIONS ...... 21 5.0 LIMITATIONS ...... 31 6.0 CONCLUSIONS ...... 33 7.0 ACKNOWLEDGEMENTS ...... 34 8.0 REFERENCES ...... 35

FIGURES

Figure 1.1 Coastal suburbs of the Wellington Harbour region...... 2 Figure 2.1 Major local active faults and other structures in central New Zealand...... 3 Figure 2.2 Overview of the geometry of the tsunami sources...... 7 Figure 2.3 Subduction interface slip rate deficit for the Hikurangi subduction interface (credit: Wallace et al., 2012)...... 8 Figure 2.4 Comparison of non-uniform slip distribution ...... 9 Figure 3.1 Nested grid setup for tsunami generation and propagation modelling...... 12 Figure 3.2 Nested grid setup for tsunami generation and propagation modelling...... 12 Figure 3.3 Nested grid setup for tsunami propagation modelling...... 13

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Figure 3.4 Nested grid setup for tsunami propagation and inundation modelling in Wellington region...... 14 Figure 3.5 Examples of computed vertical seafloor/ground displacements resulting from scenario ruptures ...... 15 Figure 4.1 Virtual tsunami gauges (black circles with red dots inside) for tsunami time history calculations, overlaid with the high-resolution DEM in Wellington Harbour region...... 17 Figure 4.2 Virtual tsunami gauges (red dots and red crosses with black dots at center), overlaid with Wellington suburb boundary map...... 18 Figure 4.3 Example of tsunami arrival time record (only the first 10 hours shown here) at the coastal front of Hataitai ...... 21 Figure 4.4 Distribution of first peak arrival times at the coastal front of Hataitai for all the Hikurangi subduction interface scenarios...... 22 Figure 4.5 An illustration of time constraints for tsunami evacuation...... 28

TABLES

Table 2.1 Tsunami observations in the 1855 Mw ~8.2 Wairarapa earthquake...... 4 Table 2.2 Local seismic source scenarios for tsunami arrival time calculation...... 6 Table 4.1 Virtual tsunami gauges for individual suburbs (see Figure 4.2)...... 19 Table 4.2 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Hikurangi subduction interface scenarios (the start times and arrival times are measured in minutes after mainshock)...... 23 Table 4.3 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Wellington Fault scenarios ...... 24 Table 4.4 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios (the start times and arrival times are measured in minutes after mainshock)...... 25 Table 4.5 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Wharekauhau Fault scenarios ...... 26 Table 4.6 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs considering all the four local fault sources ...... 27

APPENDICES A1.0 SLIP DISTRIBUTIONS IN HIKURANGI SOURCE SCENARIOS ...... 39 A2.0 ADDITIONAL TSUNAMI ARRIVAL TIMES ...... 50

A2.1 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM HIKURANGI SUBDUCTION INTERFACE ...... 51 A2.2 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM WELLINGTON FAULT...... 53 A2.3 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM WAIRARAPA FAULT ...... 55 A2.4 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM WHAREKAUHAU FAULT...... 57

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APPENDIX FIGURES

Figure A1.1 Source scenario 1 in Hikurangi subduction interface...... 39 Figure A1.2 Source scenario 2 in Hikurangi subduction interface...... 39 Figure A1.3 Source scenario 3 in Hikurangi subduction interface ...... 40 Figure A1.4 Source scenario 4 in Hikurangi subduction interface...... 40 Figure A1.5 Source scenario 5 in Hikurangi subduction interface...... 41 Figure A1.6 Source scenario 6 in Hikurangi subduction interface...... 41 Figure A1.7 Source scenario 7 in Hikurangi subduction interface...... 42 Figure A1.8 Source scenario 8 in Hikurangi subduction interface...... 43 Figure A1.9 Source scenario 9 in Hikurangi subduction interface...... 44 Figure A1.10 Source scenario 10 in Hikurangi subduction interface...... 44 Figure A1.11 Source scenario 11 in Hikurangi subduction interface...... 45 Figure A1.12 Source scenario 12 in Hikurangi subduction interface...... 45 Figure A1.13 Source scenario 13 in Hikurangi subduction interface...... 46 Figure A1.14 Source scenario 14 in Hikurangi subduction interface...... 46 Figure A1.15 Source scenario 15 in Hikurangi subduction interface...... 47 Figure A1.16 Source scenario 16 in Hikurangi subduction interface...... 47 Figure A1.17 Source scenario 17 in Hikurangi subduction interface...... 48 Figure A1.18 Source scenario 18 in Hikurangi subduction interface...... 48 Figure A1.19 Source scenario 19 in Hikurangi subduction interface...... 49 Figure A1.20 Source scenario 20 in Hikurangi subduction interface...... 49

APPENDIX TABLES

Table A2.1 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Hikurangi subduction interface scenarios...... 51 Table A2.2 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Hikurangi subduction interface scenarios ...... 52 Table A2.3 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Wellington Fault scenarios ...... 53 Table A2.4 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Wellington Fault scenarios ...... 54 Table A2.5 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios ...... 55 Table A2.6 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios ...... 56 Table A2.7 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Wharekauhau Fault scenarios ...... 57 Table A2.8 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios ...... 58

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ABSTRACT

Arrival times of tsunami from local active fault sources have been estimated for coastal suburbs of Wellington harbour region, including the start time of noticeable disturbance, the start and arrival times of the first peak, and the start and arrival times of the biggest peak. The local source scenarios that have been modelled include 20 scenarios on the Hikurangi subduction interface, 9 scenarios on the Wellington Fault, 9 scenarios on the Wairarapa Fault and 9 scenarios on the Wharekauhau Fault. The maximum potential magnitude on each fault, obtained from past studies, has been used to construct source scenarios with various slip distributions in order to account for the effect of slip heterogeneity on tsunami arrivals. This leads to a range of possible tsunami arrivals for a suburb. The earliest, 25th, 50th, 75th percentiles and the latest in the arrival time range are presented for each suburb for tsunamis from each source. Due to different positioning of the four local faults relative to Wellington suburbs, no consistent patterns of tsunami arrival time distributions have been identified for tsunamis from each of these local sources. When considering all the modelled scenarios of the four fault sources together, the modelling results show that the first peak of a tsunami would arrive at Wellington suburbs as early as in 3.5~11.4 minutes in a local earthquake event.

KEYWORDS

Tsunami simulation, Arrival times, local sources, slip variation, Wellington.

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1.0 INTRODUCTION

The Wellington region of New Zealand sits on top of the Hikurangi subduction interface where the Pacific plate is subducting beneath the Australia plate, and it is also cut by a number of major local faults, such as the Wellington Fault, Wairarapa Fault and Wharekauhau Fault. Historically, the coastal suburbs of Wellington have been affected by tsunamis originating from both local sources and distant ones (Grapes and Downes, 1997). Among these sources, local sources pose the biggest tsunami threat to the Wellington Region. The largest recorded tsunami impact was associated with the rupture of Wairarapa Fault in the great 1855AD earthquake. Tsunami heights of over a few meters were widely observed along the southern coastal suburbs of Wellington and around the inner coasts of Wellington Harbour. In Cook Strait and inside the Wellington Harbour, abnormal water oscillations lasted about 8-12 hours (Grapes and Downes, 1997).

For this type of local source tsunami, self-evacuation has been advised in New Zealand and in other parts of the world should a strongly-felt or long-duration earthquake be felt (MCDEM, 2008; 2015; Wood and Schmidtlein, 2012;2013). This is due to the limited amount of time between the generation of tsunami from local sources and the arrival of tsunami waves. For local source tsunami, successful self-evacuation, constrained by the very limited time window for people to move out of at-risk areas into safe zones, relies heavily on appropriate evaluation planning and effective community education. To achieve these, two pieces of timing information are crucially important to a coastal community: 1) the predicted tsunami arrive time(s), here defined as tsunami travel time(s) from its source to a coast front of interest, and 2) the time needed to move to designated safety zones.

In this study, we focus on providing arrival time estimates of tsunami from local seismic sources for selected coastal suburbs in Wellington region, as shown in Figure 1.1. The arrival time information includes: • Start time of noticeable disturbance (i.e., the water level anomaly, rising or ebbing). • Start time of the first peak of tsunami waves. • Arrival time of the first peak of tsunami waves. • Start time of the biggest peak of tsunami waves. • Arrival time of the biggest peak of tsunami waves.

These time estimates do not include the effects of tides and are evaluated on a suburb-by- suburb basis, including the southern coastal suburbs of Wellington (i.e., Island Bay, Lyall Bay, Kilbirnie, and Seatoun) and those around inner coasts of Wellington Harbour, e.g., Petone in Lower Hutt (Figure 1.1).

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Figure 1.1 Coastal suburbs of the Wellington Harbour region. All the coastal suburbs that are labelled are included in the arrival time calculations in this study. Abbreviations of suburb names: Korokoro (KK), Horokiwi (KW), Ngauranga (NG), Kaiwharawhara (KR), Pipitea (PT), Wellington Central (WC), Te Aro (TA), Oriental Bay (OB), Roseneath (RN), Hataitai (HT), Kilbirnie (KN), Rongotai (RT), Miramar (MM), Maupuia (MP), Karaka Bays (KB), Seatoun (ST), Breaker Bay (BB), Moa Point (MP), Lyall Bay (LB), Houghton Bay (HB), Island Bay (IB).

Tsunami source models have previously been developed for the Hikurangi subduction interface, Wellington Fault, Wairarapa Fault and Wharekauhau Fault as part of past It’s Our Fault (IOF) investigations for tsunami simulations in Mueller et al. (2014). In these source models, multiple non-uniform slip distributions were considered in each local source to account for the uncertainty in the slip distribution for potential future events.

In this investigation, the same source scenarios in Mueller et al. (2014) have been re-used in the numerical simulations of tsunami from Wellington Fault, Wairarapa Fault and Wharekauhau Fault. For the Hikurangi subduction interface, 20 slip variation scenarios have been newly created using the slip variation modelling approach in Mueller et al. (2014).

Together with these source scenarios, tsunami time history data have been simulated at pre- defined locations along coastal fronts of Wellington suburbs. The simulated time histories have then been used for the determination of the above mentioned tsunami arrival times for each suburb. This approach leads to a wide range of tsunami arrival times for an individual suburb for each tsunami source. The earliest, 25th percentile, 50th percentile, 75th percentile and the latest in the tsunami arrival time range are presented in this report.

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2.0 TSUNAMI SOURCE MODELLING

2.1 LOCAL SOURCES

The Wellington region is cut by, and underlain by, a number of active faults (e.g., Pondard and Barnes 2010; Robinson et al. 2011; Litchfield et al. 2014; Langridge et al. 2016). Among them, the Hikurangi subduction interface, the Wellington Fault, the Wairarapa Fault and the Wharekauhau Fault, as shown in Figure 2.1, have the potential to generate tsunami affecting the coastal suburbs of the Wellington Harbour region (Mueller et al., 2014). These four faults have been modelled in this study to estimate the arrival times of tsunami generated from these sources to coastal fronts of Wellington suburbs.

Figure 2.1 Major local active faults and other structures in central New Zealand. Bold blue line = Wellington Fault, bold red line = Wairarapa Fault, and bold yellow line = Wharekauhau Fault, HSZ in inset = Hikurangi subduction zone (Revised from Robinson et al., 2011).

2.1.1 Hikurangi Subduction Interface

The Hikurangi subduction interface, where the Pacific Plate is subducting beneath the Australia Plate at a rate of 20-30 mm per year in the southern North Island (Wallace et al., 2012), is one of the major tectonic features of New Zealand. This fault, expressed as the Hikurangi Trench offshore, extends nearly 1000 km along the east coast of North Island. The gently dipping fault interface extends about 150 km westwards beneath the North Island.

This is the likely source of the largest magnitude earthquakes in the study area, and a likely source of major tsunami. However, the fault’s orientation and its location are not strongly conducive to directing tsunami towards Wellington unless the rupture extends into Cook Strait. Large uncertainties exist regarding recurrence interval and rupture behaviour. It is also

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segmented, with the segments possibly rupturing independently (e.g. Wallace et al., 2009; Stirling et al., 2010; Mueller et al., 2014).

2.1.2 Wellington Fault

The Wellington Fault is the southernmost part of a distinct fault structure that extends at least 450 km from Cook Strait to Whakatane. The Wellington Fault contains three distinct sections, the “Wellington-Hutt Valley section” which extends from Cook Strait to Kaitoke, the “Tararua section” which extends from Kaitoke to Putara, and the “Pahiatua section” which extends from Putara to Woodville. Each of the sections probably ruptures independently and a rupture of any one of them may influence the timing of rupture of its neighbours. It is the Wellington-Hutt Valley segment that causes most concern, because it bisects the cities of Wellington, Lower Hutt and Upper Hutt, is close to Porirua city and a number of towns on the Kapiti coast, and also extends offshore. Thus rupture of the Wellington Fault could produce tsunami significantly affecting the coastal suburbs of Wellington.

Estimates of the maximum magnitude for a Wellington Fault rupture vary, with values ranging from Mw 7.3 to 7.6 (Van Dissen and Berryman, 1996; Langridge et al., 2011; Little et al., 2010; Rhoades et al., 2011; Stirling et al., 2012). The last rupture of Wellington-Hutt Valley segment of the Wellington Fault was between ~200 and 400 years ago.

2.1.3 Wairarapa Fault and Wharekauhau Thrust

The Wairarapa Fault is another active seismic fault in the southern part of the North Island, New Zealand. It is the southernmost part of a distinct fault structure that extends at least 300 km from Cook Strait to Hawke Bay (Carne and Little, 2012). The Wairarapa Fault extends at least 140 km northwards from the centre of Cook Strait.

The Mw ~8.2 Wairarapa Earthquake of 1885, the largest recorded earthquake in New Zealand, occurred on this fault. Past studies show that rupture along the Wairarapa Fault and Wharekauhau Fault was responsible for this Wairarapa earthquake. Right-lateral displacements of up to 18 m and vertical displacements of up to 6.4 m were observed in this event (McSaveney et al., 2006; Grapes and Downes, 1997; Rodgers and Little, 2006). A tsunami was triggered by this earthquake. Several observations of tsunami heights were documented in Table 2.1, and also refer to Grapes and Downes (1997) for more detail.

Table 2.1 Tsunami observations in the 1855 Mw ~8.2 Wairarapa earthquake.

Reported location Estimated run-up or tsunami height*

Otaki ≥ 1.0m Porirua Harbour 2.0 - 4.0 m Lyall Bay 4.0 - 6.0 m Evens Bay 3.5 – 5.0 m Lambton Quay 2.5 – 3.0 m Hutt River ≥ 2.0 m Te Kopi, Palliser Bay 9.0 – 10.0 m

* depending on data source, see Grapes and Downes (1997).

The Wharekauhau Fault is a reverse thrust fault that runs parallel to the Wairarapa Fault in the southwest corner of the Wairarapa Valley and extends offshore into Palliser Bay near Turakirae Head. It is thought that this fault typically ruptures at the same time as the Wairarapa

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Fault, as probably happened in the 1855 Wairarapa Earthquake; though it may not rupture in all Wairarapa Fault events. In this study, it is assumed that the Wharekauhau Fault ruptures individually in order to identify tsunami arrival times more specifically for each source.

2.2 TSUNAMI SOURCE MODELLING

2.2.1 Source Scenarios

For the simulations of tsunami arrival times, earthquake scenarios of any magnitude that trigger tsunami can be used to calculate the predicted (modelled) time history data and determine the arrivals of tsunami waves. However, in most applications, the magnitude of an earthquake needs to large enough so that the tsunami waves can be practically detectable. In this study, the maximum potential magnitude of each source was used for source scenario construction and tsunami simulation. The maximum potential magnitude was obtained from the New Zealand National Seismic Hazard Model (NSHM) for three of the four considered active faults (Stirling et al., 2012). The maximum potential magnitude used for the Hikurangi subduction interface was Mw = 9.0. This magnitude is considered to be at the upper bound of uncertainty regarding the maximum potential magnitude of a Hikurangi subduction interface earthquake (see Table A3.1 in Power (2013)).

It is well known that non-uniform slip, or slip heterogeneity, in an earthquake event may influence not only tsunami heights but also tsunami arrival times at a coastal location. Although there are some constraints on potential distributions of slip, it is the largest unknown parameter prior the occurrence of an earthquake and introduces a large uncertainty in source scenario development.

In the past IOF investigations for tsunami hazard assessments, source scenarios with multiple non-uniform slip distributions have previously been developed for the Hikurangi subduction interface, the Wellington Fault, the Wairarapa Fault and the Wharekauhau Fault by Mueller et al. (2014). In these source scenarios, non-uniform slip distributions have been considered for each local source to account for the uncertainty associated with slip distributions in future events.

In this study, the same source scenarios of Mueller et al. (2014) have been adopted for numerical simulations of tsunami from the Wellington Fault, the Wairarapa Fault and the Wharekauhau Fault, however the inundation results are different due to the use of more detailed topographical and bathymetrical data.

For the Hikurangi subduction interface, 20 slip variation scenarios have been newly created using the variable slip modelling approach of Mueller et al. (2014). In these scenarios, a well- accepted constraint, slip rate deficit distribution (Wallace et al., 2012), has been applied as a weighting mechanism, i.e., slip rate deficit weighting, to re-distribute the randomly generated slip distributions in order to reflect the coupling status of Hikurangi subduction interface (refer to Section 2.2.2 for detail). These 20 weighted slip variations are used as source scenarios to simulate tsunami from the Hikurangi subduction interface.

For the four local sources, the modelled source scenarios are summarized in Table 2.2.

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Table 2.2 Local seismic source scenarios for tsunami arrival time calculation.

Local source region Source scenarios

Hikurangi subduction interface Mw 9.0, with 20 weighted slip variations Wellington Fault Mw 7.6, with 9 slip variations (Mueller et al., 2014) Wairarapa Fault Mw 8.2 with 9 slip variations (Mueller et al., 2014) Wharekauhau Fault Mw 7.2 with 9 slip variations (Mueller et al., 2014)

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2.2.2 Fault Geometry, Non-uniform Slip and Slip Rate Deficit Weighting

For the Wellington Fault, the Wairarapa Fault and the Wharekauhau Fault, the fault parameters, e.g., size, orientation and geometrical parameters of the rupture plane, are extracted from the NSHM fault database (Stirling et al., 2012) through TSUNAMI-API (refer to Section 3.5 for more detail). For these faults, the fault planes are represented by a set of connected rectangles (sub-fault segments). The rectangles are connected at the surface and overlap at depth due to their dip. Before being used in tsunami modelling, the overlap, which increases with depth, is removed from the fault models to obtain a more natural representation of the faults. For the Hikurangi subduction interface, the fault geometry adopts the one developed by Williams et al. (2013), see the upper left panel in Figure 2.2.

Figure 2.2 Overview of the geometry of the tsunami sources. The Hikurangi subduction zone interface is the new representation as recently published by Williams et al. (2013). The three crustal faults considered in this investigation (Wellington, Wairarapa, and Wharekauhau faults) are taken from the National Seismic Hazard Model (Stirling et al., 2012). Please note that the manner in which the interface has been gridded gives a false visual impression of sharp changes in the shape of the interface.

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Non-uniform slip distributions on a rupture interface are generated through TSUNAMI-API (Mueller et al., 2014;2015; and also in Section 3.5 of this report) in which the methodology for slip distribution modelling follows those described by Geist (2002) as well as by Herrero and Bernard (1994). The earthquake magnitude is used to scale the slip amount appropriately on the specific scenario fault plane. In order to apply variable slip on a fault interface, the fault interface is discretised as a set of rectangular fault patches of either equal sizes or variable sizes, depending on the shape of the fault interface (Figure 2.2). The non-uniform slip distribution is then mapped onto these fault patches for seafloor/ground deformation calculations.

For the Hikurangi subduction interface scenarios, the slip rate deficit model proposed by Wallace et al. (2012), as shown in Figure 2.3, has been used as a weighting mechanism, called slip rate deficit weighting, to constrain the random distribution of non-uniform slip. Wallace et al.’s model suggests that there is an increased chance for slip to manifest predominantly in areas where the slip-rate deficit is at a maximum (e.g., the red areas in Figure 2.3). Using this slip rate deficit weighting more slip has been allocated to the areas where the slip rate deficit is higher, in order to reflect the effect that this slip rate deficit would have on slip distribution and consequent tsunami arrival time estimation in future events.

Figure 2.3 Subduction interface slip rate deficit for the Hikurangi subduction interface (credit: Wallace et al., 2012).

Figure 2.4 shows an example of non-uniform slip assigned to the Hikurangi subduction interface, one without slip rate deficit weighting (left panel) and the other with slip rate deficit weighting (right panel). It is obvious that the application of slip rate deficit weighting shifts more slips towards the southern part of the Hikurangi subduction interface in comparison with the non-uniform slip distribution without the slip rate deficit weighting.

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Figure 2.4 Comparison of non-uniform slip distribution without slip rate deficit weighting (left panel) and non- uniform slip distribution with slip rate deficit weighting (right panel) for Hikurangi subduction interface scenarios. Both scenarios have the same magnitude (Mw 9.0).

The additional slip distribution scenarios for the Hikurangi subduction interface are given in Appendix 9.1.

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3.0 TSUNAMI MODELLING

3.1 TSUNAMI MODEL

A tsunami simulation model, COMCOT (Cornell Multi-grid Coupled Tsunami), was used to simulate the generation, propagation and coastal inundation of tsunamis triggered by the four seismic tsunami sources considered in this investigation. COMCOT was originally developed by the wave research group at Cornell University, USA (Cho, 1995; Wang, 2008). It has subsequently been under continued development by Dr Wang at GNS Science, New Zealand (Wang and Power, 2011).

COMCOT is capable of simulating the entire life-span of a tsunami, including its generation by earthquakes and landslides, transoceanic propagation, coastal run-up and inundation. It uses explicit staggered leap-frog finite difference schemes to solve linear and nonlinear shallow water equations in either spherical or Cartesian coordinates to catch the dynamics of tsunami waves. The implementation of two-way nested grids allows the model to efficiently deal with the large variation of spatial and temporal scales during tsunami evolution; and a validated wetting/drying scheme has also been deployed to describe the dynamic inland flooding and withdrawing process of tsunami (Cho, 1995; Wang, 2008; Wang and Power, 2011). An improved numerical scheme has also been implemented in the model to improve its performance on the modelling of weakly dispersive tsunami waves (Wang and Liu, 2011).

The model has been widely validated and consistently shows satisfactory accuracy against analytical solutions, experimental studies and field observations (Liu et al., 1995; Cho, 1995; Wang et al., 2008). It has been successfully applied to study the propagation, run-up and inundation of historical events, including the 1960 Chilean tsunami (Liu et al. 1994) and the 2004 Indian Ocean tsunami (Wang and Liu, 2007), and to evaluate tsunami hazards by potential events in New Zealand (Power et al., 2012; Barberopoulou et al., 2014; Fraser et al., 2014).

3.2 MODELLING GRIDS

The tsunami simulation model COMCOT uses a series of nested ‘grids’, constructed from bathymetric and topographic data, to account for spatial resolution requirements by a tsunami travelling through different regions, e.g. deep ocean basins, continental shelves, nearshore regions and inlands. In this study, four levels of Digital Elevation Model (DEM, a combination of topographical and bathymetric data) grids at cascading spatial resolutions are used to simulate the generation of tsunami from the sources, offshore propagation and potential coastal flooding in Wellington suburbs. In the DEMs, both water depth and land elevation are defined in terms of Mean Sea Level (MSL).

The first level grid, grid layer 01, is derived from the NGDC ETOPO topographic and bathymetric database which covers the whole Pacific at a spatial resolution of 2 arc-minutes (~3600 m, Figure 3.1) and used to simulate tsunami generation and propagation from source regions. The second level grid, grid layer 02, is derived from LINZ Charts, the Seabed Mapping CMAP and GEBCO 08 datasets which covers the whole of New Zealand and its offshore regions at 30 arc-seconds (~640-740 m in New Zealand, Figure 3.2). The third level grid, grid layer 03, is derived from the same sources as the second level grids, covers the southern part of the North Island at a spatial resolution of 4.2 arc-seconds (~95 m in Wellington Region, Figure 3.3).

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Figure 3.1 Nested grid setup for tsunami generation and propagation modelling.The outer grid layer 01 spans the whole Pacific for tsunamis from distant sources at a spatial resolution of 2 arc-minutes. See Figure 3.2, Figure 3.3, and Figure 3.4 for closer detail of grid layers 02, 03, and 04, respectively.

Figure 3.2 Nested grid setup for tsunami generation and propagation modelling. This figure shows the nested grid layers 02 and 03 which focus on New Zealand and offshore region at increasing levels of detail. The colour scale illustrates water depth and land elevation in meters.

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Figure 3.3 Nested grid setup for tsunami propagation modelling. This figure shows nested grid layers 03 and 04 which focus on the Wellington region at increasing levels of detail. The colour scale illustrates water depth and land elevation in meters.

The fourth level grid, grid layer 04, covers the Wellington Harbour and its surrounding suburbs at a spatial resolution of about 20 meters (Figure 3.4). This high resolution DEM data is necessary for detailed tsunami propagation and inundation simulations in the coastal areas of Wellington, and is derived from a combination of LiDAR (Light Detection and Ranging) topographical data provided by Wellington Regional Council and multi-beam bathymetric survey data from NIWA (Pallentin et al., 2009) in Wellington harbour.

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Figure 3.4 Nested grid setup for tsunami propagation and inundation modelling in Wellington region. This figure shows nested grid layer 04 which has the highest level of detail. The solid black lines show the coastal lines at Mean Sea Level. The colour scale illustrates water depth and land elevation in meters.

3.3 SEAFLOOR AND GROUND DISPLACEMENT CALCULATION

For seismic sources, tsunamis are generated by large scale seafloor disturbances associated with fault ruptures. Except for some special events, such as so-called tsunami earthquakes, the rupture time and rise time in earthquake events are usually much shorter than the wave periods of tsunami. Therefore, instantaneous fault displacement is commonly assumed for tsunami generation. This assumption is also used in this study.

The seafloor/ground displacement is evaluated using Okada’s method (Okada, 1985) integrated in the COMCOT simulation package. This method calculates the displacement on the surface of an elastic half-space due to a given dislocation on a buried rectangular finite fault plane.

For all the source scenarios developed in this study, multiple fault patches (i.e., small rectangular fault planes), up to several thousands in Hikurangi scenarios, are deployed to discretize and better describe the three-dimensional down-dip profiles of source’s rupture surface. The seafloor/ground displacement for a source scenario is a linear superposition of the displacements contributed by all the fault patches that comprise the rupture surface of a specific scenario. This synthetic seafloor/ground displacement is then used as the initial condition of the tsunami simulation for the scenario.

For the local sources, the ground uplift and subsidence caused by local fault ruptures will inevitably change seafloor and ground elevation, and consequently may significantly influence the inundation range and flooding depth by the associated tsunami. Therefore, in this study, the effect of ground uplift and subsidence was considered and ground elevation modification

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was included in the simulations of tsunami flooding in order to reflect what would happen in these local events.

Some examples of the vertical seafloor/ground displacements of the source scenarios are shown in Figure 3.5.

Figure 3.5 Examples of computed vertical seafloor/ground displacements resulting from scenario ruptures of the Hikurangi subduction interface (upper left), the Wellington Fault (upper right), the Wairarapa Fault (lower left), and the Wharekauhau Fault (lower right). Note that the colour scale -8.0 m to 8.0 m is used for the left column panels and the colour scale -2.0m to 2.0m is used for the right column panels.

3.4 MODELLED DURATION AND ROUGHNESS CONSIDERATION

The observations from past tsunami events indicate that the first wave may not be the largest in a tsunami event and the highest peak may be observed many hours after the first arrival, especially in areas with particular settings such as in bays or harbours. In this study, a total duration of 30 hours has been simulated for the tsunamis from the local sources so that maximum tsunami amplitudes are sure to be captured.

In the study, DEMs used for the inundation modelling in the coastal suburbs of Wellington is a ground surface model derived from LiDAR and the retarding effect of such features as vegetation and buildings on inundation is approximated with a bottom friction model based on equivalent surface roughness estimates (Wang and Power, 2011). This is a typical and widely used approach in tsunami hazard analysis. A constant Manning’s roughness, n = 0.015, is used in the friction model in COMCOT for all the tsunami simulations in the Wellington region. This represents a lower end of Manning’s roughness values of different types of land covers. As a consequence, the modelled inundation ranges tend to be conservative (i.e., larger) which, we consider, is suitable for the purposes of evacuation mapping and tsunami hazard mitigation.

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3.5 AUTOMATION OF TSUNAMI SIMULATIONS WITH TSUNAMI-API

The effects of non-uniform slip add an additional level of complexity to tsunami source modelling and require an increased number of tsunami simulations in order to capture potentially important features of each tsunami source. A robust and efficient framework is required to manage the large amount of data and simulated scenarios. A Python-based Application Programming Interface (API) was developed that augments and drives the COMCOT tsunami simulation program, called TSUNAMI-API (Mueller et al., 2014; 2015). This enables full automation of parameter studies with COMCOT as the tsunami simulation kernel. The API manages simulations on clusters with different queuing systems, farms simulation scenarios out to clusters and collects data after simulation completion. In this study, the creation of non-uniform slip models, the rupture of the Hikurangi subduction interface and other crustal faults, rupture of fault segments and scaling with earthquake magnitude are all implemented through this TSUNAMI-API.

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4.0 TSUNAMI ARRIVAL TIME ESTIMATE

4.1 TIME HISTORY DATA MODELLING

To derive tsunami arrival times, tsunami time history data are calculated at a set of pre-defined locations at the coastal fronts of Wellington suburbs through numerical simulations. In total, 245 pre-defined locations have been chosen along the coast of Wellington Harbour and southern suburbs (Figure 4.1). These pre-defined locations, called virtual tsunami gauges, are used to record the time histories of water surface fluctuations caused by tsunami from the four local sources as described in Section 2.1.

These virtual tsunami gauges are deliberately spaced about 100 meters in front of the coast and 300 meters apart. Such gauge spacing not only allows enough resolution to resolve spatial variations of tsunami waves along the coasts, but also helps reduce the total number of virtual gauges for data recording and improves the efficiency of data processing.

For the purpose of data processing and referencing, these virtual tsunami gauges are numbered along the coast of Wellington, starting from the outlet of Lake Kohangatera to the west of Owhiro Bay for arrival time calculation (Figure 4.1 and Figure 4.2).

Figure 4.1 Virtual tsunami gauges (black circles with red dots inside) for tsunami time history calculations, overlaid with the high-resolution DEM in Wellington Harbour region.

Figure 4.2 shows a Wellington suburb boundary map overlaid with the virtual tsunami gauges. Note that for the purpose of clarity only every fifth virtual tsunami gauge is labelled in this figure.

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Figure 4.2 Virtual tsunami gauges (red dots and red crosses with black dots at center), overlaid with Wellington suburb boundary map. Note that only each fifth virtual tsunami gauge is labelled (i.e., the red crosses with black dots at center). Suburb name abbreviations: Korokoro (KK), Horokiwi (HW), Ngauranga (NG), Kaiwharawhara (KR), Pipitea (PT), Wellington Central (WC), Te Aro (TA), Oriental Bay (OB), Roseneath (RN), Hataitai (HT), Kilbirnie (KN), Rongotai (RT), Miramar (MM), Maupuia (MP), Karaka Bays (KB), Seatoun (ST), Breaker Bay (BB), Moa Point (MP), Lyall Bay (LB), Houghton Bay (HB), Island Bay (IB).

The virtual tsunami gauges are grouped for each of the selected coastal suburbs (Table 4.1). Some virtual tsunami gauges are shared by two adjacent suburbs due to their proximity to their suburb boundary. For example, gauge 117 is shared by Wellington Central and Te Aro (Table 4.1). The arrival time information obtained at these shared gauges is used by both suburbs.

Four neighbouring suburbs, Kilbirnie (KN), Miramar (MM), Rongotai (RT) and Lyall Bay (LB) as shown in Table 4.2, are combined together as one suburb group due to the low-lying, flat nature of the topography. In a large tsunami event this suburb group would be flooded together from either the northern coast inside the harbour or from the southern coast outside the harbour. Therefore, two sets of virtual tsunami gauges are assigned for this suburb group: one along its northern coast inside the harbour and the other along its southern coast outside the harbour, for arrival time determination at each of the two coasts.

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Table 4.1 Virtual tsunami gauges for individual suburbs (see Table 4.2).

Suburb Name Virtual Tsunami Gauge Numbers

Eastbourne 36 – 62 Seaview*1 63 – 68, 245 Petone*1 69 – 81, 245 Korokoro 81 – 83 Horokiwi 83 – 89 Ngauranga 89 – 98 Khandallah 98 – 101 Kaiwharawhara 101 – 102 Pipitea 103 – 115 Wellington Central 115 – 117 Te Aro 117 – 119 Oriental Bay 119 – 122 Roseneath 122 – 127 Hataitai 127 – 133 Northern coast of suburb group*2 134 – 138 Maupuia 139 – 156 Karaka Bays 157 – 162 Seatoun 163 – 169 Breaker Bay 169 – 181 Moa Point 181 – 186 Southern coast of suburb group*3 187 - 200 Houghton Bay 200 - 204 Island Bay 204 – 214 Owhiro Bay 214 – 230 Matiu Somes Island 232 – 241 *1 Virtual tsunami gauge 245 is located in Hutt River and is shared by Seaview and Petone. *2 *3 The four neighbouring suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because in a large event tsunami may flood them together from either northern coast*2 inside the harbour or southern coast*3 outside the harbour due to low-lying, flat nature of the topography.

In the numerical simulations of tsunami from the local sources, the time histories of water surface fluctuations are recorded at all the pre-defined virtual tsunami gauges for the first 30 hours after tsunami generation (i.e., the onset of an earthquake) in order to ensure that arrivals of the biggest tsunami waves are obtained. As indicated in Section 2.0, the source scenarios include 20 Hikurangi subduction interface scenarios, 9 Wellington Fault scenarios, 9 Wairarapa Fault scenarios, and 9 Wharekauhau Fault scenarios (Table 2.2).

4.2 ARRIVAL TIME IDENTIFICATION

To quantify tsunami arrival times, the following set of synthetic (modelled) timing information has been investigated in this study. • Start time of noticeable disturbance (i.e., the water level anomaly, rising or ebbing)

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This defines the first moment after an earthquake at which an abnormal water disturbance can be noticed. The disturbance can be either a water level rise or drop from local ambient water condition. • Start time of first peak This indicates the moment after an earthquake at which the water level starts to rise from local ambient water condition till the arrival of the peak of the first tsunami wave. • Arrival time of first peak This is the moment after an earthquake at which the water level reaches its peak for the first tsunami wave. • Start time of biggest peak This indicates the moment after an earthquake at which the water level starts to rise from local ambient water condition till the arrival of the peak of the biggest tsunami wave. • Arrival time of biggest peak

This is the moment after an earthquake at which the water level reaches its peak for the biggest tsunami wave.

In our modelling, a threshold value of 0.2 m water level deviation from apparent local ambient sea condition has been used to determine if a tsunami is large enough to be considered detectable in the tsunami arrival determinations. Additionally, if no tsunami elevation 0.2 m higher than ambient water level is found, a “No Value” will be assigned as the start time and arrival time of the first peak and the biggest peak of tsunami waves. The use of 0.2 m here is consistent with the tsunami elevation threshold adopted by MCDEM to classify “No Threat” and “Marine Threat” in tsunami events. Tsunami with its maximum elevation less than 0.2 m above ambient sea condition is considered no tsunami threat in the current MCDEM tsunami advisory and warning plan (MCDEM, 2014). Also note that tidal effects are not considered in the determination of tsunami wave arrivals which may affect the arrival time estimates, especially the arrival of the biggest peak.

Tsunami arrival times are extracted from the simulated time history records at the virtual tsunami gauges for all the modelled source scenarios for each of the four local active fault sources. An example of one set of time history data is shown in Figure 4.3, illustrating the identified tsunami arrival times at virtual tsunami gauge 131 at the coastal front of Hataitai (Figure 4.2) for a Hikurangi subduction interface scenario.

In this example, the start of noticeable water anomalies is identified at about 5 minutes 28 seconds after the earthquake (first solid green circle, using 0.2 m threshold); at about 18 minutes 30 seconds after the earthquake, the first detectable wave peak starts to arrive (the first solid blue square) and at about 37 minutes 10 seconds after the earthquake, the first peak arrives (the second solid blue square – in this scenario the amplitude of the first peak is ~3.7 m above MSL, accounting for any tectonic uplift/subsidence); at about 61 minutes 40 seconds after the earthquake, the biggest peak starts to arrive (the first solid red circle), and its peak arrives at 67 minutes 47 seconds after the earthquake (the second solid red circle - in this scenario the peak of the first wave has an amplitude of ~4.6 m, accounting for any tectonic uplift/subsidence).

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Figure 4.3 Example of tsunami arrival time record (only the first 10 hours shown here) at the coastal front of Hataitai (virtual tsunami gauge 131 – see Figure 4.2 and Table 4.1) for a Hikurangi subduction scenario. In this time history data, the solid green circle indicates the start of noticeable water anomalies (water ebbing for this example, using 0.2 m deviation from local ambient sea condition as a detection threshold); the two solid blue squares indicate the start time and arrival time of the first peak; the two solid red circles indicate the start and arrival of the biggest peak. Solid blue line indicates local ambient sea condition immediately after an earthquake, including the effect of uplift/subsidence of ground/seafloor and tidal condition; and solid black line represents the universal Mean Sea Level (MSL) condition.

4.3 ARRIVAL TIME ESTIMATES AND DISCUSSIONS

The modelling approach described in the earlier sections means that each Wellington suburb corresponds to multiple sets of tsunami arrival times. For example, at the coastal front of Wellington suburb Hataitai, there are 7 virtual tsunami gauges (Table 4.1 and Figure 4.2). This leads to a total of 140 sets of time history data and consequently 140 sets of tsunami arrival times for the 20 modelled slip scenarios from Hikurangi subduction interface. Each set of tsunami arrival times contains five parameters - the start of noticeable water level anomaly (rising or ebbing), start of first peak, arrival of first peak, start of biggest peak, and arrival of biggest peak. And the values of these time estimates are different from each other.

This results in a range of tsunami arrival times for an individual suburb for each active fault source, as shown in Figure 4.4.

To provide an overall picture of arrival time distributions for each suburb, the earliest (0th percentile), 25th percentile, 50th percentile (median), 75th percentile and the latest (100th percentile) of the tsunami arrival time range have been calculated and presented in this report. To determine these percentiles, for each arrival, the modelled arrival times are firstly sorted in ascending order from the earliest to the latest arrivals. Then the arrival time values at the 0th,

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25th, 50th, 75th and 100th positions in this ascending sequence are identified as the corresponding percentile arrival times, respectively. Figure 4.4 shows an example of the calculated arrival times of first peak (black circles) and the earliest (0th), 25th, 50th (median), 75th and the latest (100th) of first peak arrivals (red circles) at the coastal front of Hataitai, for all the Hikurangi subduction interface scenarios. In this example, the 25th percentile arrival time, i.e., 18.4 minutes after earthquake, means that this arrival time is larger (i.e., arriving later) than about 25 percent of all the modelled arrival times of first peak for tsunami from Hikurangi subduction interface.

Figure 4.4 Distribution of first peak arrival times at the coastal front of Hataitai for all the Hikurangi subduction interface scenarios. The modelled first peak arrival times are sorted in ascending order and presented as circles in this figure. The vertical axis indicates the arrival time in minutes after earthquake strike. The horizontal axis shows the percentile of each calculated arrival time in the sorted sequence of the first peak arrivals. From left to right, the red circles indicate the position of the earliest (0th), 25th percentile, 50th, 75th percentile and the latest (100th) arrivals of first peak. For example, the 25th percentile arrival time means that about 25 percent of all the modelled arrival times have values smaller (i.e., arriving earlier) than the 25th percentile arrival.

The earliest of the modelled tsunami arrival times are summarized on a suburb by suburb basis for each of the four local sources in Table 4.2 – Table 4.5. Considering that the earliest arrival times may be the results of only a few slip distribution scenarios, the additional 25th percentile, 50th percentile, 75th percentile and the latest tsunami arrivals are given in Table 9.1 to Table 9.8 of Appendix 9.2 for a more complete picture of tsunami arrival times for each of the four active fault sources.

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Table 4.2 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Hikurangi subduction interface scenarios (the start times and arrival times are measured in minutes after mainshock).

Start time of Start time Arrival time Start time Arrival time Suburb Name noticeable of first of first of biggest of biggest disturbance peak peak peak peak

Minutes after mainshock

Eastbourne 1.7 0.0 7.4 0.0 13.8

Seaview 2.5 0.0 8.6 0.0 11.8

Petone 1.9 0.0 7.1 0.0 9.4

Korokoro 1.8 0.0 6.9 0.0 9.4

Horokiwi 1.4 0.0 7.7 0.0 9.1

Ngauranga 1.5 0.0 8.8 0.0 25.2

Khandallah 1.5 0.0 24.3 0.0 24.3

Kaiwharawhara 1.4 0.0 24.2 0.0 24.2

Pipitea 1.4 0.0 24.0 0.0 24.0

Wellington Central 2.9 0.0 28.0 0.0 28.0

Te Aro 2.6 0.1 28.0 4.7 28.0

Oriental Bay 1.7 2.2 27.3 4.2 27.3

Roseneath 3.0 2.8 15.1 4.2 16.7

Hataitai 3.2 1.6 6.0 12.1 18.3

North of suburb group*1 2.4 0.0 7.2 16.0 19.2

Maupuia 3.2 0.0 4.7 6.0 16.6

Karaka Bays 2.4 0.0 3.6 8.4 24.0

Seatoun 2.6 0.0 4.5 21.2 23.3

Breaker Bay 2.1 0.0 4.1 0.0 9.7

Moa Point 2.7 0.0 8.9 0.0 8.9

South of suburb group*2 2.5 0.0 7.4 0.0 8.1

Houghton Bay 2.4 0.0 8.1 0.0 8.1

Island Bay 2.0 0.0 6.9 0.0 6.9

Owhiro Bay 1.6 0.0 5.6 0.0 5.6

Matiu Somes Island 4.6 0.0 9.1 0.0 9.1 *1,*2The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs. “0.0” means water level starts to rise from apparent local ambient sea condition immediately after the onset of an earthquake, and the water level keeps rising till it reaches the first peak or biggest peak.

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Table 4.3 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Wellington Fault scenarios (the start times and arrival times are measured in minutes after mainshock).

Suburb Name Start time of Start time Arrival Start time Arrival time noticeable of first time of first of biggest of biggest disturbance peak peak peak peak

Minutes after mainshock

Eastbourne 6.0 8.2 9.8 11.5 14.2

Seaview 5.4 4.3 7.8 10.3 12.7

Petone 1.4 0.0 4.8 0.0 4.8

Korokoro 1.2 0.0 4.0 0.0 4.2

Horokiwi 1.1 0.0 4.0 0.0 4.0

Ngauranga 1.0 0.0 4.6 0.0 8.8

Khandallah 1.0 0.0 3.9 0.0 10.9

Kaiwharawhara 0.9 0.0 11.7 0.0 11.9

Pipitea 0.9 0.0 11.9 0.0 11.9

Wellington Central 3.6 6.5 15.7 6.5 15.7

Te Aro 3.6 6.6 15.8 6.6 15.8

Oriental Bay 3.7 26.7 46.5 26.7 87.1

Roseneath 24.3 1.2 60.3 1.2 60.3

Hataitai 6.5 11.4 16.7 11.4 16.7

North of suburb group*1 7.4 10.7 15.8 10.7 15.8

Maupuia 4.7 3.1 5.1 10.4 17.2

Karaka Bays 15.1 13.9 19.9 30.7 32.2

Seatoun 17.0 0.0 19.7 17.1 19.8

Breaker Bay 11.4 0.0 12.8 0.0 12.8

Moa Point 10.7 0.0 13.2 0.0 13.2

South of suburb group*2 9.6 0.0 11.6 0.0 12.0

Houghton Bay 9.2 0.1 11.5 0.1 11.6

Island Bay 7.5 0.0 10.0 0.0 10.0

Owhiro Bay 3.1 0.0 6.3 0.0 6.5

Matiu Somes Island 2.3 0.0 3.7 0.0 61.6 *1,*2The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs. “0.0” means water level starts to rise from local ambient sea condition immediately after the onset of an earthquake, and the water level keeps rising till it reaches the first peak or biggest peak.

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Table 4.4 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios (the start times and arrival times are measured in minutes after mainshock).

Suburb Name Start time Start time Arrival time Start time of Arrival time of of first of first peak biggest of biggest noticeable peak peak peak disturbance

Eastbourne 1.7 0.0 15.1 0.0 15.1

Sea View 5.9 0.0 11.7 0.0 11.7

Petone 2.3 0.0 7.4 0.0 7.4

Korokoro 2.1 0.0 7.5 0.0 7.5

Horokiwi 1.8 0.0 8.4 0.0 8.4

Ngauranga 1.7 0.0 8.9 0.0 8.9

Khandallah 1.7 0.0 11.0 0.0 11.0

Kaiwharawhara 1.6 0.0 12.2 0.0 12.2

Pipitea 1.6 0.0 11.3 0.0 11.3

Wellington Central 1.9 0.0 11.4 0.0 11.4

Te Aro 4.6 0.0 11.2 0.0 11.2

Oriental Bay 5.4 1.5 10.7 1.5 10.7

Roseneath 5.5 2.5 11.3 2.5 11.3

Hataitai 4.2 4.7 15.4 4.7 15.4

North of suburb group*1 4.2 8.6 14.2 8.6 14.2

Maupuia 1.7 5.7 10.8 5.7 10.8

Karaka Bays 3.1 0.0 3.9 35.3 38.5

Seatoun 3.4 0.0 4.4 33.8 37.7

Breaker Bay 2.5 0.0 3.5 30.7 32.3

Moa Point 3.3 0.0 9.4 0.1 9.4

South of suburb group*2 2.5 0.0 5.7 0.0 6.3

Houghton Bay 3.0 0.1 4.6 31.2 32.8

Island Bay 2.8 0.0 5.1 0.0 6.0

Owhiro Bay 1.7 0.0 5.2 0.0 5.5

Matiu Somes Island 5.2 0.0 10.8 0.0 10.8 *1,*2The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs. “0.0” means water level starts to rise from local ambient sea condition immediately after the onset of an earthquake, and the water level keeps rising until it reaches the first peak or biggest peak.

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Table 4.5 Earliest tsunami arrival times at coastal fronts of Wellington Harbour suburbs for Wharekauhau Fault scenarios (the start times and arrival times are measured in minutes after mainshock).

Suburb Name Start time of Start time of Arrival time of Start time of Arrival time of noticeable first peak first peak biggest peak biggest peak disturbance

Minutes after mainshock

Eastbourne 3.1 19.2 20.8 30.2 32.4

Sea View 5.2 38.0 39.7 46.6 47.7

Petone 7.9 0.1 29.5 43.5 45.1

Korokoro 28.2 0.0 28.8 43.4 45.1

Horokiwi 27.1 0.0 27.5 43.4 46.4

Ngauranga 8.7 0.0 9.4 43.6 47.0

Khandallah 8.5 0.0 10.0 77.4 80.9

Kaiwharawhara 8.4 0.0 10.2 77.0 80.9

Pipitea 8.3 0.0 10.7 0.0 11.3

Wellington Central 8.3 0.0 12.7 105.5 107.9

Te Aro 8.3 0.0 12.7 105.2 107.9

Oriental Bay 8.6 0.0 12.5 104.7 108.4

Roseneath 8.8 0.0 12.5 0.0 13.8

Hataitai 10.2 0.0 15.3 0.0 15.6

North of suburb group*1 10.6 0.0 14.2 83.3 86.1

Maupuia 10.7 2.4 14.0 2.6 14.5

Karaka Bays 17.0 0.0 18.1 27.2 29.4

Seatoun 15.3 0.0 15.6 32.2 37.3

Breaker Bay 7.6 0.0 10.9 0.0 10.9

Moa Point 7.1 0.0 10.1 0.0 10.1

South of suburb group*2 6.2 0.0 9.2 0.0 10.3

Houghton Bay 5.9 0.1 10.0 0.2 10.3

Island Bay 5.6 0.0 8.8 0.0 8.8

Owhiro Bay 5.4 0.0 8.5 0.0 9.3

Matiu Somes Island 32.8 No Value*3 No Value*3 No Value*3 No Value*3 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs. *3 No Value means no tsunami elevation 0.2m higher than ambient sea condition was detected for this suburb among all the simulated scenarios in this source region. This implies that tsunami threat level at this suburb would fall to “No Threat” category should an earthquake with modelled magnitude occur in this source region. “0.0” means water level starts to rise from local ambient sea condition immediately after the onset of an earthquake, and the water level keeps rising until it reaches the peak of first peak or biggest peak.

Considering the fact that in a local event there would be no time and expertise for coastal communities to determine which fault has ruptured and exercise self-evacuation accordingly, the earliest of the first peak arrivals for all of the four modelled local fault sources have been

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calculated for each suburb in Table 4.6. These earliest arrivals may be treated as the worst- case constraint on evacuation planning for these suburbs.

Table 4.6 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs considering all the four local fault sources (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest arrival time of first peak Latest arrival time of first peak Minutes after mainshock

Eastbourne 7.4 163.0

Seaview 7.8 143.3

Petone 4.8 151.9

Korokoro 4.0 80.8

Horokiwi 4.0 165.2

Ngauranga 4.6 169.2

Khandallah 3.9 168.4

Kaiwharawhara 10.2 168.1

Pipitea 10.7 128.9

Wellington Central 11.4 128.9

Te Aro 11.2 128.8

Oriental Bay 10.7 88.0

Roseneath 11.3 107.4

Hataitai 6.0 352.2

North of suburb group*1 7.2 113.1

Maupuia 4.7 165.1

Karaka Bays 3.6 152.3

Seatoun 4.4 151.8

Breaker Bay 3.5 151.8

Moa Point 8.9 184.6

South of suburb group*2 5.7 185.4

Houghton Bay 4.6 77.1

Island Bay 5.1 120.6

Owhiro Bay 5.2 60.4

Matiu Somes Island 3.7 99.4 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs.

The modelling results show that when all the four fault sources are considered together the first peak of a tsunami would arrive at Wellington suburbs as early as in 3.5~11.4 minutes in a local earthquake event (Table 4.6, also Table 4.2 to Table 4.5). However, no consistent patterns of tsunami arrival time distributions have been identified among individual local fault sources due to the different positioning of the four local faults relative to the Wellington suburbs.

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Additional to the earliest arrivals, the latest arrivals of the first peaks of tsunami waves are also calculated for each suburb to illustrate the potential ranges of the first peak arrivals. However, in contrast to the arrival time estimates for each individual fault source as show in Table A2.1 to Table A2.8, no percentile arrivals have been estimated in Table 4.6 for the combination of all the four fault sources together due to the difficulty obtaining unbiased weighting of each slip variation scenario among different fault sources caused by large variations in their fault characteristics, e.g., return periods.

The tsunami arrival time estimates presented in this study determine how much time a coastal community would have to reach safety zones, such as those designated high ground, in a tsunami event originated from each of the four tsunamigenic faults local to Wellington region.

For a coastal community, effective tsunami evacuation planning is also constrained by the other piece of timing information, the time needed to evacuate to designated safety zones. Evacuation modelling, usually via least-cost based or agent-based approaches, is helpful to identify how much time it would take for people to move out of at-risk areas to designed safety zones, e.g., naturally occurring high ground. Such evacuation modelling has been carrying out for the coastal suburbs of Wellington harbour region in a separate study of IOF tsunami hazard investigations.

Figure 4.5 shows a demonstration how these two pieces of timing information would constrain tsunami evacuation planning for a coastal community with a population of 7,000.

Figure 4.5 An illustration of time constraints for tsunami evacuation. The blue line indicates the total number of residents evacuated to safety as a function of time (blue line) for an anonymous coastal community. The red line indicates the predicted arrival time of tsunami after earthquake shaking. The crossing point of the two lines shows the total number of residents able to move into safe zones by the arrival of tsunami. The residents requiring more time than the red line moment would be at risk and exposed to tsunami impact.

In this example, by the predicted tsunami arrival time, about 25 minutes after the earthquake, only 3,250 residents out of 7,000 in total would be able to reach safety zones using horizontal evacuation measures. For those that would not have enough time to evacuate into safety

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zones before the arrival of tsunami, alternative evacuation measures, such as using berms or buildings inside hazard zones as vertical evacuation refuge (Wood and Schmidtlein, 2013), may have to be considered to survive an incoming tsunami.

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5.0 LIMITATIONS

While the arrival time estimates provide valuable insights to tsunami evacuation planning and tsunami hazard mitigation, it is important that the limitations and uncertainties in this study are understood.

It is well-known that numerical simulations usually carry a number of uncertainties or inaccuracies which may lead to over- or under-estimation of arrival times of tsunami from each source (or each scenario run for a specific source) in this study. These include the limitations in numerical simulation models, the quality of Digital Elevation Models (DEM), and the uncertainties in the source parameters, such as fault plane geometries and sizes, rupture patterns and slip distributions. These effects have not been fully studied for reasons of practicality.

Among these factors, the uncertainties in source parameters likely play the most important roles in determining the accuracy of arrival time estimate. We currently assume that the effect of rupture complexity in the form of non-uniform slip is one of the most important ones (Geist, 2002) and non-uniform slip distributions have been included in the numerical calculation of arrival times of tsunami from the four local fault sources considered in this investigation. However, due to the limited number of slip variations modelled for each source region, a complete picture of the arrival time envelope may not have been obtained (and which may never be obtainable). This will rely on further investigations.

In this study, instantaneous rupture has been assumed for all the source scenarios. This may lead to an uncertainty of up to a few minutes in tsunami arrival time estimates for a Hikurangi earthquake if rupture starts at the northern end of the subduction interface. Additionally, the effect of temporal tidal level variation has not been considered, and this may significantly influence the arrival time of the biggest peak in particular, especially when the modelled biggest waves coincide with low tides.

Considering the above mentioned effects, it is not so unreasonable to assume that there is a 20% uncertainty to the arrival time estimates in this study.

While all the main local active fault sources have been considered in this study, there are other tsunamigenic sources local to Wellington region. For example, Wellington harbour region is also in close proximity to Cook Strait submarine canyon systems with abundant evidence of past submarine landslides. Strong ground shaking associated with local earthquake events could potentially trigger submarine landslides in these canyon systems, causing tsunami affecting Wellington suburbs. However, this type of landslide tsunami has not been taken into account in the current study due to the complex nature of their triggering mechanism associated with earthquakes and the limited timeframe of this project. Tsunami arrival time estimates from this type of tsunamigenic sources are therefore subject to future investigations.

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6.0 CONCLUSIONS

The arrival times of tsunami from local active fault sources have been evaluated for selected coastal suburbs in Wellington harbour region. The local fault source scenarios that have been modelled include 20 scenarios on the Hikurangi subduction interface, 9 scenarios on the Wellington Fault, 9 scenarios on the Wairarapa Fault and 9 scenarios on the Wharekauhau Fault. The maximum credible magnitude for each fault, obtained from the National Seismic Hazard Model, has been used to construct the source scenarios with variable slip distributions.

The modelled arrival time information includes the start time of noticeable water level anomaly (rising or ebbing), the start time of the first peak, the arrival time of the first peak, the start time of the biggest peak, and the arrival time of the biggest peak. The use of variable slip distributions for each local source and multiple virtual tsunami gauges along the coastal stretch of an individual suburb has resulted in multiple sets of arrival time data for each suburb, e.g., multiple arrival time values of the first peak, leading to a range of tsunami arrival times for a coastal suburb for each local source. To present an overall picture of arrival time distribution, the earliest, 25th, 50th and 75th percentile, and the latest of the modelled tsunami arrival times have been calculated for each coastal suburb of Wellington. No consistent patterns of arrival time distributions have been identified for tsunamis from these sources due to their different positioning relative to Wellington suburbs.

Considering that when a strong, hard-to-stand, or long-duration earthquake is felt, there is no time and expertise for coastal communities to differentiate which fault the tsunami would come from and exercise self-evacuation accordingly, it is preferable that local fault sources are considered together. When all the modelled scenarios of the four fault sources are analysed together, the modelling results show that the first peak of a tsunami would arrive at Wellington suburbs as early as 3.5~11.4 minutes following a local earthquake event.

The tsunami arrival time estimates in this study, interpreted together with evacuation time modelling in a separate study, will aid local authorities of Wellington in the identification of weak spots in current evacuation planning, and provide guidance where, for example, vertical evacuation options would be beneficial.

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7.0 ACKNOWLEDGEMENTS

This study was funded by It’s Our Fault project. The development of the modelling tools for this study were funded by GNS Science Capability Funds and National Natural Science Foundation of China (41528203). Their supports are gratefully appreciated. The authors also gratefully acknowledge NIWA, GWRC and the Department of Conservation (DoC) for giving permission to use their multi-beam bathymetry dataset of Wellington Harbour for this study (Wellington Harbour Bathymetry, 1m digital grid, Pallentin et al., 2009).

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8.0 REFERENCES Barberopoulou, A.; Wang, X.; Power, W. and Lukovic, B. 2014 Simulation of Tsunami Hazards Affecting the East Cape Region, New Zealand. Pure Appl. Geophys. DOI 10.1007/s00024-014-0842-7

Carne, R. C. and Little T. A. 2012 Geometry and scale of fault segmentation and deformational bulging along an active oblique-slip fault (Wairarapa fault, New Zealand). Geological Society of America Bulletin, July 2012, v. 124, no. 7-8, p. 1365-1381, first published on May 3, 2012.

Cho, Y.-S. 1995 Numerical simulation of tsunami and runup. PhD thesis, Cornell University. Mueller, C.; Power, W.L.; Fraser, S.A.; Wang, X. 2015 Effects of rupture complexity on local tsunami inundation : implications for probabilistic tsunami hazard assessment by example. Journal of Geophysical Research. Solid Earth, 120(1): 488-502; doi: 10.1002/2014JB011301 Mueller, C.; Wang, X; Power, W. L. 2014. Investigation of the effects of earthquake complexity on tsunami inundation hazard in Wellington Harbour, GNS Science Consultancy Report 2014/198. 43 p.

Fraser, S.A.; Power, W.L.; Wang, X.; Wallace, L.M.; Mueller, C.; Johnston, D.M. 2014 Tsunami inundation in Napier, New Zealand, due to local earthquake sources. Natural hazards, 70(1): 415445; doi: 10.1007/s110690130820x Geist, E. L. (2002), Complex earthquake rupture and local tsunamis. Journal of Geophysical Research, 107 (January 2001).

Grapes, R. and Downes, G. (1997) The 1855 Wairarapa, New Zealand, earthquake – Analysis of historical data. Bulletin of the New Zealand National Society for Earthquake Engineering, 30(4), 271–368.

Herrero, A., & P. Bernard (1994). A kinematic self-similar rupture process for earthquakes. Bulletin of the Seismological Society of America, 84(4), 1216–1228.

Jones, J.M., Ng, P., Wood, N.J., 2014, The pedestrian evacuation analyst—Geographic information systems software for modelling hazard evacuation potential: U.S. Geological Survey Techniques and Methods, book 11, chap. C9, 25 p., http://dx.doi.org/10.3133/tm11C9.

Langridge, R., Van Dissen, R., Rhoades, D., Villamor, P., Little, T., Litchfield, N., Clark, K., & Clark, D., (2011). Five thousand years of surface ruptures on the Wellington fault: Implications for recurrence and fault segmentation. Bulletin of the Seismological Society of America 101(5), 2088–2107. doi: 10.1785/0120100340.

Langridge, R.M., Ries, W.F., Litchfield, N.J., Villamor, P., Van Dissen, R.J., Rattenbury, M.S., Barrell, D.J.A., Heron, D.W., Haubrock, S., Townsend, D.B., Cox, S.C., Berryman, K.R., Nicol, A., Lee, J.M., Stirling, M.W., 2016, The New Zealand Active Faults Database. New Zealand Journal of Geology and Geophysics 59 (1): doi:10.1080/00288306.2015.1112818. Litchfield, N.J., Van Dissen, R., Sutherland, R., Barnes, P.M., Cox, S.C., Norris, R., Beavan, J., Langridge, R., Villamor, P., Berryman, K., Stirling, M., Nicol, A., Nodder, S., Lamarche, G., Barrell, D.J.A., Pettinga, J.R., Little, T., Pondard, N., Mountjoy, J.J., Clark, K., 2014, A model of active faulting in New Zealand. New Zealand Journal of Geology and Geophysics 57 (1): 32-56. doi: 10.1080/00288306.2013.854256.

Little, T. A., Van Dissen, R., Rieser, U., Smith, E. G. C., & Langridge, R. (2010). Co-seismic strike-slip at a point during the last four earthquakes on the Wellington fault near Wellington, New Zealand. Journal of Geophysical Research 115 (5). B05403. doi: 10.1029/2009JB006589.

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APPENDICES

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A1.0 SLIP DISTRIBUTIONS IN HIKURANGI SOURCE SCENARIOS

The slip distributions of the 20 weighted Hikurangi subduction interface scenarios are given in Figure A1.1 to Figure A1.20, along with their corresponding, non-weighted slip distribution scenarios.

Figure A1.1 Source scenario 1 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.2 Source scenario 2 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.3 Source scenario 3 in Hikurangi subduction interface Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.4 Source scenario 4 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.5 Source scenario 5 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.6 Source scenario 6 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.7 Source scenario 7 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both

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scenarios have the same magnitude (Mw 9.0).

Figure A1.8 Source scenario 8 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.9 Source scenario 9 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.10 Source scenario 10 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.11 Source scenario 11 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.12 Source scenario 12 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.13 Source scenario 13 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.14 Source scenario 14 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.15 Source scenario 15 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.16 Source scenario 16 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.17 Source scenario 17 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.18 Source scenario 18 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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Figure A1.19 Source scenario 19 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

Figure A1.20 Source scenario 20 in Hikurangi subduction interface. Left panel: slip distribution without slip rate deficit weighting; right panel: non-uniform slip distribution with slip rate deficit weighting used in this study. Both scenarios have the same magnitude (Mw 9.0).

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A2.0 ADDITIONAL TSUNAMI ARRIVAL TIMES

The tsunami arrival time calculations in this study produce a possible time range of each arrival, i.e., start time of noticeable disturbance, start time of first peak, arrival time of first peak, start time of biggest peak and arrival time of biggest peak for each suburb. The earliest arrival times are summarized in Table 4.2 to Table 4.5 in Section 4.0.

Further investigations indicate that the earliest arrivals at a site may be the result of only a few slip distribution scenarios. Therefore, the 25th percentile, 50th percentile, 75th percentile and the latest in the calculated arrival time range for first peaks and biggest peaks of tsunami are also presented in Table A2.1 - Table A2.8 in this appendix in order to provide an overall picture of of estimated tsunami arrival times in Wellington suburbs.

In these tables, it is obvious that the arrival times of biggest peak present a much wider time range than those of first peak at some suburbs (e.g., see Table A2.2). This may be due to the effects of two factors, the large variation of tsunami travel distances from slip asperities to the harbour, and the local resonances to tsunami waves. The latter largely contributes to the much later arrivals of biggest peak.

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A2.1 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM HIKURANGI SUBDUCTION INTERFACE

Table A2.1 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Hikurangi subduction interface scenarios. (The arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of first arrival time arrival time arrival time of first peak of first of first of first peak peak peak peak

Minutes after mainshock

Eastbourne 7.4 23.3 28.9 33.6 87.4

Seaview 8.6 30.2 32.7 38.3 87.5

Petone 7.1 30.8 33.7 37.1 62.5

Korokoro 6.9 30.0 33.2 35.8 64.2

Horokiwi 7.7 29.1 33.7 37.0 92.4

Ngauranga 8.8 29.4 33.2 36.4 95.3

Khandallah 24.3 28.5 31.8 34.7 95.6

Kaiwharawhara 24.2 28.8 31.6 35.0 95.9

Pipitea 24.0 30.6 33.2 35.7 96.1

Wellington Central 28.0 31.6 34.4 37.6 47.6

Te Aro 28.0 31.1 34.2 37.0 40.8

Oriental Bay 27.3 30.9 33.5 36.0 40.1

Roseneath 15.1 31.2 35.2 38.3 92.8

Hataitai 6.0 18.4 33.6 37.8 42.7

North of suburb group*1 7.2 18.7 24.9 37.2 40.0

Maupuia 4.7 22.4 27.7 36.5 92.2

Karaka Bays 3.6 21.6 24.2 26.0 30.6

Seatoun 4.5 20.6 23.3 25.7 52.5

Breaker Bay 4.1 16.3 20.6 21.8 79.2

Moa Point 8.9 13.6 20.0 20.6 111.9

South of suburb group*2 7.4 14.2 20.6 21.7 80.3

Houghton Bay 8.1 11.2 19.5 20.2 21.5

Island Bay 6.9 10.9 19.2 19.8 21.7

Owhiro Bay 5.6 9.2 18.6 19.3 36.6

Matiu Somes Island 9.1 29.2 33.4 36.2 90.6 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs.

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Table A2.2 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Hikurangi subduction interface scenarios (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of biggest arrival time arrival time arrival time of biggest peak of biggest of biggest of biggest peak peak peak peak

Minutes after mainshock

Eastbourne 13.8 31.1 38.4 57.1 87.6

Sea View 11.8 31.9 35.1 57.9 87.5

Petone 9.4 33.0 35.7 41.1 66.6

Korokoro 9.4 32.2 34.2 38.0 64.2

Horokiwi 9.1 31.4 34.8 38.3 92.7

Ngauranga 25.2 30.4 34.3 40.2 95.3

Khandallah 24.3 28.5 34.0 42.8 95.6

Kaiwharawhara 24.2 28.8 33.9 43.4 95.9

Pipitea 24.0 30.7 35.0 51.4 96.1

Wellington Central 28.0 31.7 40.3 70.7 90.4

Te Aro 28.0 31.4 39.7 70.2 90.4

Oriental Bay 27.3 31.4 38.6 59.9 91.9

Roseneath 16.7 34.6 38.6 65.5 1782.2

Hataitai 18.3 35.2 63.1 71.4 131.5

North of suburb group*1 19.2 37.9 64.9 70.5 131.9

Maupuia 16.6 30.0 39.3 65.3 1792.2

Karaka Bays 24.0 28.5 49.3 50.7 83.9

Seatoun 23.3 47.9 49.6 51.3 115.0

Breaker Bay 9.7 21.1 42.9 46.8 115.0

Moa Point 8.9 20.2 20.8 45.0 111.9

South of suburb group*2 8.1 20.9 22.0 42.1 80.3

Houghton Bay 8.1 14.3 19.6 20.3 21.5

Island Bay 6.9 12.4 19.3 19.9 47.9

Owhiro Bay 5.6 12.5 18.8 19.5 51.1

Matiu Somes Island 9.1 30.1 33.9 36.8 90.6 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs.

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A2.2 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM WELLINGTON FAULT

Table A2.3 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Wellington Fault scenarios (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of first peak arrival time arrival time arrival time of first peak of first peak of first peak of first peak

Minutes after mainshock

Eastbourne 9.8 27.8 40.0 62.0 163.0

Sea View 7.8 33.4 33.8 42.1 143.3

Petone 4.8 21.1 37.6 63.9 151.9

Korokoro 4.0 4.8 46.4 66.4 80.8

Horokiwi 4.0 9.0 48.0 54.8 165.2

Ngauranga 4.6 9.1 10.3 65.2 169.2

Khandallah 3.9 11.0 12.3 73.4 90.9

Kaiwharawhara 11.7 11.9 12.2 73.4 90.4

Pipitea 11.9 13.1 74.4 89.9 128.9

Wellington Central 15.7 45.5 61.1 89.0 128.9

Te Aro 15.8 46.5 61.1 88.0 128.8

Oriental Bay 46.5 46.9 61.4 87.1 88.0

Roseneath 60.3 63.4 75.1 85.5 88.5

Hataitai 16.7 65.3 66.8 96.2 109.5

North of suburb 15.8 64.3 65.8 108.5 109.7 group*1

Maupuia 5.1 65.0 68.7 87.6 109.3

Karaka Bays 19.9 23.4 33.0 33.6 152.3

Seatoun 19.7 20.5 23.4 32.3 151.8

Breaker Bay 12.8 17.9 27.9 29.3 151.8

Moa Point 13.2 13.5 14.3 30.3 184.6

South of suburb 11.6 16.6 28.1 29.1 185.4 group*2

Houghton Bay 11.5 11.6 12.0 12.2 77.1

Island Bay 10.0 10.5 12.0 22.1 120.6

Owhiro Bay 6.3 9.6 19.7 32.4 60.4

Matiu Somes Island 3.7 79.5 80.2 82.9 99.4 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs.

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Table A2.4 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Wellington Fault scenarios (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of biggest arrival time arrival time arrival time of biggest peak of biggest of biggest of biggest peak peak peak peak

Minutes after mainshock

Eastbourne 14.2 43.0 62.1 76.2 211.3

Sea View 12.7 100.8 101.1 101.4 143.6

Petone 4.8 67.5 85.6 99.4 225.2

Korokoro 4.2 67.5 80.4 149.3 170.8

Horokiwi 4.0 47.8 79.3 165.0 170.7

Ngauranga 8.8 10.3 133.8 134.4 170.0

Khandallah 10.9 12.4 90.6 133.4 134.2

Kaiwharawhara 11.9 12.2 89.9 133.1 133.4

Pipitea 11.9 15.1 89.6 90.5 133.5

Wellington Central 15.7 88.9 89.5 90.1 128.9

Te Aro 15.8 88.0 89.1 90.1 128.8

Oriental Bay 87.1 87.4 88.0 88.5 90.4

Roseneath 60.3 63.8 85.5 88.0 240.5

Hataitai 16.7 65.3 67.0 96.2 109.5

North of suburb group*1 15.8 64.8 81.9 108.5 109.7

Maupuia 17.2 65.3 69.9 94.4 239.0

Karaka Bays 32.2 70.2 93.1 152.1 152.4

Seatoun 19.8 25.6 70.6 151.8 153.2

Breaker Bay 12.8 28.0 30.1 88.3 151.8

Moa Point 13.2 14.3 33.0 183.9 184.9

South of suburb group*2 12.0 28.2 30.0 85.5 186.7

Houghton Bay 11.6 11.7 12.1 22.3 77.7

Island Bay 10.0 11.6 22.1 33.1 180.5

Owhiro Bay 6.5 22.2 32.4 49.3 78.5

Matiu Somes Island 61.6 79.5 80.4 82.9 99.4 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs.

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A2.3 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM WAIRARAPA FAULT

Table A2.5 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of first peak arrival time arrival time arrival time of first peak of first peak of first peak of first peak

Minutes after mainshock

Eastbourne 15.1 36.4 39.4 41.0 70.2

Sea View 11.7 14.6 16.7 24.3 49.6

Petone 7.4 9.3 9.8 24.0 27.9

Korokoro 7.5 9.3 10.3 24.0 26.9

Horokiwi 8.4 9.2 10.1 24.1 25.3

Ngauranga 8.9 9.8 12.0 23.3 25.3

Khandallah 11.0 12.5 12.7 23.2 24.1

Kaiwharawhara 12.2 12.4 12.5 23.7 24.6

Pipitea 11.3 12.2 12.7 13.8 25.1

Wellington Central 11.4 11.6 11.9 12.6 19.0

Te Aro 11.2 11.3 11.5 11.9 19.0

Oriental Bay 10.7 11.1 12.1 13.7 14.2

Roseneath 11.3 15.2 15.9 16.7 18.1

Hataitai 15.4 16.4 16.9 17.5 164.6

North of suburb group*1 14.2 16.8 18.4 18.7 113.1

Maupuia 10.8 16.5 16.9 17.4 165.1

Karaka Bays 3.9 38.9 39.5 40.2 41.2

Seatoun 4.4 37.9 39.3 40.0 41.4

Breaker Bay 3.5 33.2 35.3 37.0 40.0

Moa Point 9.4 29.0 32.9 34.1 38.9

South of suburb group*2 5.7 9.8 27.7 37.6 40.7

Houghton Bay 4.6 8.8 33.5 34.8 37.3

Island Bay 5.1 7.6 33.3 34.7 38.7

Owhiro Bay 5.2 6.2 7.2 35.7 38.9

Matiu Somes Island 10.8 11.5 18.4 22.8 24.7 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs.

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Table A2.6 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of biggest arrival time arrival time arrival time of biggest peak of biggest of biggest of biggest peak peak peak peak

Minutes after mainshock

Eastbourne 15.1 36.4 39.4 41.0 190.9

Sea View 11.7 14.6 16.7 24.3 51.7

Petone 7.4 9.4 9.9 26.7 161.9

Korokoro 7.5 9.3 10.3 24.0 26.9

Horokiwi 8.4 9.2 10.1 24.1 25.3

Ngauranga 8.9 9.8 12.0 23.3 25.3

Khandallah 11.0 12.5 12.7 23.2 24.1

Kaiwharawhara 12.2 12.4 12.5 23.7 24.6

Pipitea 11.3 12.2 12.7 13.8 25.1

Wellington Central 11.4 11.6 11.9 12.6 19.0

Te Aro 11.2 11.3 11.5 11.9 19.0

Oriental Bay 10.7 11.1 12.1 13.7 14.2

Roseneath 11.3 15.2 15.9 16.7 18.1

Hataitai 15.4 16.4 16.9 17.5 165.0

North of suburb group*1 14.2 16.8 18.4 18.7 165.8

Maupuia 10.8 16.5 16.9 17.4 165.1

Karaka Bays 38.5 39.2 39.8 40.7 41.2

Seatoun 37.7 39.1 39.8 40.7 41.4

Breaker Bay 32.3 33.9 35.5 37.0 40.0

Moa Point 9.4 29.5 33.2 34.1 38.9

South of suburb group*2 6.3 27.6 28.4 38.1 40.7

Houghton Bay 32.8 33.6 34.8 35.3 37.3

Island Bay 6.0 34.2 34.9 35.3 67.7

Owhiro Bay 5.5 25.0 35.8 37.0 137.2

Matiu Somes Island 10.8 11.5 18.4 22.8 24.7 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs.

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A2.4 ARRIVAL TIME STATISTICAL ANALYSIS FOR TSUNAMI FROM WHAREKAUHAU FAULT

Table A2.7 Statistical analysis of first peak arrival times at coastal fronts of Wellington Harbour suburbs for Wharekauhau Fault scenarios (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of first peak arrival time arrival time arrival time of first peak of first peak of first peak of first peak

Minutes after mainshock

Eastbourne 20.8 33.0 35.2 39.0 123.0

Sea View 39.7 95.2 121.1 121.7 123.2

Petone 29.5 42.8 44.4 45.6 149.5

Korokoro 28.8 45.3 45.7 46.1 48.2

Horokiwi 27.5 46.1 47.2 47.5 73.7

Ngauranga 9.4 47.3 164.4 167.6 168.2

Khandallah 10.0 10.2 112.3 168.1 168.4

Kaiwharawhara 10.2 10.5 80.9 81.0 168.1

Pipitea 10.7 12.3 28.0 80.5 109.4

Wellington Central 12.7 12.9 12.9 28.0 54.4

Te Aro 12.7 12.9 13.5 52.0 54.8

Oriental Bay 12.5 13.6 13.7 52.7 81.9

Roseneath 12.5 14.8 15.9 83.3 107.4

Hataitai 15.3 15.9 16.0 16.3 352.2

North of suburb group*1 14.2 14.5 15.0 57.5 87.0

Maupuia 14.0 15.7 16.0 56.6 139.6

Karaka Bays 18.1 19.1 28.9 29.8 39.7

Seatoun 15.6 18.3 28.8 37.4 41.2

Breaker Bay 10.9 12.0 13.4 14.6 38.3

Moa Point 10.1 11.2 11.6 12.0 28.9

South of suburb group*2 9.2 11.7 12.5 13.6 14.3

Houghton Bay 10.0 10.3 10.9 11.0 11.4

Island Bay 8.8 9.8 10.1 10.7 11.5

Owhiro Bay 8.5 9.2 9.7 10.2 10.9

Matiu Somes Island No Value*3 No Value*3 No Value*3 No Value*3 No Value*3 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs. *3 No Value means no tsunami elevation 0.2m higher than ambient sea condition was detected for this suburb among all the simulated scenarios in this source region. This implies that tsunami threat level at this suburb would fall to “No Threat” category should an earthquake with modelled magnitude occur in this source region.

GNS Science Report 2016/03 57

Table A2.8 Statistical analysis of biggest peak arrival times at coastal fronts of Wellington Harbour suburbs for Wairarapa Fault scenarios (the arrival times are measured in minutes after mainshock).

Suburb Name Earliest 25th 50th 75th Latest arrival time percentile percentile percentile arrival time of biggest arrival time arrival time arrival time of biggest peak of biggest of biggest of biggest peak peak peak peak

Minutes after mainshock

Eastbourne 32.4 35.9 59.2 94.2 342.6

Sea View 47.7 121.2 121.4 122.0 317.4

Petone 45.1 73.5 146.6 148.0 173.3

Korokoro 45.1 45.7 48.2 165.5 166.1

Horokiwi 46.4 47.3 47.5 48.2 322.3

Ngauranga 47.0 166.6 167.6 167.8 168.3

Khandallah 80.9 167.9 168.1 168.3 168.4

Kaiwharawhara 80.9 81.0 167.8 167.9 168.2

Pipitea 11.3 109.0 109.2 109.4 184.8

Wellington Central 107.9 108.4 108.7 109.0 109.4

Te Aro 107.9 108.2 108.4 108.6 108.8

Oriental Bay 108.4 108.6 108.9 109.1 110.1

Roseneath 13.8 15.3 107.3 108.6 352.8

Hataitai 15.6 86.5 86.9 87.3 352.8

North of suburb group*1 86.1 86.4 86.8 87.3 103.5

Maupuia 14.5 16.1 86.9 104.5 352.4

Karaka Bays 29.4 39.8 40.6 160.1 160.6

Seatoun 37.3 38.0 39.3 40.0 42.7

Breaker Bay 10.9 35.6 36.6 37.6 68.6

Moa Point 10.1 26.1 27.8 28.0 29.0

South of suburb group*2 10.3 27.4 28.1 28.6 34.3

Houghton Bay 10.3 22.8 23.2 25.5 65.2

Island Bay 8.8 10.7 24.3 34.7 76.7

Owhiro Bay 9.3 26.2 35.4 36.2 92.9

Matiu Somes Island No Value*3 No Value*3 No Value*3 No Value*3 No Value*3 *1,*2 The four adjacent suburbs, Kilbirnie, Miramar, Rongotai and Lyall Bay, are grouped together as one suburb group. This is because a large tsunami event may flood them together from either northern coast*1 or southern coast*2 due to low-lying, flat nature of the topography for these four neighbouring suburbs. *3 No Value means no tsunami elevation 0.2m higher than ambient sea condition was detected for this suburb among all the simulated scenarios in this source region. This implies that tsunami threat level at this suburb would fall to “No Threat” category should an earthquake with modelled magnitude occur in this source region.

58 GNS Science Report 2016/03

Principal Location Other Locations

1 Fairway Drive Dunedin Research Centre Wairakei Research Centre National Isotope Centre Avalon 764 Cumberland Street 114 Karetoto Road 30 Gracefield Road PO Box 30368 Private Bag 1930 Wairakei PO Box 31312 Lower Hutt Dunedin Private Bag 2000, Taupo Lower Hutt New Zealand New Zealand New Zealand New Zealand T +64-4-570 1444 T +64-3-477 4050 T +64-7-374 8211 T +64-4-570 1444 www.gns.cri.nz F +64-4-570 4600 F +64-3-477 5232 F +64-7-374 8199 F +64-4-570 4657