Is the Wairarapa Fault Slip Rate Decreasing to the North? GNS Science Consultancy Report 2008/170

CONFIDENTIAL

This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to the Earthquake Commission. Unless otherwise agreed in writing, all liability of GNS Science to any other party other than the Earthquake Commission in respect of the report is expressly excluded.

The data presented in this Report are available to GNS Science for other use from July 2008

BIBLIOGRAPHIC REFERENCE

Villamor, P.; Langridge, R. M.; Ries, W.; Carne, R.; Wilson, K.; Seebeck, H.; Cowan, L. 2008. It’s Our Fault – Wairarapa Fault Slip Rate Investigations Task: Completion Report – Is the Wairarapa Fault slip rate decreasing to the north? GNS Science Consultancy Report 2008/170. 58p.

Project Number: 430W3110 Confidential 2008

CONTENTS

EXECUTIVE SUMMARY ...... IV 1.0 INTRODUCTION ...... 1 2.0 SCOPE OF WORK...... 2 3.0 THE WAIRARAPA FAULT: AGES OF DISPLACED GEOLOGIC FEATURES FAULT, SLIP RATE, AND SINGLE EVENT DISPLACEMENT...... 2 3.1 Ages of displaced geomorphic features ...... 2 3.2 Fault slip rate...... 3 3.3 Right-lateral displacements along the Wairarapa Fault ...... 3 4.0 RE-ASSESSMENT OF DISPLACEMENTS ALONG THE NORTHERN SECTOR OF THE WAIRARAPA FAULT ...... 4 5.0 NEW SLIP RATE ESTIMATES ALONG THE NORTHERN SECTOR OF THE WAIRARAPA FAULT ...... 5 5.1 Kopuaranga site ...... 5 5.2 Ruamahanga site ...... 7 6.0 DISCUSSION ...... 9 6.1 Is fault slip rate decreasing to the north? ...... 9 6.2 Are lateral displacement values along the Wairarapa Fault diminishing to the north? ...... 10 6.3 Is single event displacement associated with the 1855 Wairarapa earthquake surface rupture diminishing north?...... 11 6.4 How can slip rate decrease northwards? ...... 11 6.5 Where does the Wairarapa Fault slip rate decreases and why? ...... 12 7.0 CONCLUSIONS ...... 13 8.0 RECOMMENDATIONS ...... 14 9.0 ACKNOWLEDGEMENTS ...... 14 10.0 REFERENCES ...... 15

TABLES

Table 1 Lateral and vertical displacement and slip rate values at Ruamahanga and Kopuaranga sites...... 17 Table 2 Radiocarbon ages...... 18 Table 3 OSL ages...... 18 Table 4 Future study sites ...... 18 Table A1 OSL samples collected ...... 42 Table A2 Radiocarbon samples collected ...... 43 Table1-Appendix 4 Doserate contribution of cosmic radiation...... 46 Table2-Appendix 4 Radionuclide and water contents...... 46 Table3-Appendix 4 Measured a-value and equivalent dose, doserate and luminescence age ...... 46

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FIGURES

Figure 1 Active faults in the Wellington region from GNS Active Fault database (http://data.gns.cri.nz/af/). Stars are main towns. Numbers are previously published slip rate values in mm/yr from Van Dissen and Berryman (1996; Tea Creek site), Schermer et al, (2004; Alfredton trench site), Wellman (1972; Waiohine site), Wang and Grapes (2007; Waiohine site) and Little et al. (in prep; Waiohine site). Insert: tectonic setting of New Zealand. Plate motion directions and rate according to DeMets et al. (1994). Coordinates are New Zealand Map Grid...... 19 Figure 2 Simplified geology map showing relationships between Wairarapa Fault (bold red line) and greywacke-dominated basement rocks, overlaying Tertiary sediments and Quaternary alluvial terraces and fans. Mapping and ages of quaternary alluvial terraces and fans from Begg & Johnston (2000) and Lee & Begg (2002). Faults from GNS active fault database (http://data.gns.cri.nz/af/)...... 20 Figure 3 Plot of displacement values along the fault trace from published literature and this study (coloured by source). For more details on displacements see Appendix 1. Digital Elevation Model from LINZ...... 21 Figure 4-A & B Plots of horizontal displacement data available along the Wairarapa Fault. Displacement data classified by author (both plots contain same data but are plotted with different scales). See also Appendix 1...... 22 Figure 4-C & D Plots of available horizontal displacement data along the Wairarapa Fault. Selected displacement data for all geomorphic features classified by age (both plots contain same data but are plotted with different scales). Ages assigned to displacements are taken from existing mapping, most of which have not being verified with absolute ages. See also Appendix 1...... 23 Figure 4-E & F Plots of available horizontal displacement data along the Wairarapa Fault. Selected displacement data for streams and channels classified by age (both plots contain same data but are plotted with different scales). Ages assigned to displacements are taken from existing mapping, most of which have not been verified with absolute ages. See also Appendix 1...... 24 Figure 5 Kopuaranga site: A, Quaternary geology (based on Townsend et al., 2002) laid over aerial photo (NZ Aerial Mapping).Labels in brackets and italics indicate newly assigned terrace ages from this study. White circle is location of stratigraphic column in Figure 6. B, Digital terrain model for the Kopuaranga site with location of channels, piercing points (maximum and minimum estimates shown by white lines), and assessed lateral displacements for channel 1 and 2 (yellow and pink arrows, respectively). Trenches are shown with black rectangles...... 25 Figure 6 Stratigraphy of the main terrace at Kopuaranga (Penn) site. Composite stratigraphic column of the terrace exposure at the man made canal. Upper 5.5 m consists of loess deposits. These are not exposed in the canal but inferred from stratigraphy in trenches. Elevations of channel units exposed in different trenches are shown. Elevation for “channel axis bottom” indicates depth of incision of channel into the terrace loess cover...... 26 Figure 7-A Kopuaranga site trenches. Penn 1 trench log. See Figure 5 for location and Appendix 2 for unit descriptions...... 27 Figure 7-B Kopuaranga site trenches. Penn 2 trench log. See Figure 5 for location and Appendix 2 for unit descriptions...... 28 Figure 8 Ruamahanga site: A, Quaternary geology of the area (based on Townsend et al 2002) laid over aerial photo (NZ Aerial Mapping). Labels in brackets and italics indicate newly assigned terrace ages from this study (see Appendix 5 for details). B, Digital terrain model for the Kopuaranga site with location of channels; piercing points (small line across fault indicate maximum and minimum values) used for lateral displacement measurements. Trenches are shown with black rectangles...... 29 Figure 9A Ruamahanga site, Barton 1 trench log. See Figure 8 for location and Appendix 2 for unit descriptions...... 30 Figure 9B Ruamahanga site Barton 2 trench log. See Figure 8 for location and Appendix 2 for unit descriptions...... 31 Figure 9C Ruamahanga site Barton 3 trench log. See Figure 8 for location and Appendix 2 for unit descriptions...... 32 Figure 10 Variation of slip rate and youngest single event displacement along the Wairarapa Fault...... 33 Figure 11 Hypothetical model for rupture history at Kopuaranga site. The stratigraphy in Penn 1 and Penn 2 trenches (Fig. 7) shows a repeated sequence of gravel overlain by silt and/or sand, which represents repeated cycles of increased to decreased sediment load in the channel (each cycle is presented here by a different coloured channel unit). We hypothesise that each sequence is related to an earthquake event on the Wairarapa Fault. After each earthquake the stream could have a period of high sediment load (caused by landsliding in the catchment of the stream and exposure of

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sediments at fault trace) followed by a period of decreased sediment load (fault quiescence). In this preliminary rupture model, we can account for 5 surface ruptures. Penn 1 trench has 4 sequences (1 to 4 from bottom to top; Fig 7a) while Penn 2 trench has two sequences (5 to 6 from bottom to top; Fig 7a). The first sequence in a newly formed stream might not necessarily need to be related to an earthquake, so it is possible that sequence 1 (Penn 1 trench) is not related to an earthquake. However, in a newly formed stream, when the older stream is beheaded by faulting, the first gravel package (sequence 5 in Penn 2 trench) is related to the event that beheaded the older stream and it is formed prior to the new channel being offset. Alternatively, deposition of these sequences (or some of them) could be related to storms. A paleoseismic trench across the fault trace to investigate the number of event associated to the ~ 40m displacement could elucidate this hypothesis...... 34 Figure 12 Kinematic model for the transfer of lateral slip rate from the Carterton, Masterton and Mokonui faults (CMM) onto Wairarapa Fault. Estimates of amount of slip rate transferred from CMM to central sector of the Wairarapa Fault for a combined slip rate for CMM of 5 mm/yr or 4 mm/yr (this latter in brackets)...... 35 Figure A1-A Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Location of profiles 1 to 11...... 49 Figure A1-B Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Correlation of terraces across the Ruamahanga River...... 50 Figure A1-C Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Topographic profiles along different age terraces of Ruamahanga River...... 51 Figure A1- D Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Topographic profiles along different age terraces of Ruamahanga River, and along Kopuaranga River. Profiles start at point where both rivers merge...... 52 Figure A1- E Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Correlation of Ruamahanga River terraces with Kopuranga River terraces...... 53

APPENDICES

Appendix 1 Database of displacement measures along the Wairarapa Fault...... 37 Appendix 2 Unit descriptions ...... 38 Appendix 3 Samples collected...... 42 Appendix 4 Report on OSL dating ...... 44 Appendix 5 Correlation of terraces of Ruamahanga and Kopuranga Rivers ...... 47

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EXECUTIVE SUMMARY

We have assessed the dextral slip rate of the Wairarapa Fault along its northern sector, north of the intersection with the Carterton Fault, at two new sites: Kopuranga River and Ruamahanga River terraces. Prior to this study, sparse information concerning the slip rate of the Wairarapa Fault pointed to a possible northward decrease of slip rate. The slip rate on the central sector of the fault has been estimated at ~8-13 mm/yr at the well-known location of the Waiohine terraces. On the northern sector at Tea Creek site, a ~ 6 mm/yr slip rate has been published. We have obtained new slip rate values on the northern sector to better assess the possible decrease of slip rate to the north. At the Kopuaranga site we have obtained a maximum slip rate value of ~8 mm/yr and at the Ruamahanga site we have obtained a minimum value of 1.2-1.6 mm/yr. While we hope to obtain a representative minimum value for the Kopuaranga site with results from a newly submitted OSL sample, we cannot constrain the values at the Ruamahanga site with anymore certainty. Slip rate values based on well-constrained offset channels at the Ruamahanga site are much lower than expected (~1.2-1.6 mm/yr). We present possible explanations that could explain this value but we cannot fully confirm any of them.

Our preliminary values agree with slip rate data previously published from the Tea Creek site and thus we propose that the slip rate along the Wairarapa Fault could decrease northwards. The exact decrease is hard to evaluate at this stage within the uncertainties of our results but the current data suggest a 1.5-7.1 mm/yr decrease from the central to the northern sector. This statement may have to be re-assessed when results from new work at Waiohine terraces (Its Our Fault- Wairarapa Pat Surface Rupture Timing Task) are final. Through a reassessment of the published displacement values along the fault undertaken as part of this study, and with some assumptions about the value of the single event displacement along its northern sector, we can explain a ~4 mm/yr decrease in slip rate as a consequence of the decrease in displacement per event along strike. This implies that the Wairarapa Fault always ruptures with lengths similar to the 1855 earthquake. However, with the uncertainty of our values we can not fully rule out the possibility that part of the decrease is achieved by segmented ruptures of the fault so that the southern sector ruptures more frequently. Fault segmentation could be assessed with new investigations of the rupture history along the northern sector of the Wairarapa Fault.

Within the uncertainties of our results and of some of the published data, we propose that deformation is transferred between the Carterton, Masterton and Mokonui faults onto the central sector of the Wairarapa Fault. If we assigned a ~4-5 mm/yr to the combined lateral slip rate of the Carterton, Masterton and Mokonui faults as proposed by previous studies, the amount of lateral slip transferred from the Carterton, Masterton and Mokonui faults to the central sector of the Wairarapa Fault is ~3.6 to 4.5 mm/yr. In this kinematic model, decrease in lateral slip rate along the northern section of the fault takes place at each intersection with the Carterton, Masterton and Mokonui faults. Most of this decrease probably takes place at the intersection with the Carterton Fault (which has the highest slip rate of the three). Slip rate decrease at the other two intersections is possibly much smaller and lies within the uncertainty of slip rate data.

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1.0 INTRODUCTION

This report presents results from It’s Our Fault-Wairarapa slip rate investigations task. The aim of this project is assessing the possible northwards decrease in slip rate along the Wairarapa Fault as suggested by scarce published data. For that purpose we have assessed the slip rate at two new sites, Ruamahanga and Kopuaranga sites, to confirm lower slip rate values along the northern sector of the fault. Northwards decrease in slip rate along the Wairarapa Fault cold be related to possible westward transfer of deformation from the Masterton, Carterton and Mokonui faults to the Wairarapa Fault (Fig. 1)

Despite being one of the major active faults in the North Island, New Zealand, and having ruptured in historical times (the M 7.9-8.2 1855 Wairarapa Earthquake; Darby and Beanland, 1992; Grapes and Downes 1999), slip rate and single event displacement of the Wairarapa Fault at its northern sector (north of the intersection with the Carterton Fault; Fig. 1) are poorly understood. During the late 1960’s to the 80’s, several studies measured displacement of geomorphic features along the fault (Lensen, 1968, 1969; Lensen and Vella, 1971; Wellman, 1972; Grapes et al., 1984 and Grapes and Wellman, 1988). More recent studies (Van Dissen and Berryman, 1996; Rodgers and Little, 2006; Carne et al., 2007; Wang and Grapes, 2008) have concentrated on better quantifying slip rate, single event displacement and earthquake history for specific locations on the fault (Fig. 1). Only one of these recent studies (Van Dissen and Berryman, 1996) is located within the sector of interest of this study.

Published Wairarapa Fault slip rate values (Lensen and Vella, 1971; Van Dissen and Berryman, 1996; Carne et al., 2006; Wang and Grapes, 2008; Little et al., in prep; Fig.1) point to a possible decrease in slip rate of the Wairarapa Fault from south to north (Beanland, 1995). If this is correct and not an artefact of sparse data, the decrease in slip rate has important implications for the kinematics of the area as well as for hazard estimates.

At the northern sector of the Wairarapa Fault trace, several NE striking faults splay towards the east (Fig 1: Carterton, Masterton and Mokonui faults; see Langridge et al., 2005 and references therein). These active faults could play an important role in the transfer of deformation form the reverse tectonic province of the onshore accretionary prism in the Hawke’s Bay area to the North Island Dextral Fault Belt (i.e., Wairarapa Fault) (Beanland & Haines, 1998; Wallace et al., 2004). The way that slip rate is distributed along the Wairarapa, Carterton, Masterton and Mokonui faults could help our understanding of this kinematic interaction.

In terms of hazard estimates, it is crucial to understand whether the Wairarapa Fault always ruptures with lengths similar to the 1855 earthquake (Schermer et al., 2004, postulated that the southern part of the Alfredton Fault ruptured in 1855; Fig. 1), or if shorter segments of the fault rupture independently. Assessment of slip rate decrease can bring insights into fault segmentation.

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2.0 SCOPE OF WORK

In this study, we intend to analyse if slip rates on the northern sector of the Wairarapa Fault have lower values than slip rate in the central sector (Waiohine terraces; Fig. 1) We define the northern sector of the fault from its intersection with the Carterton Fault to the Alfredton Fault (Fig 1). The central sector extends from the intersection with the Carterton Fault to the northern end of the Wharekauhau thrust (Fig. 1). For the purpose of this study, we have revisited sites with published measurements of displaced geomorphic features along the northern sector of the Wairarapa Fault to evaluate the validity of previous estimates. We have also measured new sites, and selected two sites for detailed studies. These two sites are located at the Ruamahanga River terraces and the Kopuranga River terraces (Fig. 1).

At the two selected sites, we have produced detailed digital terrain models, excavated trenches across offset streams and attempted to date the stream deposits. With this information, we have been able to estimate a new slip rate for the northern sector of the Wairarapa Fault at the Kopuranga and Ruamahanga sites. We also present some preliminary interpretations of the single event displacement based on comparison between our new displacement data and existing data along the whole fault. Results are presented in this report. The report has been internally reviewed by Russ Van Dissen and Dougal Townsend.

3.0 THE WAIRARAPA FAULT: AGES OF DISPLACED GEOLOGIC FEATURES FAULT, SLIP RATE, AND SINGLE EVENT DISPLACEMENT

The Wairarapa Fault is a prominent feature in the landscape of the Wairarapa region (Figs.1 and 2). The southernmost sector the Wairarapa Fault extends from offshore in Cook Strait (Barnes et al., 2008) to onshore where it crosses the Rimutaka Ranges up to the southern end of Lake Wairarapa. At this location the Wairarapa Fault merges with the Wharekauhau thrust, the eastern boundary of the ranges. The central sector of the fault extends from the southern end of Lake Wairarapa to the intersection with the Carterton Fault. Here, the Wairarapa Fault itself is the boundary between the greywacke-dominated Rimutaka Ranges on the west and the Quaternary alluvial deposits of the Wairarapa plains to the east. North of this intersection, the fault trace diverges from the eastern boundary of the greywacke ranges and crosses remnants of old Quaternary terraces and Pliocene and Miocene deposits (Begg & Johnston, 2000; Zachariasen et al., 2000; Lee & Begg, 2002; Townsend et al., 2002; Fig. 2). In this northern sector, the Carterton, Masterton and Mokonui faults splay from the Wairarapa Fault with a NE to ENE strike (Langridge et al., 2005). North of Mauriceville (Fig. 1), the fault splays into two faults, the Pa Valley Fault and the Alfredton Fault. The northern extent of the 1855 Wairarapa earthquake surface rupture extended into at least the southern part of the Alfredton Fault (Schermer et al., 2004).

3.1 Ages of displaced geomorphic features

The Wairarapa Fault displaces right-laterally and vertically different geomorphic elements. The ages assigned to these geomorphic features are crucial for estimates of fault slip rate. In some areas large rivers, such as the Tauherenikau, Waiohine, and Ruamahanga rivers (Fig.

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1) have deposited extensive greywacke gravel dominated alluvial fans, terraces and floodplains. The main alluvial terraces date from Isotope Stage 8 (~300 ka) to the present and have been assigned ages based on isotope stages (e.g., Q8a to Q1a, Begg and Johnston, 2000; Lee and Begg, 2002; Townsend et al., 2002). Ages of most of these terraces and fans are based on regional mapping with scarce absolute dates. In the North Island, the most extensive terrace (Q2a) is the aggradation terrace associated with the last glacial period, which is known as the Ohakean surface (Milne, 1973). A general age for the Ohakean surface across the whole North Island has been estimated at 18 ± 3 ka (Litchfield and Berryman, 2005). In the Wairarapa area, the most extensive terrace is locally known as the Waiohine surface. Ages of the Waiohine surface in the Wairarapa area range from ~28 to 10 ka (see Wang & Grapes, 2008 and references therein; Little et al., in prep.). In the south- eastern Wairarapa region, Formento-Triglio et al. (2002) distinguish between 30-15-kyr-old Ohakean gravels (that they define as “older Waiohine gravels”) and 13 to <10 k/yr-old Waiohine gravels based on Optically Stimulated Luminesce (OSL) dates.

3.2 Fault slip rate

Slip rate has previously been calculated at several locations along the Wairarapa Fault. The best known site is the Waiohine River terraces (Fig. 1), south of the intersection of the Carterton Fault with the Wairarapa Fault. At this site, estimates of right-lateral slip rate range from ~ 8 to13 mm/yr (Lensen and Vella, 1971; Grapes and Wellman, 1988; Wang, 2001; Carne et al., 2007; Wang and Grapes, 2008; Little et al., in prep). Farther north at the Tea Creek site (Fig. 1), close to the intersection with the Mokonui Fault, estimates are somewhat smaller and range from 5.9 to 6.5 mm/yr (Van Dissen and Berryman, 1996). In the north, the Wairarapa Fault splays to become the Pa Valley and Alfredton faults. Estimates of right- lateral slip rate on the Alfredton Fault yield c. 3 mm/yr (Schermer et al., 2004; Fig.1) but, because there is no available data on the slip rate of the Pa Valley fault, we cannot fully compare slip rate in the north with that calculated farther south. Clearly, more information is required to confirm and quantify the possible variation of slip rate along the Wairarapa Fault.

3.3 Right-lateral displacements along the Wairarapa Fault

Geomorphic and geologic features that are offset along the fault include a wide range of single event displacement values associated with the 1855 Wairarapa earthquake, as well as larger values representing accumulated displacement (Lensen, 1968, 1969; Grapes et al., 1984; Grapes & Wellman, 1988; Van Dissen & Berryman, 1996; Little & Begg, 2005; Rodgers & Little, 2006). In Figure 3, we present the location of published offset values and plot these data against distance from the coast along the fault in Figures 4A to 4F.

Recent studies have shown that the surface rupture of 1855 Wairarapa earthquake produced one of the largest single event displacements associated with a strike-slip fault worldwide. South of the intersection with the Carterton Fault, Rodgers and Little (2006) estimated an average single event displacement for the central sector of the fault of 15.5 ± 1.4 m. This estimate is larger and supersedes displacement values reported by previous studies (e.g., Grapes & Wellman, 1988; Grapes, 1999; see Rodgers & Little, 2006 for detailed discussion).

Along the Alfredton Fault, the 1855 earthquake produced a co-seismic displacement of 4-7 m (Schermer et al., 2004), much smaller than the displacement reported for the central sector

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of the Wairarapa Fault (15.5 ± 1.4 m: Rodgers and Little, 2006). Although these data suggest a northward decrease in the single event displacement associated with the 1855 earthquake, it is unclear how this decrease is achieved. On the Wairarapa Fault and north of the intersection with Carterton Fault, there are no reports on the displacement associated with the 1855 earthquake. At the Tea Creek site (Fig. 1), Van Dissen and Berryman (1996) report a displacement of ~40 m of a stream across the Wairarapa Fault that was achieved in 4 surface rupturing events. This suggests a mean single event displacement of ~10 m at this location.

4.0 RE-ASSESSMENT OF DISPLACEMENTS ALONG THE NORTHERN SECTOR OF THE WAIRARAPA FAULT

We undertook field reconnaissance to re-measure some of the published offsets and to select sites for detailed studies. Most of those measurements were undertaken with the intention of replicating published values so that the source for discrepancies between different authors could be better understood. We took ~30 measurements (Fig. 3, Appendix 1) using a tape measure and estimated uncertainties in the field based on the shape of the displaced geomorphic features. These measurements have large uncertainties associated with them but give an indication of which of the published values are closer to our values and could be used in this study.

We have produced several plots from the lateral offset database compiled for this study to assess variation of strike-slip along the fault (Fig. 4). Because of the local nature of vertical displacements along a strike-slip fault in general, we have not analysed variation of vertical displacement along the Wairarapa Fault. Figure 4A and 4B present the data contained in our database classified by author. For the northern sector of the Wairarapa Fault, we find that measurements from Grapes and Wellman (1988) range from 11.5 to 380 m (Fig. 4A) and present 3 well defined clusters at ~12, 24 and 65 m. Values from Lensen (1968, 1969) range from 4.5 to 140 m, and present no clear clusters. New measurements from this study are similar to those presented by Lensen (1968). It is remarkable that Grapes and Wellman’s cluster values are poorly represented in the other two datasets (Lensen’s and this study). Similar discrepancies are reported in Little & Rodgers (2008) for their re-assessment of the most recent displacements in the central sector of the fault compared to Grapes & Wellman (1988).

Figures 4C to 4F were produced from our new data, and some of the validated published data. From the published data, and for sites that were not re-visited during this study, we have selected the most recent published offset values (e.g., Rodgers & Little, 2006 values from Pigeon Bush site) or values which, through our previous evaluation, we believe to be more comparable with our data (e.g., we have favoured Lensen, 1968 and 1969, values to those of Grapes and Wellman, 1988, for the same site).

We have also re-assessed the age of the geomorphic features displaced by the Wairarapa Fault at preferred sites. This has been done by re-interpretation of some of the ages assigned to these sites in the literature, and analysis of the measurement location with respect to new published maps (Begg & Johnston, 2000; Zachariasen et al., 2000; Lee & Begg, 2002; Townsend et al., 2002; Fig. 2). For all sites along the northern sector of the

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Wairarapa Fault, we have also reviewed 1:16,000-scale aerial photographs.

Figures 4C and 4D show displacement values for all types of geomorphic features in the database with respect to their age. Figures 4E and 4F present data from offset streams and channels. We regard the latter as more representative of the true displacement values compared with displacement values of other geomorphic features (such as terrace risers) that can be subjected to post-displacement erosion. Although the ages assigned are very uncertain (no absolute dates for most sites), Figures 4C and 4D show a general tendency for maximum values of offsets on Q2a and Q3a to decrease northwards. This tendency is not clear on Figures 4E and 4F. Some of these preferred values are clearly questionable as exemplified by a very large displacement of ~ 280 m associated with a Q2a surface compared to other Q2a offset values (Fig. 4C).

For displacements of ~Q1a age there is no clear decrease of maximum displacement but there does seem to be a reduction of the minimum displacement (possibly representative of the last single event displacement) north of the intersection with the Mokonui Fault. This reduction seems to be more pronounced on Figures 4C and 4D than on Figures 4E and 4F.

5.0 NEW SLIP RATE ESTIMATES ALONG THE NORTHERN SECTOR OF THE WAIRARAPA FAULT

We selected two sites, Kopuaranga and Ruamahanga (Fig. 1), for detailed studies of lateral slip rate on the Wairarapa Fault. These sites are located in two different geological settings. The Kopuaranga River is sourced in the Tertiary hill country to the north (e.g., Figs. 1 & 2). This river does not produce extensive terraces such as those of the Ruamahanga River to the south. Kopuaranga River terrace deposits comprise greywacke and limestone gravels as well as thick silt layers and are mostly eroded from Tertiary mudstones and conglomerates. By contrast, the Ruamahanga River is one of the main rivers sourced in the Axial Ranges, and has formed extensive greywacke gravel-dominated alluvial terraces.

5.1 Kopuaranga site

At the Kopuaranga site, the Wairarapa Fault is sub-parallel to the general course of the sinuous Kopuaranga River. The prominent fault scarp on a high terrace indicates a dip-slip component (upthrown to the west) and the streams, flowing east into the river from the Tertiary hill country, show right-lateral offset (Fig. 5). The streams (channels 1, 2 and 3 in Fig. 5) flow into a larger stream (channel 4 in Fig. 5A) which is parallel to the fault and which grades into the current river bed on the downthrown side. Channel 2 crosses the fault trace and shows a ~9.6 m lateral offset at the intersection with the fault. Channel 1 (Fig. 5B, only present on the eastern, downthrown, fault block), seems to have been beheaded from its original source, which was likely channel 2 on the western fault block. A third stream, channel 3 (Fig. 5B), is only present on the eastern block. It has possibly been offset from its source on the western side of the fault somewhere to the north, but the source is now eroded by a meander of the Kopuaranga River (Fig. 5A). These streams are clearly seen on old aerial photos (NZ Aerial mapping photo 898/15, dating from 1943). Subsequently, a man- made canal, excavated along channel 4, has modified the site substantially. The excavation has exposed the older Kopuaranga River terrace deposits.

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In Figure 5B we present a digital terrain model (DTM) produced from a combination of differential GPS and EDM (Electronic Distance Meter) data. We have used the DTM to analyse the geometric shape of the channels in order to restore their lateral and vertical offsets. To account for the uncertainty in the lateral displacement values, we have used the thalweg of the stream as the piercing point but also considered a maximum and a minimum value. In the maximum and minimum values we account for the bend in the stream at the fault intersection. This is important especially for older offsets where the stream may have been straighter where it intersected the fault. For vertical displacements we have added an uncertainty value equivalent to 10% of the measured scarp height to allow for irregularities along the scarp (similar to Villamor and Berryman 2006). The alternative distances are shown on Figure 5B and offset values and their uncertainties are given in Table 1.

The ages of the channels and of the main terrace where channels are incised are based on radiocarbon dates and OSL dates from the sediments found in 4 trenches excavated across the streams and from the canal exposure (Fig. 5; Tables 2 and 3). Trenches Penn 1, Penn 2 and Penn 3 were excavated across channels 1, 2 and 3, respectively, on the eastern (downthrown) block of the fault with the aim of dating the channel deposits. Penn 4 trench was excavated on the western block of the main terrace away from the stream with the intention to identify and distinguish terrace deposits from channel deposits.

In Penn 4 trench, the stratigraphy correlated with massive weathered silt deposits at the base of Penn 1 and 2 trenches and to the sides of the channels units in Penn 1, 2 and 3 trenches. This deposit has very distinct weathering (iron stained) throughout the unit that is not found in the silt-rich layers of the channel deposits. This deposit is also very firm compared to channel silts. This silt possibly correlates with the silt deposits that commonly underlie the Kopuaranga River terraces and which are exposed in the canal (Unit A, Fig. 6; Appendix 2). No channel deposits were found in trench 4 and thus the trench walls were not logged. Penn 3 trench was also no logged because the analysis of the DTM revealed that we could not assess displacement associated to channel 3. Some samples for potential dating were collected from trenches 3 and 4 (Appendix 3).

Penn 1 and 2 trenches clearly display lens-shaped channel deposits, overlying the silt-rich sequence of the Kopuaranga River terrace. The western walls of both trench Penn 1 and Penn 2 trenches were logged and described, and numerous samples were collected (Figs. 7A-B). The stratigraphy in both trenches consists of alternating coarse and fine sediments. Coarse sediments are mainly medium gravels with limestone, sandstone and greywacke clasts and the fine sediments are fine sand to clay. (See unit descriptions in Appendix 2). We have grouped the channel deposits into sequences that are fining upwards, as shown on Figs. 7A and B. Each sequence represent periods of high to low energy in the channel, represented by a coarse basal deposit (gravel; e.g. Unit 8 of sequence 5; Fig. 7B) overlain by fine grained units (sand or silt; e.g. Unit 7 in sequence 5; Fig. 7B).

In general, the age of a channel is defined by the age of the oldest channel-filling deposits (minimum age) and the age of the youngest sediments of the terrace where the stream is cut into (maximum age). The oldest channel deposit in Penn 1 trench is unit 13. Although this trench did not fully expose the whole lateral extent of the channel units, it exposed the basal contact of unit 13, during excavation and with a small man-dug pit. The basal contact of Unit 13 is located at the base of the trench as per Fig. 7A. This relation was also clear on the east

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wall. The oldest date that we could obtain from channel 1 is ~ 5620 yr (sample P1-5 in Unit 9 in Fig. 7A; Table 2; Appendix 3). To date the terrace we collected an OSL sample from silt above the gravels that underlie the terrace exposed at the man-made canal (sample River Bank -1, Fig. 6; Table 3: Appendices 3 & 4). The OSL age of ~70 ka suggests that the terrace is equivalent to Q4a (Porewan ~71 to 59 ka). Previously this terrace was assigned a Q3a-Q4a age based on surface morphology and relative height above the river level (Townsend et al., 2002).

Although sample River Bank -1 does not date the youngest sediments (loess deposits) that cover the terrace, we prioritised dating it because we consider it more important to date the age of abandonment of the terrace by the Kopuaranga River. This date is the first available age for the Kopuaranga River terraces and helps with relative dating for the whole river terrace sequence. Because of the large difference in age between the abandonment of the river terrace (~70 ka) and the oldest deposits of the stream (~5.6 ka) we cannot use the 70 ka age as the maximum age for the channel. We have now sought funding and submitted another sample that will date younger sediments of the terrace cover (sample P3O-1, Fig. 6). The results will not be available for at least 12 months.

At the Kopuaranga site, we have calculated the maximum fault strike-slip rate of 7.8 ± 1.9 mm/yr (obtained from a 44 ± 10 m lateral displacement and a 5.6 ± 0.4 ka age; Table 1). This is a maximum fault slip rate because the age assigned to this displacement is a minimum age. A vertical slip rate for the terrace assuming that the terrace is the same on both sides of the faults is 0.09± 0.02 mm/yr (obtained from a 6.5 ± 1.6 m vertical displacement and a 69.9 ± 4.1 ka age; Table 1). We have used the methodology presented in Villamor and Berryman (2006) to estimate fault slip rate and to propagate uncertainties.

5.2 Ruamahanga site

At the Ruamahanga site, the Wairarapa Fault intersects the Ruamahanga River at almost right angles. The site preserves offset streams which have cut into an extensive greywacke gravel-dominated alluvial terraces of the Ruamahanga River. Figure 8 shows a series of terrace risers and stream channels that have been right-laterally offset. These features are located on a terrace that has been mapped either as Q1a_deg (early Holocene degradation terrace; Townsend et al., 2002) or Q2a (Lee & Begg, 2002) based on aerial photography review.

We have used a DTM constructed from differential GPS and EDM data to analyse the lateral and vertical displacements associated with terrace risers and stream channels (Fig. 8B). For horizontal displacements of terraces risers we have allowed for an uncertainty of ± 2 m. For horizontal displacements of stream channels we have estimated maximum and minimum value to account for the uncertainty in the shape of the channel (Fig. 8). For vertical displacements we have added an uncertainty value equivalent to 10% of the measured scarp height to allow for irregularities along the scarp (similar to Villamor and Berryman 2006).

Displacement values are presented in Table 1. Most of the lateral displacement values are within a range of ~20 to 35 m (Fig. 8B). Only the measurement associated with the terrace Riser 1 is uncertain. The point where Riser 1 intersects the fault trace on the up-thrown side of the fault has been modified, but it is possible that there is a ~29 m displacement.

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We dug three trenches across one of the channels (Figs. 8B) one on the up-thrown side of the fault (Barton 1 trench, Fig. 9B) and two on the down-throw side (Barton 2 and 3 trenches; Figs. 9B and 9C). Barton 1 and 3 trenches expose an entire cross-section of the channel. Barton 2 trench (Fig. 9B) is a deeper trench and was excavated close to Barton 3 to expose river terrace deposits so that they could be distinguished from deposits associated with the channel. Ideally it is best if the trenches are situated very close to the fault trace, in order to project any piercing points (e.g., sand lenses, see below) a short distance to the fault trace. Although the selected sites are located some distance from the fault, they showed high potential to contain fine grained sediments with organic matter for radiocarbon dating and fine grained sands or silts for OSL dating.

Barton 1 and Barton 3 trenches show very similar stratigraphy (Fig. 9; Appendix 2). The basal gravels are exposed at bottom and lateral ends of the trenches. They consist of coarse greywacke gravels and are interpreted as part of the gravels that form the main terrace deposits. Above the gravel there are lenses of medium to coarse sand that are interpreted as lag deposits of the channel. These lenses are located in the lowest points of the channels. Massive silt overlies the sand lenses and terrace gravel. This silt is either overbank silt deposited by the Ruamahanga River and/or aeolian loess deposits.

We collected multiple samples for OSL dating to date the channel and terrace deposits, three of which where analysed. An age of ~28 ka (Sample B2O-4 from Barton 2 trench; Figure 9B; Table 3; Appendices 3 & 4) dates unit 7, a sandy lens interbedded within the coarse gravel (Fig. 9B). The channel sediments were dated with two OSL samples: one collected from a sand lens at Barton 3 trench (B3O-1, Fig. 9C); and another from the silt overlying the terrace gravel at Barton 2 trench (B2O-1, Fig. 9B). The ages of these two channel samples were ~18 and 14 ka, respectively. Thus the channel was incised sometime between 28 and 18 k/yr.

The Barton site terrace has been mapped as a Q1a degradational terrace (Townsend et al., 2002). The ages obtained here point to a Q2a degradational terrace instead. The deeper sediments exposed in the Barton 2 trench (units 7, 8 and 9) could be Q2a to Q3a gravels (27.7 k/yr) partially eroded during Q2a_deg river incision. Unit 5 could represent fluvial deposits associated with this incision period. This terrace was abandoned by the river some time before 18 ka except for some streams depositing fine overbank silt large floods (units 4 and possibly part of 3). When the terrace was fully abandoned loess (unit 3) could have been deposited and preserved in these stream channels.

At the Ruamahanga site, the oldest bracketing age (~28 k/yr, unit 7) for the displacement measured is probably too old (gravels are not related to the formation of Q2a_deg), but we could not date unit 5 (Q2a_deg gravels), which would have been more representative. Using the OSL ages above (18 k/yr as minimum and 28 k/yr as a maximum), we obtain a lateral slip rate of 1.2-1.6 mm/yr from displaced channels (Table 1). This slip rate (1.2-1.6 mm/yr) is very low compare with other values along the northern section of the Wairarapa Fault (~6 at tea Creek, Van Dissen & Berryman, 1996). We discuss this further in the next section.

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6.0 DISCUSSION

6.1 Is fault slip rate decreasing to the north?

Based on existing data (Tea Creek site: 5.9-6.5 mm/yr, Waiohine terraces: 8-13 mm/yr) and results from this study (Kopuaranga River: <7.8±1.9 mm/yr) we propose that the slip rate along the Wairarapa Fault decreases northwards by ~ 1.5 to 7.1 mm/yr (Fig. 10). This value is mainly obtained from the comparison between the Waiohine terraces and Tea Creek sites. Our results also suggest a northward decrease of slip rate but the exact amount cannot be confirmed within our uncertainties. At this stage, we do not use the results from the Ruamahanga River site because we cannot explain why are they so low (~1.2-1.6 mm/yr) At the Kopuaranga site, we have obtained a maximum slip rate of 7.8±1.9 mm/yr. The lower range of the value at the Kopuaranga site (~6.1 mm/yr) is comparable to the mean value at Tea Creek (~ 6.2 mm/yr). We have submitted a new OSL sample with which we hope will help us to constrain the minimum slip rate of the Wairarapa Fault at Kopuaranga site.

The slip rate estimated at the Ruamahanga site from displaced streams appears too low (1.5 - 2.5 mm/yr) compared with other sites along the fault (Fig. 10). For this reason, we will not regard this value as representative of the slip rate of the Wairarapa Fault at this location. This low slip rate value could be caused by various factors, such as:

1. The slip rate value is not representative of the real slip rate at this site because the terrace is younger than the Q2a age obtained from OSL samples at the site, meaning the OSL ages are in error, or 2. The slip rate value is not representative of the real slip rate at this site because the traces that we have studied do not include all the deformation associated with the Wairarapa Fault at this location (i.e., additional fault traces have not been identified), or 3. The slip rate value is representative of the real slip rate at this site and it is small because deformation is distributed between the Wairarapa and the Mokonui faults.

Of these three possible explanations we can not fully confirm or rule out any. Our preliminary analysis of the elevations of the terraces around the Ruamahanga site, presented in Appendix 5, indicates a Q2a_deg (degradation terrace incised in Q2a terrace) age for the site terrace as suggested by our OSL dates. However, there are some inconsistencies on the mapping of river terraces around the site that hinder full confirmation of this age. Absolute dating of other surfaces would be necessary to calibrate our ages.

If the OSL ages at the Ruamahanga site are correct, then we need to explain why there is such low slip on such an old terrace at the site. It is possible that the fault diverges into several strands at this location and that we have missed slip. Other fault splays are present in the area as shown by the small splays to the south of the trace main on the DTM produced (Fig. 8). Channels across these traces do not show lateral displacement. It is also possible that the fault traces with vertical offset mapped on the higher terrace to the north-west of the site (Fig. 8) extend into the terrace of our study. However, we can not distinguish any lateral or vertical offsets in the area where those traces would cross the terrace of our study. It is likely that if those fault traces displaced the lower (site) terrace they would show vertical displacement as they do on the higher terrace.

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An alternative explanation is that the slip is distributed between the Wairarapa Fault and the Mokonui Fault. Slip may be transferred back onto the Wairarapa Fault between the Ruamahanga site and the Kopuaranga site, where some strands of the Mokonui Fault apparently merge with the Wairarapa Fault (Fig. 1). Studies on the Mokonui Fault account for >0.3-0.7 mm/yr of slip rate and a ratio of vertical to lateral displacement of 1:1 (Langridge et al., 2003). Profile 9 in Appendix 5 also show a minimum vertical offset of the Barton site terrace across the Mokonui Fault of 5-8 m. This number agrees with published results if the Barton site terrace has an 18-23 ka age (see more information on age in Table 1). Combining both slip rates, the total slip rate for the Wairarapa-Mokonui system is ~ >1.8-3.2 mm/yr. This number is still low compared to Tea Creek and Kopuaranga sites but it is a minimum, and thus it is possible that slip transfer between the Mokonui and the Wairarapa Fault may account for the low slip rate found at the Kopuaranga site.

6.2 Are lateral displacement values along the Wairarapa Fault diminishing to the north?

An alternative way to investigate if there is a slip rate decrease to the north is to determine whether decrease in displacement values for similar-age geomorphic features along the fault exists. We have explored this by compiling and analysing a database of published offset measurements (Figs. 2 & 4). In general, the older (late 1960’s to 80’s) and most abundant values within this dataset are difficult to interpret because how those values were obtained is not well documented. Also for most of the values there is not an associated age with the geomorphic feature displaced. Nonetheless, there are a few important outcomes from this review.

In general, maximum offsets for different geomorphic surfaces seem to decrease to the north (Fig.4C). This is clear for Q2a surfaces and, to a certain extent, for surfaces older than Q2a. The Quaternary mapping along the Wairarapa Fault (Fig. 2) and Figures 4C to 4F show that there are no geomorphic surfaces older than Q2a on the central part of the Wairarapa Fault. Therefore, decrease in offset for surfaces >Q2a in the northern sector is interpreted by comparison with Q2a values at the central sector. Decrease in maximum displacement values is not clear for Q1a surfaces.

Although the mapping of Quaternary geomorphic surfaces used here is up-to-date and very sound, it is clear from the study of terraces around the Ruamahanga site (Appendix 5) that mapping of terraces based on aerial photo interpretation alone could be misleading. It is possible that the mapping that we have used here has still some misinterpretations of surface ages. The tectonic uplift and subsidence of terraces and fans close to the Wairarapa Fault, and in some cases close to other faults (Carterton, Masterton, Mokonui faults), makes it difficult to correlate distant surfaces. Only with absolute dating techniques and high resolution digital elevation data (higher resolution than used here, i.e., 30-m-pixel SRTM, TOPSAR or LIDAR data) could these correlations be attempted. This should be taken into account for siting of future studies and when attempting to assign ages to a site that are obtained from terraces of supposedly the same age but at distant locations.

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6.3 Is single event displacement associated with the 1855 Wairarapa earthquake surface rupture diminishing north?

If we assume that the smallest lateral offset measured along the Wairarapa Fault represents the 1855 event, these values fall in the range from 4.5 to 12 m for the northern sector of the fault, depending on the author. Figure 4D and 4F show a northward decrease of the minimum offset of the fault. The decrease in the minimum offset value takes place north of the intersection with the Masterton Fault as shown by Figures 4D and 4F, but we believe that it could take place north of the intersection with the Carterton Fault. The minimum offset values in the section of the fault between the Carterton and Mokonui faults (Figs. 4C and 4F) come from one of the clusters of Grapes and Wellman (1988) that we could not identify in aerial photo review.

At the Kopuaranga site, the smallest displacement (~9.6 m) could be associated with the 1855 surface rupture if other displacements of streams in the area of ~4.5 m are statistically representative of the lower displacement value for the 1855 earthquake in this area. Alternatively, it could represent two events if ~4.5 m is a mean value for single event displacement in this area. Without trenches across the fault trace it is difficult to assess how many events represent that displacement, but with the information available we have attempted a hypothetical model of earthquake history for this site, based on the stratigraphy found in the trenches excavated across channels. The earthquake history can help us to assess whether the smallest displacement value at the Kopuaranga site is representative of the displacement associated with the 1855 earthquake.

In this preliminary rupture model, we can account for 5 surface rupture events associated with the 44 m right-lateral offset, based on trench stratigraphy (see Fig. 11 for explanation). For the 9.6 m displacement we only account for one event, which could be the 1855 earthquake. If we divide 44 m of total displacement by 5 events the single event displacement mean value is 8.8 m, which is similar to our smallest displacement at the site. This model could be confirmed with paleoseismic trenches across the fault and dating of some of the samples collected from Penn1 and 2 trenches.

Comparison of our preliminary results with existing data suggests that single event displacement for the 1855 surface rupture could have decreased to the north (Fig. 10). On the Alfredton Fault, 4-7 m of lateral displacement occurred during the 1855 earthquake (Schermer et al., 2004). Along the northern sector of the Wairarapa Fault there is no documentation on displacement associated with the 1855 earthquake. If we assume that the smallest displacements (4.5 to 12 m) along the northern sector of the Wairarapa Fault (including ~9.6 m observed at the Kopuaranga site) formed during the 1855 earthquake, we can infer that the slip associated with the 1855 earthquake decreased to the north (slip per event at Pigeon Bush was 15 to 17 m; Rodgers & Little, 2006).

6.4 How can slip rate decrease northwards?

There are several ways that the Wairarapa Fault can accumulate less slip on the northern sector and thus slip rate can decrease northwards: if the fault always ruptures the same length but single event displacement decreases northwards; or if the fault ruptures in different segment lengths with more ruptures on the southern side; or a combination of both

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options. Preliminary results (with assumptions) from this study indicate a possible decrease in single event displacement to the north as mentioned above. Also at Pigeon Bush, Rodgers and Little (2006) reported that the penultimate event was of the same size (slip per event) as the 1855 earthquake, implying that the Wairarapa Fault possibly ruptured the whole length in the penultimate event too.

The ratio between large single event displacement in the central sector and smaller displacement in the northern sector of the Wairarapa Fault based on available data could be of the order of 1.7:1 (16 m: 9 m), which is similar to the possible decrease in the mean slip rate form 10.5 (Waiohine site) to 6.2 (Tea creek site) mm/yr (a ratio of 1.7:1). From this comparison we could conclude that a decrease in single event displacement is responsible for the decrease in slip rate. However, our data and interpretations have large uncertainties and assumptions and thus we cannot exclude the possibility of segmented ruptures on the Wairarapa Fault contributing to this decrease. This possibility could be investigated if more earthquake history studies were available for the northern sector of the fault.

6.5 Where does the Wairarapa Fault slip rate decreases and why?

The decrease in slip rate along the Wairarapa Fault could take place at the intersection with the Carterton Fault. At this location, the character of the fault changes greatly. To the south of the intersection, the fault is represented by one main N047°E striking fault which bounds high ranges to the west and a subsiding basin (Lake Wairarapa) to the east (Fig. 3). To the north of the intersection, the fault strikes N042°E and several other N053-074°E striking splays diverge from the fault towards the east (Carterton, Masterton, Mokonui, and Alfredton faults). In the north, the Wairarapa Fault is not the boundary between western high ranges and the eastern basin but crosses through the Tertiary hill country. The landscape is still somewhat higher to the west and lower to the east of the fault just north of the intersection but this difference diminishes to the north.

The Carterton, Masterton and Mokonui faults have been defined as oblique slip faults. The Carterton Fault is the best characterised of these faults with dextral and vertical slip rates of 2-4 mm/yr and 0.1-0.5 mm/yr, respectively (Zachariasen et al., 2000). The Masterton and Mokonui Faults yield vertical slip rates of c. 0.3-0.7 mm/yr and 0.2-0.5 mm/yr, respectively (Zachariasen et al., 2000; Langridge et al., 2003). However, because the ratio of horizontal to vertical slip for the Masterton and Mokonui faults is unknown (1:1 is being proposed with scarce data; Begg et al., 2000, Langridge et al., 2003), it is not possible to estimate their strike-slip and total slip rate. Nevertheless, their strike-slip rate is currently considered to be less than that of the Carterton Fault. Modelling of seismic fault slip and GPS data (Wallace et al., 2004) predicts a total of c. 5 mm/yr of dextral strike-slip and <1 mm/yr of extension across the area encompassing these faults, implying that the largest portion of the strike-slip deformation for these faults is accommodated by slip on the Carterton Fault.

Within the uncertainties of our results and of some of the published data, we propose that deformation is transferred from the Carterton, Masterton and Mokonui faults onto the central sector of the Wairarapa Fault. To give a first order estimate of the amount of transfer, we assume that the combined lateral slip rate of the Carterton, Masterton and Mokonui faults is 4-5 mm/yr and the angle between the central sector of the Wairarapa Fault (N047°E) and a generalised trend for the Carterton, Masterton and Mokonui faults (N74°E) is 27° (Fig. 11).

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We do not use the vertical component of the Carterton, Masterton and Mokonui faults in this calculation because the fault dip is not known, and we thus cannot estimate the slip rate horizontal component that is normal to the faults.

The amount of lateral slip transferred from the Carterton, Masterton and Mokonui faults to the central sector of the Wairarapa Fault is ~3.6 to 4.5 mm/yr (Fig. 11). In this kinematic model, decrease in lateral slip rate along the northern section of the fault takes place at each intersection with the Carterton, Masterton and Mokonui faults. Most of this decrease probably takes place at the intersection with the Carterton Fault (which has the highest slip rate of the three). Slip rate decrease at the other two intersections is possibly much smaller and lies within the uncertainty of slip rate data.

7.0 CONCLUSIONS

We have obtained two new preliminary slip rate estimates for the northern sector of the Wairarapa Fault. At the Kopuaranga River we obtain a maximum right-lateral slip rate value of ~8 mm/yr (vertical of ~0.1 mm/yr). We are still waiting for results on an OSL date to constrain the minimum slip rate at this site. At the Ruamahanga River we obtain a horizontal slip rate of ~1.2 to 1.6 mm/yr. These slip rate values are much lower than other values along the same sector of the fault. At this stage we can not fully explain these values, but they could be related to slip transfer between the Wairarapa and the Mokonui Fault. Additional work is required to understand the slip rate value at Ruamahanga site.

Comparison of these results with published slip rate values and slip magnitude values suggests slip rate decreases northward by ~ 1.5 to 7.1 mm/yr from the central to the northern sector of the Wairarapa Fault (Figs.1 & 10). With the data available and using several assumptions as described above, this decrease could be explained by either a decrease in slip per event along fault in successive whole fault rupture, similar to the 1855 earthquake, or a segmented rupture with fewer surface ruptures in the north. Because of the lack of earthquake history data for the northern sector of the Wairarapa Fault (north of Tea Creek site), we cannot rule out either mechanism as the reason for the observed decrease in slip rate.

Within the uncertainties of our results and of some of the published data, we propose that deformation is transferred between the Carterton, Masterton and Mokonui faults, and the central sector of the Wairarapa Fault. If we assigned a combined lateral slip rate of ~4-5 mm/yr to the Carterton, Masterton and Mokonui faults as proposed by previous studies, the amount of lateral slip transferred from the Carterton, Masterton and Mokonui faults to the central sector of the Wairarapa Fault is ~3.6 to 4.5 mm/yr (Fig. 11). In this kinematic model, decrease in lateral slip rate along the northern section of the fault takes place at each intersection with the Carterton, Masterton and Mokonui faults. Most of this decrease probably takes place at the intersection with the Carterton Fault (which has the highest slip rate of the three). Slip rate decrease at the other two intersections is possibly much smaller and lies within the uncertainty of slip rate data.

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8.0 RECOMMENDATIONS

More slip rate assessment sites along the northern sector of the Wairarapa Fault should be investigated to reduce the uncertainties associated with existing information, and to fully evaluate and quantify the amount and location in fault slip rate decrease along the fault. During the field reconnaissance along the northern sector of the Wairarapa Fault, several potential sites for slip rate investigations were identified (Table 4).

To make the most of the effort, new investigation sites (Table 4) can be chosen that combine potential results for slip rate and for paleo-earthquake rupture history. These sites will not only help to confirm segmented vs. non segmented models as a source for slip rate decrease, but they will also increase the dataset on earthquake history along the fault. These new sites did not appear to have the same potential for slip rate assessment as the two sites chosen to investigate.

The results presented here and potential new results should incorporate future with results from re-evaluation of slip rate at the Waiohine terraces site (Carne et al., work in progress; IOF-Wairarapa Fault Past Surface Ruptures Timing Task).

We recommend next targeting sites that are located in the Tertiary hill country and that have more likelihood of preserving organic matter that can be radiocarbon dated. Unfortunately, these sites are often more easily eroded and water logged than site located on extensive gravel terraces. As a consequence, some of them are greatly deteriorated or modified at present in comparison with aerial photos from 1943 and 1963.

When sites on extensive greywacke gravel terraces that have little potential for radiocarbon dating (e.g., Ruamahanga site) are studied, we recommend that OSL samples are not interpreted in isolation. We thus recommend that other terraces in the area are also dated and correlated. An important result from this study is the complexity of Quaternary terrace mapping close to faults. Localised tectonic uplift and subsidence make it difficult to correlate terraces across and along fault lines by using exclusively aerial photo review. We recommend the use of high resolution digital elevation models (ideally from LIDAR data) to undertake the terrace correlation (e.g., Litchfield et al., 2007).

9.0 ACKNOWLEDGEMENTS

We thank landowners for access to their properties. Special thanks to the Barton and Penn families for allowing us to excavate trench. José Martínez-Díaz, Carol Canora and Jose Antonio Alvarez Gómez (University Complutense of Madrid) helped us with preliminary field reconnaissance. Colorado College lecturers, Christine Smith-Siddoway and Eric Leonard, and their fourth year students assisted in logging trenches and acquiring data for DTMs. Victoria University of Wellington allowed used of their electronic distance-meter. This research was funded by EQC (It’s Our Fault Project) and FRST (C05X0402).

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10.0 REFERENCES

Barnes, P M., Pondard, N., Lamarche, G., Mountjoy, J., Van Dissen, R., Litchfield, N. 2008 It’s Our Fault: Active Faults and Earthquake Sources in Cook Strait. NIWA Client Report: WLG2008-56. Beanland, S. 1995. The North Island Dextral Fault Belt, Hikurangi Subduction Margin, New Zealand. Unpublished PhD thesis, Victoria University of Wellington, New Zealand. Beanland, S., and J. Haines (1998). The kinematics of active deformation in the North Island, New Zealand, determined from geological strain rates. New Zealand. Journal of Geophysical Research, 41, 311– 323. Begg, J.G., Johnston, M. 2000. Geology of the Wellington area. Institute of Geological & Nuclear Sciences 1:250 000 geological map 10. Carne, R.C. Little, T.A., Van Dissen, R.J., Schermer, L. 2007. Refined late Holocene history of surface rupture and slip rates on the southern Wairarapa Fault. Geological Society of New Zealand Miscellaneous Publication 123A: 24. Darby, D.J., Beanland, S. 1992. Possible source models for the 1855 Wairarapa earthquake. New Zealand. Journal of Geophysical Research 97: 12,375-12,389. DeMets, C., Gordon, R.G., Argus, D.F., Stein, R. 1994. Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions. Geophysical Research Letters 21: 2191–2194. Grapes, R.H., 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. Grapes, R.H., Wellman, H. 1988. The Wairarapa Fault. Research School of Earth Sciences, Victoria University of Wellington. Publication 4. Grapes, R.H., Hardy, E.F., Wellman, H. 1984. The Wellington, Mohaka and Wairarapa Faults. Geology Department. Victoria University of Wellington, Publication n. 28. Langridge, R.M., Townsend, D., Persaud, M. 2003. Paleoseismic assessment of the active Mokonui Fault, Wairarapa. GNS Client Report 2003/68. Langridge, R.M., Van Dissen, R.J., Cochran, U.A., Litchfield, N.J., Berryman, K.R., Begg, J.G., Villamor, P., Heron, D., Nicol, A., Townsend, D. 2005. Active faulting and paleoearthquakes in the Wairarapa and Wellington regions. In Proceedings of The1855 Wairarapa Earthquake Symposium, September 2005, Published by Greater Wellington Regional Council: 66-71. Lee, J.M., Begg, J.G. 2002. Geology of the Wairarapa area. Institute of Geological & Nuclear Sciences 1:250 000 geological map 11. Lensen, G.J. 1968. Late Quaternary tectonic map of New Zealand, 1:63,360, Sheet N158, Masterton. New Zealand Geological Survey, Department of Scientific and Industrial Research, Wellington, New Zealand. Lensen, G.J. 1969. Late Quaternary tectonic map of New Zealand, 1:63,360, Sheet N153, Eketahuna. New Zealand Geological Survey, Department of Scientific and Industrial Research, Wellington, New Zealand.

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Lensen, G, Vella, P. 1971. The Waiohine River faulted terrace sequence. Recent Crustal Movements, Royal Society of New Zealand Bulletin, 9: 117-119. Litchfield, N., Van Dissen R., Nicol, A. 2007 Reassessment of slip rate and implications for surface rupture hazard of the Martinborough Fault, South Wairarapa, New Zealand New Zealand Journal Of Geology And Geophysics, 50: 239-243. Little T.A., Begg, J. 2005 All-day Field-trip to the Wairarapa Fault and 1855 rupture sites. In Proceedings of The1855 Wairarapa Earthquake Symposium. September 2005. Published by Great Wellington regional Council: 28p. Little, T. A., Van Dissen, R., Schermer, E., Carne, R. 2008 in prep. Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: Link between earthquakes and the raising of beach ridges on a rocky coast. To be submitted to GSA. Milne, J.D.G. 1973. Map and sections of river terraces in the Rangitikei Basin, North Island, New Zealand. New Zealand. Soil Survey Report 4. Rodgers, D.W., Little, T.A. 2006. World's largest coseismic strike-slip offset: The 1855 rupture of the Wairarapa Fault, New Zealand, and implications for displacement/length scaling of continental earthquakes Journal of Geophysical Research-Solid Earth, v.111: B12; Article Number: B12408 Schermer, E.R., Van Dissen, R., Berryman, K.R., Kelsey, H.M., Cashman S.M. 2004. Active faults, paleoseismology and historical fault rupture in the northern Wairarapa, North Island, New Zealand. New Zealand Journal of Geology and Geophysics 47: 101-122. Townsend, D., Begg, J., Villamor, P., Lukovic, B. 2002. Late Quaternary displacement of the Mokonui Fault, Wairarapa, New Zealand: a preliminary assessment of earthquake generating potential. Institute of Geological and Nuclear Sciences Client Report 2002/58 prepared for Wairarapa Engineering Lifelines Association. Van Dissen, R.J., Berryman, K.R. 1996. Surface rupture earthquakes over the last c. 1000 years in the Wellington region, New Zealand, and implications for ground shaking hazard. Journal of Geophysical Research 101 (B3): 5999-6019. Villamor, P. & Berryman, K.R., 2006. Late Quaternary geometry and kinematics of faults at the southern termination of the Taupo Volcanic Zone, New Zealand, N. Z. J. Geol. Geophys., 49 (1), 1-21. Wallace, L.M., Beavan, J., McCaffrey, R., Darby, D. 2004. Subduction zone coupling and tectonic block rotations in the North Island, New Zealand. Journal of Geophysical Research 109: 1-24. Wang, N. 2001. Optically Stimulated Luminescence dating techniques, over the last ~100 years in the Wellington region, New Zealand, and their application to dating the loess in Southern North Island. Unpublished MSc Thesis, Victoria University of Wellington, 131p. Wang, N., Grapes, R. 2007. Infrared-stimulated luminescence dating of late Quaternary aggradation surfaces and their deformation along an active fault, southern North Island of New Zealand. Geomorphology, 96: 86-104. Wellman, H.W. 1972. Rate of horizontal fault displacement in New Zealand. Nature, 237: 275-277 Zachariasen, J., Villamor, P., Lee, J.M., Lukovic, B., Begg, J.G. 2000. Late quaternary faulting of the Masterton and Carterton faults, Wairarapa. GNS Client Report 2000/71.

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Table 1 Lateral and vertical displacement and slip rate values at Ruamahanga and Kopuaranga sites Feature Lateral Vertical Age Lateral Vertical displacement displacement Slip rate Slip rate (m) (m) *** (mm/yr) (mm/yr) Kopuaranga 9.5±3* Channel 2 Kopuaranga 44±9.5* >5.6±0.4 <7.8±1.9‡ Channel 1 Kopuaranga 6.5±1.6 69.9±4.1 0.09±0.02 Terrace (Q4a) Ruamahanga ? 4.5±1.1 Terrace 1 (main fault) Ruamahanga 0 3±0.8 Terrace 1 (secondary fault 1)** Ruamahanga 0 1±0.3 Terrace 1 secondary fault 2** Ruamahanga 8.5±1.4 23.0±2.3+ ? 0.37±12 Terrace 1 all faults** Ruamahanga 29.5±2.5* 2.7±0.7 Riser 1 main fault Ruamahanga 30.5±4.5* 3±0.8 Riser 2 main fault Ruamahanga 22.1±12.9* 1.9±0.5 Channel 1 main fault Ruamahanga 33.5±11.5* 1.8±0.5 Channel 2 main fault Ruamahanga 3±0.8 Terrace 2 main fault Combined from 5 previous rows 28.8±19.4 3.1±1.5 main fault Ruamahanga 0 0.4±0.2 Terrace 2 secondary fault 1** Ruamahanga 0 0.2±0.2 Terrace 2 secondary fault 2** Ruamahanga 28.8±19.4 3.7±1.5 23.0±2.3+ 1.25±0.85 0.16±0.08 Terrace 2 all faults** 18.3±1.2++ 1.57±1.07 0.20±0.1

*Channel offsets: measured maximum and minimum are assumed to be 2 standard deviations. Riser offsets: mean value is obtained from average value of offsets at top and bottom of riser; an additional 2 m uncertainty is added to the variability of these two values. ** Secondary faults 1 and 2 have opposite throws in Terrace 1 (graben) compared to terraces 2 and 3 (horst) *** Uncertainties in vertical offset are assessed as 25 % of the scarp height plus a ~ 0.2 m uncertainty of the DTM scale. + Age =bracketed between 27.7± 2.0 and 18.3± 1.2 ++Age= oldest post Q2a gravels age. ‡ Maximum value

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Table 2 Radiocarbon ages

NZA 14C age Cal. age Material dated, location Trench Sample ID (Lab Number) (yr BP) (cal. yr BP) and significance

Penn 1 P 1-4 27639 (29528/1) 2753 ± 30 2864 to 2747 Wood sample from unit 7

Plant material (wood pieces) Penn 1 P 1-5 27689 (29528/3) 4953 ± 35 5659 to 5581 within clayey soil from unit 9

Twiggy plant material within Penn 2 P 2-4 27688 (29528/2) 1224 ± 30 1175 to 979 dark brown soil from unit 8

Table 3 OSL ages

VUW OSL-age Material dated, location and Trench Sample ID Lab Sample No. (ka) significance

River Bank- Silt. 30 cm above gravels, 70 cm below Penn 1 WLL667 69.9 ± 4.1 ground surface

Silt over gravel. 1.2 m below ground Barton Barton 2-1 WLL668 14.1 ± 1.1 surface

Barton Barton 2-4 WLL669 27.7 ± 2.0 Fine-med sand over gravel 2.47 m below ground surface

0.94 m below ground surface, from fine Barton Barton 3-1 WLL670 18.3 ± 1.2 sand over gravel

Table 4 Future study sites

Site* Geographic area Coordinates Potential study

6 Carterton Bush S26/210240 Slip rate (& earthquake timing?)

21 Cootes_Ngararu S26/299341 Slip rate (& earthquake timing?)

20 Cootes_Ngararu S26/301345 Slip rate (& earthquake timing?)

18 Ngararu T26/306352 Slip rate & earthquake timing

North 18 Ngararu_Waipoua T26/308356 Slip rate& earthquake timing

10 Kopuaranga T25/358411 Earthquake timing

13 Kopuaranga T25/364420 Earthquake timing (Penn site) * Label as per database of Appendix 1 (this study)

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2675000 2700000 2725000 2750000 2775000

175° E 6075000 6075000

35° S

Ft lley Va a 47 mm/yr P Alfredton trench Taranaki c.3 mm/yr Basin Ft Ruamahanga River n to 6050000 6050000 40° S lfred

h River Waiohine A g u o Mauriceville Tr

Waipoua River Kopuaranga rn ikurangi e H th Ruamahanga

r River Kopuaranga 41 mm/yr o W a N in r Tea Creek g e a t iv wa i F onu R 6.2 ± 0.3 mm/yr ok u Ri M h v e er a g Ft n n t a o n F h n Ft rto rterto ngt ste W Ca li Ma r Masterton ve 6025000 Wel i 6025000 rapa Ft R a u ir er w Wa au Waiohine Perrys Road T Carterton 3 ± 1 mm/yr Tauheranikau River l a tr Greytown n r e e C iv Featherston a R g

amahan u R 6000000 6000000

irarapa a rn W Martinborough e ke Towns th t u s La o ru S h Published Data T u a k u a k Study Sites re a h W Active Fault River 02010 Km 5975000 5975000

2675000 2700000 2725000 2750000 2775000

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2675000 2700000 2725000 6050000 6050000

rn e h rt o N 6025000 6025000

l a tr n e C

6000000 Lake 6000000 Wairarapa

rn e th u o S

River

Wairarapa Fault

Other Fault

01020Km 5975000 5975000

2675000 2700000 2725000

Q1 Present day river deposits and Q7 River terraces and fans formed between early Holocene river terraces and fans 186000-245000 years Q2 River terraces and fans formed during Q8 River terraces and fans formed between the late Last Glacial (14000-24000 years) 245000-303000 years Q3 River terraces and fans formed between Tertiary 24000-59000 years (Last Glacial) Q4 River terraces and fans formed between Mesozoic Greywacke-dominated bedrock 59000-71000 years (Last Glacial) Q5 River terraces and fans formed between 710000-128000 years (Last Glacial) Q6 River terraces and fans formed during the Waimea Glacial (128000-186000 years)

Figure 2 Simplified geology map showing relationships between Wairarapa Fault (bold red line) and greywacke-dominated basement rocks, overlaying Tertiary sediments and Quaternary alluvial terraces and fans. Mapping and ages of quaternary alluvial terraces and fans from Begg & Johnston (2000) and Lee & Begg (2002). Faults from GNS active fault database (http://data.gns.cri.nz/af/).

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2675000 2700000 2725000 2750000 2775000

AUTHOR Villamor, P., et al. this study Rodgers, D.W., Little, T.A. 2006 Van Dissen, R.J., Berryman,.K.R. 1996 t y F le Little, T., Begg, J.G. 2005 al V a Grapes, R.H., Wellman, H.W. 1988 P Grapes, R.H., Hardy, E.F., Wellman, H.W. 1984 t F ton Lensen, G.J. 1968 d lfre A Lensen, G.J. 1969

Masterton

Carterton

Greytown

Featherson

Martinborough

Towns River

5975000 6000000 6025000 6050000 Active Fault 5975000 6000000 6025000 6050000

01020Km

2675000 2700000 2725000 2750000 2775000

Figure 3 Plot of displacement values along the fault trace from published literature and this study (coloured by source). For more details on displacements see Appendix 1. Digital Elevation Model from LINZ.

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400.00 50.00 Van Dissen, R.J., Berryman, K.R. 1996 Van Dissen, R.J., Berryman, K.R. 1996 Grapes, R.H., Hardy, E.F., Wellman, H.W. 1984 Grapes, R.H., Hardy, E.F., Wellman, H.W. 1984 45.00 350.00 Grapes, R.H., Wellman, H.W. 1988 Grapes, R.H., Wellman, H.W. 1988 Lensen, G.J.1968 &1969 Lensen, G.J. 1968 &1969 40.00 Little, T., Begg, J.G. 2005 Little, T., Begg, J.G. 2005 300.00 Rodgers, D.W., Little, T.A. 2006 Rodgers, D.W., Little, T.A. 2006 35.00 Villamor, P. et al., this study Villamor, P. et al., this study

250.00 30.00

200.00 25.00

20.00

Horizontal offset150.00 (m) Horizontal offset (m) 15.00 100.00 10.00

50.00 5.00

0.00 0.00 0.00 20.00 40.00 60.00 80.00 100.00 0.00 20.00 40.00 60.00 80.00 100.00 Distance from coast (km) Distance from coast (km)

Figure 4-A & B Plots of horizontal displacement data available along the Wairarapa Fault. Displacement data classified by author (both plots contain same data but are plotted with different scales). See also Appendix 1.

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Figure 4-C & D Plots of available horizontal displacement data along the Wairarapa Fault. Selected displacement data for all geomorphic features classified by age (both plots contain same data but are plotted with different scales). Ages assigned to displacements are taken from existing mapping, most of which have not being verified with absolute ages. See also Appendix 1.

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E F

Horizontal offsets from streams and channels vs distance from coast, trusted data, Horizontal offsets from streams and channels vs distance from coast, trusted data, classified by possible offset age classified by possible offset age

400.00 50.00

45.00 Unknown 350.00 Unknown Q1a Q1a Q2a 40.00 Q2a 300.00 35.00

) 250.00

m 30.00

200.00 25.00

20.00 150.00 Horizontal offset ( Horizontal offset (m) 15.00 100.00 10.00

50.00 5.00

0.00 0.00 0.00 20.00 40.00 60.00 80.00 100.00 0.00 20.00 40.00 60.00 80.00 100.00 Distance from coast (km) Distance from coast (km)

Figure 4-E & F Plots of available horizontal displacement data along the Wairarapa Fault. Selected displacement data for streams and channels classified by age (both plots contain same data but are plotted with different scales). Ages assigned to displacements are taken from existing mapping, most of which have not been verified with absolute ages. See also Appendix 1.

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B 5

. ig F

0 20 40 Meters A 070140Meters B N N

Figure 5 Kopuaranga site: A, Quaternary geology (based on Townsend et al., 2002) laid over aerial photo (NZ Aerial Mapping).Labels in brackets and italics indicate newly assigned terrace ages from this study. White circle is location of stratigraphic column in Figure 6. B, Digital terrain model for the Kopuaranga site with location of channels, piercing points (maximum and minimum estimates shown by white lines), and assessed lateral displacements for channel 1 and 2 (yellow and pink arrows, respectively). Trenches are shown with black rectangles.

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Figure 6 Stratigraphy of the main terrace at Kopuaranga (Penn) site. Composite stratigraphic column of the terrace exposure at the man made canal. Upper 5.5 m consists of loess deposits. These are not exposed in the canal but inferred from stratigraphy in trenches. Elevations of channel units exposed in different trenches are shown. Elevation for “channel axis bottom” indicates depth of incision of channel into the terrace loess cover.

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P10-1

OSL Sample C14 Sample P10-2

P1-1

P1-4 P 1-2

P1-3 P1-8 P1-7 P10-3 1m P1-6 xx x P1-5

Figure 7-A Kopuaranga site trenches. Penn 1 trench log. See Figure 5 for location and Appendix 2 for unit descriptions.

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Penn 2 Trench - West Wall

P2-1 P20-1 P2-16 P2-15

P2-14

P2-5 P2-13 P2-11 P2-6 1m Sequence 1

P20-2 P2-12 P2-2 P2-8 P2-9 P20-4 1m P2-17 P2-7 P2-3 P2-4 Sequence 2 9a Overbank silty Clay with fine Sand P20-2 or Loess (Main terrace) on South Wall 9b Overbank coarse Sand with clayey Silt [775 AD to 971 AD] or Loess (Main terrace) [1175 to 979 cal BP]

Figure 7-B Kopuaranga site trenches. Penn 2 trench log. See Figure 5 for location and Appendix 2 for unit descriptions.

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Ruamahanga - Barton Site Riser 1 Q1a Q2a 29.5±2.5

Fig.8B e c a tr lt u a f in a M Channel 1 22.1±12.9

1 ce ra lt t au y f Barton1 ar Q1a deg Riser 2 nd co 2 30.5±4.5 Se ce ra t t (Q2a deg) ul fa ry da on Q1a ec Q2a Barton 2 S

Barton 3 Channel 2 33.5±11.5 43.5 & 54m Q1a Riser 1 lt u a R F u a a p m ra a a h ir a a n Channel 1 g W a Ri ve r Q2a (Q1a) Riser 2 0 50 100 Meters 0 150 300 Meters Channel 2 A Q1a N B N

Figure 8 Ruamahanga site: A, Quaternary geology of the area (based on Townsend et al 2002) laid over aerial photo (NZ Aerial Mapping). Labels in brackets and italics indicate newly assigned terrace ages from this study (see Appendix 5 for details). B, Digital terrain model for the Kopuaranga site with location of channels; piercing points (small line across fault indicate maximum and minimum values) used for lateral displacement measurements. Trenches are shown with black rectangles.

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B1O-1

1m 1 2 3 4 5

1m B1-2

Equivalent of grey sand in T3 1m

Figure 9A Ruamahanga site, Barton 1 trench log. See Figure 8 for location and Appendix 2 for unit descriptions.

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1 1 Topsoil 2 2 Subsoil 3 Silt 3 A Bench B2O-2 A 3 Silt 4 B2O-3 3 4 Mixed grey Sand

B2O-1

5 5

5 Gravel 6 5 7 7 6 Mixed interfingered Sand 1m 6 7 B2O-4 7 Well sorted fine-medium Sand 8 8 Cobble to boulder Gravel with medium to fine sand matrix 1m

[14.1±1.1 ka] [27.7±2.0 ka]

Figure 9B Ruamahanga site Barton 2 trench log. See Figure 8 for location and Appendix 2 for unit descriptions.

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1m 1 2 1m 3 4 5

1m B3O-1

18.3±1.2 ka

Figure 9C Ruamahanga site Barton 3 trench log. See Figure 8 for location and Appendix 2 for unit descriptions.

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2675000 2700000 2725000 2750000 2775000

Site type - Slip rate

6075000 Extensive greywacke gravel terraces 6075000 Terraces in Tertiary countryside Ft Tertiary lley a V Future study sites Pa

t Alfredton trench F on c.3 mm/yr dt

6050000 e 1855: 4-7m 6050000 fr Al Kopuaranga Ruamahanga 1.9 ± 0.7 mm/yr 7.9 ± 1.8 mm/yr 1855?: 9.6m Cootes/Ngararei i Ft Te a Cre ek onu Mok 6.2 ± 0.3 mm/yr Ft ton 1855?: 8-15m ster t Ma ton F arter

6025000 t Carterton bush C 6025000 F ton g Perrys Road in ll Ft e a Waiohine 3 ± 1 mm/yr W ap ar 10.5 ± 0.5 mm/yr ir a 13.5 ± 0.5 m W

13 ± 0.5 m

6000000 18.7 ± 1 m 6000000 15.1 ± 1 m

12.9 ± 2 m

5975000 17.5 ± 1.5 m 5975000 01020Km 16.4 ± 2 m 1855 displacement data from Rodgers & Little, 2006

2675000 2700000 2725000 2750000 2775000

Figure 10 Variation of slip rate and youngest single event displacement along the Wairarapa Fault.

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Event 1

Sequence 6 Sequence 5

~ 8m Event 3 Event 2 Event 4

Sequence 4 Sequence 3 Sequence 2

Event 5-1855?

Sequence 1

~ 40m

Figure 11 Hypothetical model for rupture history at Kopuaranga site. The stratigraphy in Penn 1 and Penn 2 trenches (Fig. 7) shows a repeated sequence of gravel overlain by silt and/or sand, which represents repeated cycles of increased to decreased sediment load in the channel (each cycle is presented here by a different coloured channel unit). We hypothesise that each sequence is related to an earthquake event on the Wairarapa Fault. After each earthquake the stream could have a period of high sediment load (caused by landsliding in the catchment of the stream and exposure of sediments at fault trace) followed by a period of decreased sediment load (fault quiescence). In this preliminary rupture model, we can account for 5 surface ruptures. Penn 1 trench has 4 sequences (1 to 4 from bottom to top; Fig 7a) while Penn 2 trench has two sequences (5 to 6 from bottom to top; Fig 7a). The first sequence in a newly formed stream might not necessarily need to be related to an earthquake, so it is possible that sequence 1 (Penn 1 trench) is not related to an earthquake. However, in a newly formed stream, when the older stream is beheaded by faulting, the first gravel package (sequence 5 in Penn 2 trench) is related to the event that beheaded the older stream and it is formed prior to the new channel being offset. Alternatively, deposition of these sequences (or some of them) could be related to storms. A paleoseismic trench across the fault trace to investigate the number of event associated to the ~ 40m displacement could elucidate this hypothesis.

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42°

r) to c e s rn e h Lateral slip rate transferred to rt o central sector of the Wairarapa Fault n ( a p ra 74° a ir r a /y r) m /y W m m .5 m 4 .6 3 ( CMM 27° r m/y 5m r) m/y (4m r) to c Combined lateral slip rate of se Carterton, Masterton and Mokonui faults l a 4-5 mm/yr tr n e c ( a p ra a ir a W

47°

Figure 12 Kinematic model for the transfer of lateral slip rate from the Carterton, Masterton and Mokonui faults (CMM) onto Wairarapa Fault. Estimates of amount of slip rate transferred from CMM to central sector of the Wairarapa Fault for a combined slip rate for CMM of 5 mm/yr or 4 mm/yr (this latter in brackets).

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APPENDICES

Appendix 1 - Displacement database Appendix 2 - Unit descriptions Appendix 3 - Samples collected

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APPENDIX 1 DATABASE OF DISPLACEMENT MEASURES ALONG THE WAIRARAPA FAULT

A digital file (shape and Microsoft excel formats named Wairarapa_Points_database) is provided in the enclosed CD. The fields correspond to the point attribute coverage of the GNS Active fault database (Jongens & Dellow, 2003). The last two fields have been added for the purpose of this study and they represent the following:

Age: Age assigned to the displaced feature in this study

Supersede: No= data point is up-to-date and has been used in Figure 4. Yes= data is been supersede and has not been used in Figure 4.

Jongens R., Dellow, G. 2003. The Active Faults Database of NZ: Data dictionary. Institute of Geological and Nuclear Sciences, Science report 2003/17.

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APPENDIX 2 UNIT DESCRIPTIONS

Penn #1 Trench

1. Modern root rich layer in matrix of fine Sand. Colour: dark greyish brown 2.5Y 4/2. Root size ranges from .5-10 cm in diameter. Primary internal structures overprinted by modern roots. Loose, dry. Sharp basal contact

2. Massive very stained (iron oxide) clayey Silt. Colour: olive 5Y 5/4. Moderately loose to moderately dense, and moist. Gradual basal contact

3. Massive slightly clayey silty fine Sand. Colour: light olive brown 2.5Y 5/3. Moderately dense to moderately loose. Dry. Sharp basal contact.

4. Massive very fine sandy pebbly Gravel. Colour: light olive brown 2.5Y 5/3. Matrix-support to clast-supported. Maximum clast size: 5 cm. Poorly sorted. Subangular clasts. Iron oxide staining throughout. Moderately lose. Moderately moist. Sharp, undulating basal contact.

5. Slight sandy clayey Silt with sparse gravel. Colour: dark grey 5Y 4/1. Planar stratification that disappears to the top of the layer. Moderately lose. Moderately moist. Gradational basal contact.

6. Massive coarse sandy pebbly Gravel. Colour: olive 5Y 4/3. Clast-supported. Maximum clast size: 6 cm. Very poorly sorted. Subangular clasts. Iron oxide staining throughout. Moderately loose to moderately dense. Moderately moist. Sharp, ondulating basal contact.

7. Massive silty Clay. Colour: grey 7.5YR 5/1, becoming gray 5Y 5/1 to left of vertical. Organic matter rich. Moderately loose. Moderately moist. Sharp basal contact.

8. Massive silty pebbly Gravel. Colour: dark grey 2.5Y 4/1. Matrix-supported. Maximum clast size: 8 cm. Very poorly sorted. Subangular clasts. Iron oxide staining throughout. Moderately dense. Moderately moist. Sharp, ondulating basal contact

9. Well-bedded to slightly bedded very fine sandy Clay. Colour: olive 5Y 4/3. Organic matter rich. Lamination and organic matter content diminish left of vertical 1. Moderately loose. Moderately moist. Sharp basal contact.

10. Very firm clayey Silt. Colour: greenish grey GLEY 6/10Y. Iron stain throughout. Loess cover on terrace and rip-up clast into channel deposits. Very dense, and moist.

11. . Well sorted medium to fine Sand with sparse clasts. Colour: olive grey 5Y 5/2. Subrounded clasts. Moderately loose. Moderately moist. Sharp basal contact.

12. Massive very fine sandy Silt. Colour: grey 5Y 5/1. Well sorted. Moderately dense. Moderately moist. Gradational basal contact.

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13. Massive very fine sandy clayey pebbly Gravel. Colour: grayish brown 2.5Y 5/2. Matrix to clast-supported. Maximum clast size: 4-5 cm. Poorly sorted. Subangular clasts. Iron oxide staining throughout. Moderately dense. Moderately moist. Sharp, ondulating basal contact

Penn # 2 Trench

1. Modern Root rich layer in matrix of silty, fine Sand. Root size ranges from .5-10 cm in diameter. Primary internal structures overprinted by modern roots. Moderately dense to moderately loose, moist to dry. Sharp basal contact

2. Massive sandy Silt in an intermediate stage of soil development with some modern root intrusions <5 cm in diameter. Colour: greyish brown 2.5Y 5/2. Moderately loose to moderately dense, and moist. Gradual and undulating basal contact

3. Gradational layer from silty, fine Sand to fine sandy Silt with patches of modern root intrusions. Patches of iron oxidation. Colour: Olive 5Y 4/3. Moderately dense and moist. Erosional sharp basal contact.

4. Silty, clayey, fine Sand, with organic material. Slightly laminated. Colour: dark greyish brown 2.5Y 4/2. Moderately dense, moderately moist. Gradational basal contact.

5. a. Clayey Silt with fine sand and minor patches of iron oxidation. Massive and well sorted. Colour: dark greyish brown 2.5Y 4/2. Moderately dense, moderately moist.

b. Clayey Silt with fine sand and minor patches of iron oxidation. Massive and well sorted. Colour: light olive brown 2.5Y 5/3. Dense to moderately dense. Moderately moist.

c. Matrix supported, poorly sorted Gravel. Sub-angular clasts ranging from 1-5 cm of greywacke and clay rip-up clasts. Matrix is composed of silty sand. Matrix colour: Olive 5Y 4/4. Moderately loose to loose, moderately moist. Bottom of gravel layer have sharp, undulating, erosional contact. Top of gravel layer boundary is more planar

d. Clayey Silt with fine sand similar to 5a. Colour: Olive grey 5Y 4/2. Moderately dense, moderately moist.

e. Matrix supported Gravel. Very poorly sorted at base of lens with rip-up clay and angular greywacke clasts up to 5 cm. Fining upwards and presence of laminations at top of lens. Moderately loose, moderately moist. Bottom of gravel layer have sharp, undulating, erosional contact. Top of gravel layer boundary is more planar.

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1) a. Massive homogenous layer that is organic rich Clay with fine sand and silt. Laterally discontinuous with 6b where unit 5e cuts into layer. Colour: dark olive brown 2.5Y 3/3. Moderately stiff, moderately moist. b. Similar to 6a, massive homogeneous layer that is organic rich Clay with fine sand and silt. Also, interfingers with unit 7a. Colour: dark olive brown 2.5Y 3/3. Moderately stiff, moderately moist.

2) a. Silty fine to medium Sand, fining upwards. Interfingers with unit 6b. Presence of small 5cm-long lenses of clay organic rich material. Colour: very dark grey 5Y 3/1. Moderately loose, moderately moist.

b. Fine sand intermixed with lenses of Clay, organic rich material (like 7a). Note: a 25 x 30 cm lens of well sorted fine sand under a 40 x 5 cm lens of clay rich with fragments of organic material. Colour: very dark grey 5Y 3/1. Moderately loose, moderately moist.

3) Matrix supported Gravel. Sub-rounded to sub-angular greywacke clasts that range from gravel to medium gravel size intermixed with abundant clay rip-up clasts. Matrix consists of silty clay with fine sand. Matrix Colour: olive 5Y 5/3. Moderately loose, moderately moist. Bottom of gravel layer have sharp, undulating, erosional contact. Top of gravel layer boundary is more planar

4) a. Massive silty Clay with fine sand with patches of clayey silt. No internal bed forms, large-scale iron oxidation. Colour: pale olive 5Y 6/3-4. Stiff and moist to dry b. Lens within 9a. Massive, matrix supported coarse sand with clayey silt. Lens is 35 x 8 cm. Basal contact:

Below horizontal 1.6: Bottom of channel sharp, undulating, erosional contact. Above horizontal 1.6: Southern edge – contact is gradational. Horizontal 2 – top: Northern side – gradational boundary.

Penn canal exposure (River-bank section)

Unit A) Massive grey to bluish grey fine sandy clayey Silt with pods of coarse to medium sandy clayey Silt and sparse greywacke and mudstone clasts. Locally there are lenses of poorly to moderately sorted, clayey, coarse Sand to fine gravel. Well developed iron-staining throughout. Clasts are rounded to sub-rounded. Very sharp basal contact.

Unit B: Massive orange/red brown to dark red pebbly to cobbly Gravel. Poorly sorted. Clast- supported. Mean clast size: 5 cm. Maximum clast size: 25 cm at the basal lag deposit. Clast composition: greywacke, sandstone and limestone. Matrix is very scarce and consists of fine to coarse sand. Very sharp and erosional basal contact.

Barton trenches

1. Very dark grey massive sandy Silt with abundant roots. Blocky soil structure. Moderately dense. Dry. Gradational basal contact. A soil horizon.

2. Dark grey massive slightly clayey sandy Silt with sparse roots, and sparse (trenches # 1 & 2) to mod. abundant (trench # 3) sub-rounded gravel clasts (pebble size except for

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trench 1 where there are at least 10-20 cm). Abundant burrows. Moderately dense. Dry. Gradational basal contact. B soil horizon.

3. Massive light brown clayey Silt with sparse (~5%) small pebbly gravel clasts (up to 3-4 cm). Sparse charcoal and roots. Moderately dense. Dry. Sharp undulating basal contact.

4. Well sorted light brown to grey brown medium Sand with sparse sub-angular greywacke clasts. Very loose to loose. Dry. Sharp erosional basal contact. Unit is sandier in Barton # 2 trench with smaller gravel clasts. Sand lens in thalweg.

5. Massive light brown very fine sandy cobble to boulder Gravel. Clasts are sub-rounded to rounded greywacke (dominant) and mudstone clast. Mean clast size is 15 cm, maximum size is 30cm. Poorly sorted. Matrix-supported. Loose to moderately loose. Dry. Sharp, undulating basal contact (exposed in Barton #2 trench). Possibly Q2a deg terrace gravels.

6. Massive grey clayey, silty fine Gravel to gravelly coarse Sand. Clast are greywacke (dominant) and mudstone. Mean clast size: 5 mm. Contains small lenses of clay and silt. Gravel is matrix-supported and poorly sorted. Moderately dense to moderately loose. Moderately dry to moderately moist .Sharp undulating basal contact. Possibly Q2a to Q3a alluvium.

7. Massive greyish brown clayey fine Sand. Moderately loose. Moderately moist. Sharp undulating basal contact. Possibly Q2a to Q3a alluvium.

8. Massive light grayish brown fine sandy Gravel. Poorly sorted with clasts ranging from granule to cobble size. Mean clast size: 7-8 cm. Maximum clast size: 20 cm. Clast are greywacke (dominant) and mudstone. Matrix to clast-supported. Moderately dense to moderately loose. Moderately dry to moderately moist. Possibly Q2a to Q3a alluvium.

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APPENDIX 3 SAMPLES COLLECTED

Table A1 OSL samples collected

VUW OSL-age Material dated, location and Trench Sample ID Lab Sample No. (ka) significance E 2736500 N 6042400, 30 m above Penn River Bank-1 WLL667 69.9 ± 4.1 gravels, 70 cm below ground surface Penn 1 P 10-1 From unit 2 Penn 1 P 10-2 From unit 5 Penn 2 P 20-1 From unit 2 Penn 2 P 20-2 From unit 9a back of south wall Penn 2 P 20-3? Penn 2 P 2T-1 Tephra? From unit 2 (&3?) east wall Penn 3 P 30-1 67 cm below surface Penn 4 P 40-1 0.73 cm below surface Penn 4 P 40-2 195 cm below surface E 2732560 N 6037661, 1.2 m below Barton Barton 2-1 WLL668 14.1 ± 1.1 ground surface, from silt over gravel Barton Barton 2-4 WLL669 27.7 ± 2.0 E 2732560 N 6037661, 2.47 m below ground surface, from fine-med sand over gravel and below coarse sand E 2732550 N 6037670, 0.94 m below ground surface, from fine sand over Barton Barton 3-1 WLL670 18.3 ± 1.2 gravel

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Table A2 Radiocarbon samples collected

14 NZA C age Cal. age Material dated, location Trench Sample ID (Lab Number) (yr BP) (cal. yr BP) and significance

Penn 1 P 1-1 Bulk from base of unit 5 Penn 1 P 1-2 Bulk from unit 7 Penn 1 P 1-3 Bulk from unit 9 Penn 1 P 1-4 27639 (29528/1) 2753 ± 30 2864 to 2747 Wood sample from unit 7 Plant material (wood pieces) within clay like soil from unit Penn 1 P 1-5 27689 (29528/3) 4953 ± 35 5659 to 5581 9 Penn 1 P 1-6 Wood from unit 9 Penn 1 P 1-7 Wood from unit 9 Penn 1 P 1-8 From unit 9 Penn 2 P 2-1 From unit 2 Penn 2 P 2-2 From unit 6b Penn 2 P 2-3 Modern root? From unit 8 Twiggy plant material within Penn 2 P 2-4 27688 (29528/2) 1224 ± 30 1175 to 979 dark brown soil from unit 8 Penn 2 P 2-5 Modern root? from unit 9a Penn 2 P 2-6 Modern root? From unit 9a Penn 2 P 2-7 Modern root? from unit 7a Penn 2 P 2-8 Modern root? From unit 5e Penn 2 P 2-9 Modern root? From unit 5e Opposite wall, equivalent to Penn 2 P 2-10 unit 8, just above 9a Penn 2 P 2-11 Bulk sediment from unit 5d Penn 2 P 2-12 Bulk sediment from unit 6a Wood from unit 4 (fine grained clay/silt layer, ~4 cm Penn 2 P 2-13 thick, at top of gravels) Bulk, may contain modern roots, from unit 4, close to gradational boundary with Penn 2 P 2-14 5a Penn 2 P 2-15 Wood/charcoal? From unit 3 Penn 2 P 2-16 Bulk sediment from unit 2 Penn 2 P 2-17 Bulk sediment from unit 7b

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APPENDIX 4 REPORT ON OSL DATING

(Note that sample WLL611 does not belong to this project)

Luminescence dating of 5 samples / fault studies

Report by: Dr. Uwe Rieser Luminescence Dating Laboratory School of Geography, Environment and Earth Sciences Victoria University of Wellington e-mail: [email protected] tel: 0064-4-463-6125 fax: 0064-4-463-5186

Summary 5 samples (laboratory code WLL611 and WLL667-670) were submitted for Luminescence Dating by Pilar Villamor (GNS-Science). All deposition ages have been determined using the silt fraction. The palaeodose, i.e. the radiation dose accumulated in the sample after the last light exposure (assumed at deposition), was determined by measuring the blue luminescence output during infrared optical stimulation (which selectively stimulates the feldspar fraction). The doserate was estimated on the basis of a low level gammaspectrometry measurement. All measurements were done in Victoria Universities Dating Laboratory.

Procedure / Luminescence measurements Sample preparation was done under extremely subdued safe orange light in a darkroom. Outer surfaces, which may have seen light during sampling, were removed and discarded. The actual water content and the saturation content were measured using 'fresh' inside material. The samples were treated with 10%HCl to remove carbonates until the reaction stopped, then carefully rinsed with distilled water. Thereafter, all organic matter was destroyed with 10%H2O2 until the reaction stopped, then carefully rinsed with distilled water. By treatment with a solution of sodium citrate, sodium bicarbonate and sodium dithionate iron oxide coatings were removed from the mineral grains and then the sample was carefully rinsed again. The grain size 4-11 m was extracted from the samples in a water-filled (with added dispersing agent to deflocculate clay) measuring cylinder using Stokes' Law. The other fractions were discarded. The samples then are brought into suspension in pure acetone and deposited evenly in a thin layer on 70 aluminum discs (1cm diameter). Luminescence measurements were done using a standard Riso TL-DA15 measurement system, equipped with Kopp 5-58 and Schott BG39 optical filters to select the luminescence blue band. Stimulation was done cw at about 30mW/cm2 with infrared diodes at 880 80nm. -irradiations were done on a Daybreak 801E 90Sr,90Y -irradiator, calibrated against SFU, Vancouver, Canada to about 3% accuracy. -irradiations were done on a 241Am irradiator supplied and calibrated by ELSEC, Littlemore, UK.

The Paleodoses were estimated by use of the multiple aliquot additive-dose method (with late-light subtraction). After an initial test-measurement, 30 aliquots were -irradiated in six groups up to six times of the dose result taken from the test. 9 aliquots were -irradiated in

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three groups up to three times of the dose result taken from the test. These 39 disks were stored in the dark for four weeks to relax the crystal lattice after irradiation. After storage, these 39 disks and 9 unirradiated disks were preheated for 5min at 220C to remove unstable signal components, and then measured for 100sec each, resulting in 39 shinedown curves. These curves were then normalized for their luminescence response, using 0.1s shortshine measurements taken before irradiation from all aliquots. The luminescence growth curve ( -induced luminescence intensity vs. added dose) is then constructed by using the initial 10 seconds of the shine down curves and subtracting the average of the last 20 sec, the so called late light which is thought to be a mixture of background and hardly bleachable components. The shine plateau was checked to be flat after this manipulation. Extrapolation of this growth curve to the dose-axis gives the equivalent dose De, which is used as an estimate of the paleodose. A similar plot for the alpha-irradiated discs allows an estimate of the -efficiency, the a-value (Luminescence/dose generated by the -source divided by the luminescence/dose generated by the -source).

Fading test Samples containing feldspars in rare cases show an effect called anomalous fading. This effect inhibits accurate dating of the sample, as the electron traps in the crystal lattice of these feldspars are unable to store the age information over longer periods of time. None of your samples gave an indication of this problem.

Procedure / Gamma spectrometry The dry, ground and homogenised soil samples were encapsulated in airtight perspex containers and stored for at least 4 weeks. This procedure minimizes the loss of the short- lived noble gas 222Rn and allows 226Ra to reach equilibrium with its daughters 214Pb and 214Bi.

The samples were counted using high resolution gamma spectrometry with a broad energy Ge detector for a minimum time of 24h. The spectra were analysed using GENIE2000 software. The doserate calculation is based on the activity concentration of the nuclides 40K, 208Tl, 212Pb, 228Ac, 214Bi, 214Pb, 226Ra.

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Results

Table1-Appendix 4 Doserate contribution of cosmic radiation 1 Sample depth dDc/dt (Gy/ka) Field code no. below surface (m) WLL611 1.0 0.1935±0.0097 Bennet-5 WLL667 0.7 0.1875±0.0094 RiverBank-1 WLL668 1.2 0.1749±0.0087 Barton2-1 WLL669 2.47 0.1474±0.0074 Barton2-4 WLL670 0.94 0.1813±0.0091 Barton3-1 1 Contribution of cosmic radiation to the total doserate, calculated as proposed by Prescott & Hutton (1994), Radiation Measurements, Vol. 23.

Table2-Appendix 4 Radionuclide and water contents Sample Water U ( g/g) U ( g/g)2 U ( g/g) Th ( g/g)2 K (%) Field code no. content from 234Th from 226Ra, from 210Pb from 208Tl, 1 214Pb, 214Bi 212Pb, 228Ac WLL611 1.510 2.21±0.16 1.87±0.11 2.33±0.14 7.52±0.10 0.87±0.02 Bennet-5

WLL667 1.164 2.84±0.28 2.73±0.19 3.01±0.26 7.80±0.14 2.06±0.05 RiverBank-1

WLL668 1.209 2.70±0.20 2.74±0.13 2.24±0.17 11.75±0.14 2.06±0.04 Barton2-1

WLL669 1.185 2.40±0.27 2.39±0.18 2.29±0.25 10.34±0.16 2.22±0.05 Barton2-4

WLL670 1.118 2.33±0.28 2.09±0.18 2.73±0.26 10.32±0.16 2.36±0.05 Barton3-1 1 Ratio wet sample to dry sample weight. Errors assumed 50% of ( -1). 2 U and Th-content is calculated from the error weighted mean of the isotope equivalent contents

Table3-Appendix 4 Measured a-value and equivalent dose, doserate and luminescence age

Sample no. a-value De (Gy) dD/dt (Gy/ka) OSL-age (ka) Field code

WLL611 0.068±0.007 26.3±2.2 1.80±0.19 14.6±2.0 Bennet-5

WLL667 0.044±0.002 248.0±4.8 3.55±0.20 69.9±4.1 RiverBank-1

WLL668 0.058±0.007 54.5±2.1 3.88±0.26 14.1±1.1 Barton2-1

WLL669 0.040±0.003 101.6±4.0 3.67±0.23 27.7±2.0 Barton2-4

WLL670 0.043±0.004 73.7±3.8 4.02±0.18 18.3±1.2 Barton3-1

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APPENDIX 5 CORRELATION OF TERRACES OF RUAMAHANGA AND KOPURANGA RIVERS

Because the amount of lateral offset of the studied channel at the Ruamahanga site is much smaller than other location along the fault on Q2a surfaces, we have further explored whether the OSL age obtained for the site agrees with the relative dating of mapped surfaces in the area and with our OSL age for the Kopuaranga River site. OSL samples from similar river terraces have sometimes yielded older than expected ages because the sediment grains were not fully exposed to light (bleached) before deposition. In these cases, the OSL ages are older than the sediment that they were collected from (e.g., Te Maura terraces, IOF-Wellington Fault subproject, Russ Van Dissen pers. com.).

We have used the existing mapping of Quaternary terraces (Fig. 2) and Shuttle Radar Topography Mission (SRTM) 90 m Digital Elevation Data (http://srtm.csi.cgiar.org/) to assess the relative age between different terraces. Although resolution of the DEM is below that which is desirable to undertake this type of study, it is useful in that the elevation of a pixel is representative of the real height in that pixel as recorded from remote sensing. This is an advantage over other publicly available DEMs (e.g. LINZ) where values in relatively flat areas are sometimes extrapolated from distant contour lines. We have produced topographic profiles across and along different terraces to analyse their relative height.

Comparison of different topographic profiles across and along the Ruamahanga and Kopuaranga rivers (Figs. A1-A, B & C) shows that correlation of river terraces across the fault may previously have been misinterpreted. From aerial photo review, the terrace of underlying the Barton trenches (hereafter referred as “Barton site terrace”) and the terrace to the east of it have been assigned a Q1a-deg and Q2a age, respectively (by Townsend et al., 2002) or, alternatively a Q2a and Q3a age, respectively (Lee and Begg, 2002) (Fig. A1- A). These two studies also assign a Q2a age to the extensive terrace on the southwest side of the river (Fig. A1-A)

Our new correlations suggest that the terrace mapped as Q2a on the south-west side of the river is probably younger on the downthrown side of the fault than on the up-thrown side. Also this terrace is apparently not correlative to the Barton site terrace (or the terrace to the east of the site) on the downthrown side of the fault. Profiles 1 and 2 (Fig. A1-B) are taken across the Ruamahanga River on the down-thrown and up-thrown sides of the Wairarapa Fault, respectively. Profile 1 shows that the Barton site terrace and the one to the east of the site are 3 to 5 m higher than the “Q2a” terrace on the south-western side of the river (Lee and Begg, 2002; Townsend et al., 2002) on the down-thrown side of the fault. Profile 2 on the up-thrown side of the fault presents a different story. There the “Q2a” terrace mapped on the south-western side of the river could be the equivalent terrace to the Barton site one.

The non-correlation between mapped Q2a terraces across the Wairarapa Fault is also clear on profiles 3 to 6, which are parallel to the river (Fig. A1-C). The fault scarp on the Q2a terrace on the southwest bank of the river (profile 3) is higher or of the same order (~10 m) as nearby terraces that are older (e.g., Ruamahanga site: 4.5 m, profile 4; Q2a-Q3a? terrace east of Ruamahanga site: ~10 m as shown on profile 5). Because the fault scarp height

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should be progressively higher on older terraces, we believe that the surface mapped as Q2a on the south-west side of the river (profile 3) is in fact two different surfaces across the fault trace, i.e., Q2a on the up-thrown side and Holocene on the downthrown side. The anomalous large size of the fault scarp across Q2a terrace on the southwest river bank can, alternatively, be caused by the great variability in vertical displacement, and thus fault scarp height, typical of strike-slip faults. We think that the lack of features such as push-ups in this terrace suggests that the anomalously large fault scarp at this location is more likely to be related to juxtaposition of different aged terraces across the fault.

Although we have now a better understanding of the relative height (and thus relative age) of the river terraces around the Barton site, we still cannot confirm if the obtained OSL age represents the real age of the terrace there. To explore this further, we have attempted to establish a comparison between the heights of terraces of the Ruamahanga and Kopuaranga rivers. In the Kopuaranga site area, four levels of terraces are preserved above river level and may correspond to Q4a (site itself, dated ~70 ka in this study), Q3a (next level down) and Q2a (second lowest level and more extensive) and Q1a (lowest) (Fig. 5). Ages assigned to the Kopuaranga site, together with elevation data as shown on profile 8 (Fig. A1-D) suggests that the main terrace where the Ruamahanga and Kopuaranga rivers merge is Q2a, as mapped by Lee and Begg (2002) (Fig. A1-A).

To compare the relative terrace height of our two sites, we have plotted together the possible Q2a terrace of both rivers from the point where they merge. On profiles along the Ruamahanga River terraces (profiles 7, 9 and 10 Fig. A1-D) it is not possible to determine which of the three terrace levels grades to the merging point (and thus to Q2a) because these profiles are al displaced vertically where they cross the Mokonui Fault. Although Figure A1-D cannot help us to establish the correlations sought, they clearly display the difficultly in correlating terraces in areas with local uplift and subsidence caused by active faulting.

To avoid having to correlate terraces across active faults, we have produced profile 11 (Fig. A1-E) which is a composite of profile 1 (Fig. A1-B), part of profile 10 (Fig. A1-D) and a profile linking these two. Profile 11 (Fig. A1-E) shows that the Q2a terraces on the northeast side of the Ruamahanga River could either correlate with, or be older (higher) than, the Q2a terraces of the Kopuaranga River at the linking section of the profile. This uncertainty is caused by the low resolution of the DEM used to make the profiles. We can not be more precise in the analysis of the surfaces in the linking section and thus we can not confirm the real inclination of terraces (i.e., if they are sub-horizontal terraces or steep fans) to establish a unique correlation.

Nevertheless, the Barton (Ruamahanga) site terrace seems to stand high in the landscape compared with other more extensive terraces, which suggests that it is unlikely that it has a Q1a age, and thus we conclude that a Q2a age is plausible. The terrace was mapped as a degradation terrace based on the texture of the surface and its height below the Q2a terrace (Q1a_deg; D. Townsend pers. comm.). A degradational origin for this terrace is also suggested by the stratigraphy and the ages of samples in the Barton trenches as explained in main text (Fig. 9, Appendix 2). We therefore assign a Q2a_deg age to the terrace and a tentative Q2a age to the terrace east of the site. This latter uncertain age based on a small difference in height between the two terraces on the up-thrown side of the fault.

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2730000 2732000 2734000 2736000 2738000 2740000 6042000 Kopuaranga Site Q1a Q2a deg Q2a Q3a

6040000 Q4a 6040000 Q6a Q8a

lt u a F a p 11 ra a ir a 6038000 6038000 W

Ruamahanga Site ) 3 (

t l

o 2 k o

M 1 5 6036000 6036000

4 8 6 Lee & Begg (2002) Begg & Lee Townsend et al. (2002) et al. Townsend 3 ) ult (1 ui Fa okon 6034000 M 6034000 ) 7 (2 lt au F 9 ui on ok M

10 Profiles River 6032000 Fault 6032000 Map Boundary 012Km

2730000 2732000 2734000 2736000 2738000 2740000

Figure A1-A Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Location of profiles 1 to 11.

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SW Ruamahanga NE Downthrown Profile 1 Site Q2a/Q3a? NE 197 Q1a/Q2a? NE 196

) 195

m Ruamahanga River (

194

n Q2a SW

o 193 i

t Waipoua River

a 192

v e l 191 Q1a E 190 189

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Distance (m)

SW Upthrown Profile 2 NE 220 Ruamahanga Waipoua River Site

) 215

m (

Q2a/Q3a? NE

n 210 o

i Q2a SW Ruamahanga River t Q1a/Q2a? NE

a 205 v

e Q1a l E 200 Q1a

195

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

Distance (m)

Figure A1-B Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Correlation of terraces across the Ruamahanga River.

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Profiles 3, 4, 5 & 6 SW NE 220 Wairarapa Fault Prof3=Q2a (SW) Prof4 =Site, Q1a-Q2a? (NE) Prof5=Q2a-Q3a? (NE)

210 Prof6=Q1a

) m

( 200

e l

190 E

180

Upthrown Downthrown 170 0 500 1000 1500 2000 2500 3000 Distance (m)

Figure A1-C Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Topographic profiles along different age terraces of Ruamahanga River.

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Profiles 7, 8, 9 & 10 230 SE Q4-Q8 terraces NW 90° change in trend of profile 220 Q2a-Q3a? (NE) Barton site Penn site Q1a-Q2a? (NE) 210 Wairarapa Fault

200 Mokonui Fault (3) Q2 190

) Mokonui Fault (3) m 180 ( 2a

n o

i Q2a? (SW)

t 170

v e l a Q2 E 160 Mokonui Fault (2) Prof 7 (Q2a-Q3a? Ruamahanga NE) 150 Q2a Mokonui Fault (1) Prof 8 (Q2a Kopuaranga)

140 Prof 9 (Q1a-Q2a? deg Ruamahanga NE) Prof 10 (Q2a Ruamahanga SW)

130 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000

Distance (m)

Figure A1- D Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Topographic profiles along different age terraces of Ruamahanga River, and along Kopuaranga River. Profiles start at point where both rivers merge.

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Profile 11

SW NE

Ruamahanga Site

Q4a

)

m (

Terrace

i t

a Q2a-Q3a? (NE) Fan? v

e Q2a?

l E

Ruamahanga River

Profile 1 Profile 10 from 90° bend to Wairarapa Fault Ruamahanga River Kopuaranga River

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Distance (m)

Figure A1- E Correlation of river terraces around the Ruamahanga and Kopuaranga sites. Correlation of Ruamahanga River terraces with Kopuranga River terraces.

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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