Assessment of active fault and fold hazards in the area, Mackenzie District, South Canterbury Report No. R10/25 ISBN 978-1-877542-91-6

Prepared for Environment Canterbury by DJA Barrell GNS Science Consultancy

May 2010

Report R10/25 ISBN 978-1-877542-91-6

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Assessment of active fault and fold hazards in the Twizel area, Mackenzie District, South Canterbury D.J.A. Barrell

GNS Science Consultancy Report 2010/040 Environment Canterbury Report No. R10/25 May 2010

DISCLAIMER

This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to Environment Canterbury (ECan). Unless otherwise agreed in writing by GNS Science, GNS Science accepts no responsibility for any use of, or reliance on, any contents of this report by any person other than ECan and shall not be liable to any person other than ECan, on any ground, for any loss, damage or expense arising from such use or reliance.

The data presented in this report are available to GNS Science for other use from May 2010

BIBLIOGRAPHIC REFERENCE

Barrell, D.J.A. 2010. Assessment of active fault and fold hazards in the Twizel area, Mackenzie District, South Canterbury. GNS Science Consultancy Report 2010/040. 22 p.

© Environment Canterbury Report No. R10/25 ISBN 978-1-877542-91-6

Project Number: 440W1435 2010

CONTENTS

EXECUTIVE SUMMARY ...... II 1. INTRODUCTION ...... 1 2. INVESTIGATION METHODS...... 1 3. GEOLOGICAL SETTING...... 2 4. RECOGNITION OF ACTIVE FAULTS AND FOLDS...... 6 5. FAULT AND FOLD HAZARD ASSESSMENT ...... 8 5.1 Data collection...... 8 5.2 Summary description of fault deformation features...... 9 5.3 Do the fault-related features in the Ostler Fault Zone represent hazards?...... 11 5.4 Types of fault and fold hazards ...... 12 5.5 Fault avoidance zones ...... 13 5.6 Implications for planning and land-use...... 14 5.7 Hazard assessment...... 16 6. CONCLUSIONS ...... 19 ACKNOWLEDGEMENTS...... 20 REFERENCES...... 20 FIGURES APPENDIX A – DEFORMATION ANALYSES APPENDIX B – GIS LAYERS & REPORT PDF

TABLES (in text)

Table 1: Ground classification Table 2: Building Importance Categories and representative examples Table 3: Relationships between recurrence interval and Building Importance Category Table 4: An example of resource consent categories in relation to fault complexity classes for the Ostler Fault Zone

PHOTOS (in text)

Photo 1: Panorama of the Ruataniwha and Y faults, looking southwest from Mt Ostler Photo 2: Aerial view looking northeast across the investigation area. Photo 3: Detail of the surface expression of the Ruataniwha and Y faults Photo 4: Fault scarp formed on the Chelungpu Fault during the magnitude 7.6 Chi-Chi Earthquake, Taiwan, 1999 Photo 5: The Haybarn Fault Photo 6: Tilted deposits in the cliff beside Fraser Stream

FIGURES (following text)

Figure 1: Location and geological setting Figure 2: Ground classification Figure 3: Fault complexity classification Figure 4: Fault avoidance zonation

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EXECUTIVE SUMMARY

This report presents an assessment of active geological fault and fold hazards in the vicinity of Twizel, South Canterbury. The feature of interest is the Ostler Fault Zone, which has offset or buckled a series of geologically-young landforms in a zone stretching from the west side of Lake Pukaki southwest to the Omarama area.

Geological evidence from radiocarbon dating indicates that on average, the Ostler Fault Zone experiences an earthquake large enough to cause rupture (i.e. offset) and buckling of the ground surface every 3,000 ± 1,000 years. The most recent known surface-rupture earthquake was about 3,600 years ago. While this does not mean that an earthquake is imminent, it leaves no room for complacency. It should be assumed that significant and abrupt ground breakage, perhaps involving up to several metres of movement, and buckling of the ground, will occur within the Ostler Fault Zone during the next rupture. We do not know when that may be, but the geological evidence implies that this is a credible hazard that could happen at any time in the future. Minimising or avoiding the adverse effects of a ground-rupturing earthquake should therefore be a priority issue in land-use planning and development in the Twizel area.

The approach to active fault and fold hazard assessment described in this report involves three principal components:

• mapping and description of physical landform features in the vicinity of the fault zone

• interpretation and classification of the landform features that relate to past ground deformation events

• relating this information to standard hazard planning guidelines, in order to produce a hazard zonation map that can be used to assist in land-use zonation and for assessing the suitability of different areas for development, such as rural residential subdivision.

This work has highlighted areas that, according to guidelines and interpretations of legislation, are sufficiently hazardous as to warrant avoidance of certain types of buildings. Various uncertainties in the landform evidence of past movements are accommodated by placing hazard avoidance set-back distances from the most hazardous areas. Further, more detailed site investigations may be able to locate more precisely the extents of hazardous areas, and may provide a basis for narrowing the width of hazard avoidance zones. More detailed assessments may be able to show that some areas subject to less severe ground deformation hazards are suitable for certain categories of buildings.

The approaches described in this report should be applicable elsewhere in the general region, on the Ostler Fault Zone or on other active fault or fold systems.

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1. INTRODUCTION

One of the ’s better known active geological faults, the Ostler Fault Zone, lies, in part, just to the west of Twizel township in the of South Canterbury. The spectacular open landscape surrounded by mountains and proximity to sporting and recreational opportunities have created demand for residential development in Twizel, in particular to the west of the town. Mackenzie District Council (MDC) is considering a change to land-use zoning in an area surrounding and west of Twizel. Part of that area (proposed plan change area) lies within the Ostler Fault Zone (Fig. 1).

The main potential hazard associated with active faults is the occurrence of a sudden slip event (‘rupture’) on the fault, causing a large earthquake. Ruptures commonly begin deep in the earth’s crust and, if of large size, may extend up to the ground and cause ‘surface rupture’, which involves a sudden offset (faulting) or buckling (folding) of the ground surface by up to several metres. Buildings situated within a zone of sudden offset and/or buckling are likely to suffer serious damage or even destruction. This poses a significant threat of injury or death to occupants of any such buildings.

Guidelines for planning to mitigate the threat to life safety due to active fault ruptures are presented in a Ministry for the Environment report on “Planning for development on or close to active faults” (Kerr et al. 2003). To aid in the implementation of these guidelines, a number of active fault hazard assessments have been undertaken elsewhere in , such as Wellington City (Kerr et al. 2003) and Kapiti Coast District (Van Dissen and Heron 2003, Van Dissen et al. 2004, 2006).

In consultation with MDC, Environment Canterbury engaged the Institute of Geological and Nuclear Sciences Limited (GNS Science) to provide an assessment of active fault and fold hazards in relation to the proposed plan change area. This report presents the results of that hazard assessment. Information in this report is intended to provide a technical basis for the avoidance or mitigation of active fault and fold hazards in the proposed plan change area.

2. INVESTIGATION METHODS

This assessment of active geological fault hazard is based on:

• examination of existing publicly-available information on the Ostler Fault Zone, including hydro-electricity investigation/construction reports, and aerial photographs;

• results from a previous site ‘walkover’ examination of geological features, and high- precision Global Positioning System (GPS) surveying of geological features, in the ‘Ruataniwha area’ (Barrell 2005), as shown in Fig. 2;

• additional mapping based on interpretation of aerial photos in the Twizel area (Fig. 2);

• evaluation and interpretation of the information obtained.

The investigations were confined to surface observations and measurements. No

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excavations or other subsurface investigations were undertaken.

3. GEOLOGICAL SETTING

Twizel lies on glacial meltwater outwash plains formed by the Ohau River during ‘Ice Age’ glacial events in the region (Photos 1 & 2). During Ice Age cold climate conditions, glaciers extended into the area of ‘glacial’ lakes Ohau and Pukaki, depositing moraines at the ice termini and building broad plains of river gravel downstream of the moraines. Intervening periods of warm climate, such as the present day, saw the glaciers retreat far back into the mountains, while the rivers cut deep terraced valleys downstream of the lakes. Each major glacier advance has been responsible for the build-up of river gravel within former river valleys and the formation of broad outwash plains.

Near Twizel, there are two main glacial outwash terraces, which in the Mackenzie Basin are named after geographic locations near Lake Tekapo. The older, higher, ‘Balmoral’ outwash plain (Fig. 1) is thought to be at least 60,000 years old (Cox & Barrell 2007). Cut below the level of the Balmoral terraces is the ‘Mt John’ outwash plain. The Mt John outwash plain was formed at the peak of the last Ice Age, approximately 20,000 years ago (Schaefer et al. 2006; Cox & Barrell 2007). As the Ohau glacier began to retreat, a series of river-cut small terrace steps were formed on the Mt John outwash plain (see Fig. 2). The final event of the last Ice Age was a smaller re-advance of the glaciers about 18,000 years ago (Schaefer et al. 2006) which formed the ‘Tekapo’ outwash plain. At the head of Lake Ruataniwha, the Tekapo outwash plain lies about 30 m below the level of the Mt John outwash plain.

Photo 1: View southwest from Mt Ostler, looking across the approximately 20,000 year old Mt John outwash plain. The Ruataniwha Fault scarp (red arrows) forms the sinuous line trending up the photo, while the Y Fault scarp forms a less distinct sinuous line trending up the photo centre (pink arrows). The darker green lines running right to left are relict channels on the former river plain (blue arrows). All of the ground in this view to the right of the Ruataniwha Fault scarp has been deformed to some extent by fault offset, warping or tilting of the ground surface.

Windblown river silt has formed a blanket of yellow-grey loess on many parts of the landscape. A metre or more of loess mantles the Balmoral outwash plain, giving it a rather featureless and subdued appearance compared to the prominent braided river channel patterns on the Mt John and Tekapo outwash plains, which have only thin patches of loess.

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Photo 2: Aerial view northeast across Ohau A power station, Ohau and Pukaki canals and part of the investigation area. Photo from Macfarlane (1981).

The Ostler Fault Zone extends from the west side of Lake Pukaki southwest to the Ohau River (now Lake Ruataniwha) (Fig. 1) and continues southwest to the Ahuriri valley (e.g. Read 1984; Blick et al. 1989; Davis et al. 2005; Amos et al. 2007; Ghisetti et al. 2007;

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McClymont et al. 2008; Campbell et al. 2010; Amos et al. in press). The Ostler Fault Zone is one of the most spectacular active fault lines in New Zealand, because it crosses a large area of unforested landforms (Last Ice Age and earlier). Progressive displacement and buckling of the ground surface over more than 18,000 years is preserved pristinely on outwash plains (Mansergh & Read 1973; Read 1984; Macfarlane 1981; see Photos 1 to 3). The fault zone comprises at least three, partly overlapping, strands. The northernmost strand is the Haybarn Fault, while the central strand is the Ruataniwha Fault (Fig. 1). The Y Fault is probably a subsidiary break-out from the Ruataniwha Fault at depth. On the Haybarn Fault, the amount of offset of landforms decreases southwest of Fraser Stream, and decreases rapidly between Ben Ohau Road and Manuka Terrace. Near Fraser Stream, the Ruataniwha Fault appears to the east of the Haybarn Fault (Fig. 1). The offset of landforms across the Ruataniwha Fault increases progressively southwestward. At the Ohau River, the Haybarn Fault has died out and the Ruataniwha Fault is the main feature of the Ostler Fault Zone.

As a whole, the Ostler Fault Zone is inclined (‘dips’) towards the west and the sense of fault movement is ‘reverse’, which means that the west side has been progressively thrust up over the east side. West of Twizel, movements associated with the Ruataniwha and Y faults have broken and warped the ground surface at several locations within a ~1 km wide zone parallel to the fault (e.g. Read 1984; see Appendix and Photo 1). In this area, the maximum total vertical uplift across the fault zone over the past 20,000 years is of the order of 20 m, which equates to an average long-term rate of vertical slip of about 1 mm per year.

It is generally considered that active faults that have broken the ground surface evolved by sudden displacements causing large earthquakes (Photo 4), with intervening long periods of no displacement. The Ostler Fault Zone is unusual because, in the Lake Ruataniwha to Fraser Stream areas, the ground has been bulging by ‘creep’ over the past few decades (e.g. Blick et al. 1989, Van Dissen et al. 1993 – see Appendix). The creep has been detected near the Ruataniwha and Y Faults, but has not been detected across the Haybarn Fault. Although this episode of creep has been at a rate similar to the long term slip rate of the fault (Blick et al. 1989), it does not mean that large earthquakes do not occur on the fault.

An investigation trench excavated across the Haybarn Fault in the Twizel River headwaters revealed a series of abrupt offsets within river sediments, indicating that large surface- rupturing earthquakes have occurred on the fault (Van Dissen et al. 1993; see Fig. 1). Based on radiocarbon dating, Van Dissen et al. (1993) concluded that the most recent ground- rupturing earthquakes occurred at about 3,600, 6,000 and 10,000 years ago. This implies an average recurrence interval of ground-rupturing earthquakes of about 3,000 ± 1,000 years. This led Van Dissen et al. (2003) to classify the Ostler Fault Zone as a Recurrence Interval (RI) Class II active fault (i.e. it has an average recurrence interval that lies in the range of 2,000 to 3,500 years).

We do not know whether the Haybarn and Ruataniwha faults rupture in unison, or whether they rupture independently at different times. As there is no direct evidence for the timing of ruptures on the Ruataniwha Fault, usual practice is to assume that the history of movement recorded in the upper Twizel River also applies in the Twizel area, and that the Haybarn and Ruataniwha faults (as well as their subsidiary faults and folds), all experience offset or growth in unison. In other words, at least three ground-surface rupture earthquakes are assumed to have occurred in about the last 10,000 years on the Ostler Fault Zone near Twizel.

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Photo 3: Top: View southwest along the Ruataniwha Fault scarp (red arrows) showing eastward tilted ground extending many tens of metres right (west) of the fault. Bottom left: The Y Fault scarp (pink arrows) cuts across a broad, 3 m high, approximately 20,000 year old, river terrace edge (blue arrows) that runs from bottom left up to centre middle. Here, the fault is marked by a sharp 3.5 m high scarp, with a broad zone of anticlinal upfolding on the west (upthrown) side of the fault – far distance. Bottom right: Farther north along the Y Fault, less of the offset is in the fault scarp (pink arrows) and more is in the anticlinal upfold. Note the fence disappears from sight over the crest of the upfold. An additional small 0.5 m high fault scarp (orange arrows) lies just in front of the vehicle, and indicates that at least one fault rupture involved migration of movement about 14 m out from the main fault scarp.

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Photo 4: Fault scarp formed on the Chelungpu Fault during the magnitude 7.6 Chi-Chi Earthquake, Taiwan, 1999. The disrupted running track shows damage typical of a ground-surface rupture. This location lies on a stream terrace that is younger than the last rupture event on the fault, so that there was no scarp here before the earthquake. Vertical offset is about 3 m. This example illustrates the sorts of effects that can be expected at the Ostler Fault Zone when its scarp(s) grow in the next surface rupture earthquake (whenever that may be). Photo and information from Kelson et al. (2001).

4. RECOGNITION OF ACTIVE FAULTS AND FOLDS

A distinction is made in regard to the style of active deformation, whether predominantly by fault offset of the ground (fault scarp), or whether by folding (buckles, tilts or warps) of the ground. Folds are subdivided into ‘one-sided folds’, or monoclines, and ‘two-sided upfolds’, or anticlines. Active downfolds (synclines) are not a prominent feature of the Twizel area.

The key evidence for recognising active faults or folds is the offset or buckling of landforms or young geological deposits. This is seen most clearly on old river terraces or plains, where the original channel and bar patterns of the former riverbed are ‘fossilised’ in the landform (Photos 1 & 2). Topographic steps or rises that cut across such river-formed features cannot have been formed by the river, and therefore result from subsequent deformation of the ground. As long as factors such as landsliding can be ruled out, these topographic features may be attributed with confidence to active fault or fold movements.

There is rarely an exact distinction between a fault and monocline. Fault scarps are commonly associated with some buckling of the ground and near-surface layers, particularly on the upthrown side of the fault (Photos 4 & 5). In some cases, part of the fault movement may have broken out on smaller subsidiary faults in the vicinity of the main fault (Photo 3).

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Photo 5: The Haybarn Fault. Top left: Looking west from atop Mt Ostler, the Haybarn fault scarp, with a dotted red line at its base, has offset the Balmoral outwash plain (right of dotted blue line marking the top of the terrace edge) by about 18 m, while the fault scarp is no more than about 6 m high on the Mt John outwash plain (left of the blue line). Note the location of Ben Ohau Road crossing the fault scarp (other photos). Bottom left: Looking west towards the morphologic crest of the upthrown side of the Haybarn Fault scarp on Ben Ohau Road (vehicle sits at approximate location of the crest). Although the main fault offset is not exposed (the road cutting is not deep enough to reveal it), the top of Balmoral outwash gravel (thick orange line) has been warped down towards the viewer. At least 4 steeply west-dipping subsidiary faults (red lines) displace the top of the gravel. Upper right: Tape is ~1 m long. This subsidiary fault shows ~ 0.6 m normal displacement (west side down, tensional movement). Lower right: This other subsidiary fault (and two others) shows ~ 0.4 m reverse displacement (west side up, contractional movement). Windblown silt has accumulated on the downthrown sides of these faults, smoothing and obscuring their expression on the ground surface. Thin orange lines mark geologic boundaries within the outwash gravel, which have been offset by the faults.

In the case of monoclines or anticlines, subsidiary faults may also occur over buried faults that underlie these folds, resulting in small ground surface offsets (e.g. Kelson et al. 2001). The important message is that on any active fault or fold, there commonly are elements of both faulting and folding close to the ground surface, and the proportions of faulting to folding deformation may vary over short distances. Typically on active faults and folds like those of the Ostler Fault Zone, surface deformation is widest on the upthrown side of the fault or on

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the up-flexed side of a monocline.

In practice, where the zone of ground deformation is quite narrow, we interpret it as a fault, and where it is broad, we interpret it as a fold (e.g. monocline). The best way to determine the accuracy of this interpretation is by excavating a trench across the deformed zone to try and see the extents to which the near-surface deposits have been offset, or merely folded.

River-related landforms are ideal for detecting and measuring fault-related deformation. Generally speaking, river beds have a uniform and predictable slope down-valley. Localised departures from this may reflect folding or buckling. However, braided river systems pose a challenge by way of their channel and bar morphology. On the Mt John outwash plain, there is commonly as much as 2 m of relief between channels and bars, creating ‘noise’ that may obscure the effects of deformation. For the purposes of fault hazard assessment in relation to the Ostler Fault Zone, the spacing of twenty-metre interval topographic map contours (NZMS 260 series: H38, 1:50,000 scale) between 1 and 3 km west of the Haybarn Fault, and near Twizel township, at locations thought to be beyond the influence of deformation associated with the Ostler Fault Zone, indicate an original slope of the Mt John outwash plain of approximately between about 0.5° and 0.6° towards the east. Departures from these values illustrate fault- and fold-related deformation of the outwash plain surfaces.

5. FAULT AND FOLD HAZARD ASSESSMENT

5.1 Data collection

Detailed field surveys were carried out near the Ruataniwha and Y faults in 2005 (Barrell 2005; Ruataniwha area, Fig. 2). All identifiable fault and fold features were mapped, including the tops and bases of fault scarps and monoclinal flexures, and crests of anticlines. River-cut terrace features were also mapped, as these provide reference indicators for determining the amounts of deformation (Fig. 2). In areas of obviously tilted ground, slope angles and directions were measured using a hand-held clinometer. Hand-held GPS units were used for general surveying, but key features, including crests and bases of folds or faults and river-cut terraces, were surveyed using differential GPS, which has height accuracy of approximately ± 2 cm and even better horizontal accuracy.

Faults and folds are typically mapped as lines, and this was done in generalised form (Fault mapping; Fig. 2). However, even the narrowest and sharpest of fault–related features are, from top to base, at least several metres wide when viewed from above (i.e. in map or plan view). Therefore, in addition, the faults and monoclines were also mapped as zones, or areas (Ground classification; Fig. 2). The line depiction is useful at generalised scales, while the mapped areas suit more detailed map scales.

Beyond the coverage of the 2005 work, for the purposes of this report the remainder of the proposed plan change area (Twizel area - Fig. 2) was mapped in a more generalised manner using information from existing reports (e.g. Macfarlane 1981) as well by interpretation of vertical aerial photographs, viewed stereoscopically (which provides a 3-dimensional view of the ground). The best resolution aerial photos in the GNS Science collection are NZ Aerial Mapping SN2723, photos 15 to 17, taken in 1959 and therefore providing a record of the

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natural landscape before the extensive earthworks associated with hydro-electricity development in the 1970s. Faults and monoclines were mapped as lines and the main fault scarp also mapped as an area.

The ground was classified as shown in Fig. 2 and Table 1. The ground classification is based on physical characteristics of the ground surface (e.g. slope angle) within a context of ground surface age and the origin of its form. The locations of past fault rupture breakage (see Photo 4) or flexure-related cracking of the ground have long since healed over, and the soil re- formed, since the last surface rupture (several thousand years ago). Therefore, the mapping and classification rely largely on the steepness of the ground slope, as a means of interpreting the locations and extents of fault scarps and monoclinal flexures.

Landforms of Balmoral age are much more deformed than those of Mt John or Tekapo age. This is because the Balmoral landforms are significantly older, and have therefore accrued a greater amount of deformation. The classification of hazards in areas of ‘younger land surfaces’ is made difficult where the landforms are younger than the most recent fault deformation event(s), and thus conceal the fault or fold locations. In the cases of gullies or steep slopes, these landforms are not as uniform in character as Ice Age outwash plains, and are less suitable for showing the effects of small amounts of deformation.

5.2 Summary description of fault deformation features

Mt John/Tekapo outwash plains

Deformation associated with the Ruataniwha and Y faults shows a great deal of complexity and variability over short distances (Photos 1 & 3; Fig. 2). The Ruataniwha Fault is sharpest at Max Smith Drive, where it has a well-defined, narrow scarp about 20 m high, offsetting both the Tekapo and Mt John outwash plains by about the same amount. Farther north it devolves into a monoclinal flexure with a low fault scarp at its foot (Photo 3), and is quite sinuous in map view. The Y Fault shows similar types of variability. In addition, there are localised fault scarps and flexures that have the opposite sense of upthrow, that is, up to the east rather than the west. The surface deformation suggests that the fault movements, generated at depth (i.e. several kilometres), have emerged rather diffusely near the ground surface, creating a complex array of fault scarps, flexures, anticlines and tilted ground.

Balmoral outwash plain

Balmoral landforms show much greater amounts of fault offset and tilting than do the Mt John and Tekapo landforms. Extensive areas of ‘moderately tilted’ ground north of Mt Ostler are a significantly older (and therefore more tilted) equivalent to slightly-moderately tilted’ ground on the Mt John outwash plain south of Mt Ostler. Steep-sided gullies cut into the Balmoral outwash plain north of Mt Ostler (Fig. 2) are much younger landforms than the tilted Balmoral landforms into which they are cut. The complexity of deformation represented by the Y fault is not so clearly expressed on the Balmoral outwash plain. Reasons for this may include the loci of deformation having been obscured by loess (Photo 5). Alternatively the deformation may have died out northwards towards Fraser Stream, or perhaps converged back onto the Ruataniwha Fault. At Fraser Stream, deformation to the west of the Ruataniwha Fault is predominantly tilting and no significant subsidiary faulting is evident (Photo 6).

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Table 1: Ground classification (see Fig. 2)

Ground Description Likely effects on Fault classification (slope angles are relative to original dwellings in a ground- complexity

ground slope of ~0.5°) deforming earthquake class (Fig. 3)

Older land surfaces (sufficient age to exhibit effects of past fault surface-rupture events)

Fault scarp Well defined, indicating physical Very severe damage or breakage of the ground by fault destruction; major life safety movement at the base of or within threat to occupants. Well-defined the feature. Associated warping or deformation tilting. Slope generally at least 10° steeper than original ground slope.

Monoclinal Broader well-defined feature with Slight to severe damage; flexure (and clear tilting (generally 2 to 10° possible life safety threat to Distributed broad fault slope) of original ground surface. occupants from toppling or deformation scarp) Possibly affected by diffuse ground sliding furniture. breakage as well as tilting.

Moderately Clearly defined tilt (generally 2 to Slight to moderate damage; Uncertain tilted ground 4°) of original ground slope. In possible life safety threat from deformation – places may grade into a flexure. toppling or sliding furniture. constrained Slightly to Slight but noticeable tilt (typically Building out of plumb. Ground moderately 0.5 to 1°) relative to the original shaking effects dominate life tilted ground ground slope. safety hazard to occupants. Uncertain deformation – Slightly tilted Slight tilt (typically <0.5°) relative to Building slightly out of plumb. poorly ground the original ground slope. Ground shaking effects constrained dominate life safety hazard to occupants. Not deformed The ground surface has not been Ground shaking the sole deformed or tilted by fault-related source of damage and life No hazard movements. safety hazard to occupants. Younger land surfaces Ground Natural ground surface removed or modified by obscured by human activity. earthworks Stream Active land surface feature As per floodplain or adjacent lake surface zone(s). Characteristics of adjacent Relative youth Gully floor Floor of gully incised into Balmoral older ground surfaces prevents outwash plain on Mt Ostler block assumed to apply. reliable Steep slope Steep slopes cut into Balmoral assessment of outwash plain on Mt Ostler block deformation Abandoned River floodplain not currently active river or stream plain

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The large scarp of the Ruataniwha Fault, 60 to 80 m high, at the eastern margin of the Mt Ostler block, is classified as a ‘broad fault’ in the accompanying GIS data set, and included within the distributed deformation class in Fig. 3. A number of minor fault scarps, perhaps up to several metres high, are evident on the broad fault scarp, particularly near its crest (Fig. 2). They may be comparable to the minor faults illustrated in Photo 5 of the Haybarn Fault.

5.3 Do the fault-related features in the Ostler Fault Zone represent hazards?

A key question is whether or not large ground-rupture earthquakes in fact occur on the Ostler Fault Zone. The evidence for ongoing slow ground deformation near the Ruataniwha and Y faults (Appendix) could suggest that all of the deformed land surfaces are the result of slow creep, rather than sudden ruptures. Three lines of evidence favour the occurrence of large earthquakes, as the origin of at least some, if not most, of the ground deformation features;

• Van Dissen et al. (1993) found evidence for apparently abrupt offsets of subsurface deposits, which tends to support the occurrence of large ground-rupture earthquakes;

• There is no difference in the amount of deformation of the Mt John and Tekapo outwash surfaces at the Ruataniwha or Y faults. These outwash surfaces are at least 2,000 years different in age (Schaefer et al. 2006). If steady slow creep were the source of all the deformation, there should be a noticeable difference in the amounts of deformation of each surface. More likely the deformation has occurred in sudden earthquakes, and no earthquake happened to occur during that 2,000 year period.

• The occurrence in a few places of more than one sharp step in the Ruataniwha and Y fault scarps (as described by Barrell 2005, also see Mansergh & Read 1973 and Read 1984), suggests the occurrence of discrete earthquake ruptures. If all deformation were due to steady slow creep, in all likelihood the creep would have emerged on a single shear plane, and there would not be multiple steps.

On this basis, it seems reasonable to assume that large ground-rupture earthquakes have occurred in the past, and will occur again at some time in the future.

Photo 6: View southwest towards the stream-cut cliff beside Fraser Stream, west (upstream) of the Ruataniwha Fault. Photo from Macfarlane (1981).

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5.4 Types of fault and fold hazards

In relation to a feature such as the Ostler Fault Zone, there are at least three types of categories of fault-related hazard.

Ground deformation hazard (see Table 1 & Fig. 3)

‘Fault complexity’ is a useful way of defining ground deformation hazard. Where fault-related deformation is distributed over a wide area, the amount of deformation at a specific locality within the distributed zone is less compared to where the deformation is concentrated on a single well-defined sharp fault scarp. The relative fault rupture hazard is therefore less within a zone of distributed deformation than within a narrow well defined zone. Following the approach used for the Kapiti Coast District (Kerr et al. 2003; Van Dissen & Heron 2003; Van Dissen et al. 2004), the categories of fault complexity used in this report are listed below. a) ‘Well-defined’ deformation zones, potentially subject to fault rupture (sudden physical displacement or deformation of the ground due to a fault breaking through to the ground surface and forming a fault scarp, or increasing the size of an existing scarp), along with flexing (sudden physical buckling or tilting of the ground over a fairly narrow zone, probably including localised subsidiary ground rupture). b) ‘Distributed’ deformation zones, potentially subject to significant flexing or tilting (sudden physical buckling or tilting of the ground over a fairly wide area, possibly including localised subsidiary ground rupture). c) ‘Uncertain’ deformation zones, potentially subject to slight or moderate tilting (sudden physical tilting of the ground over a wide area).

In this report, fault scarps are considered to be ‘well-defined deformation’ because their boundaries are quite clearly defined on the ground surface. Monoclinal flexures and broad fault scarps have more diffuse or ill-defined boundaries so are regarded as ‘distributed deformation’. Categories of ‘extended’ deformation (precisely-located features extrapolated to areas where the mapping is less detailed), ‘moderately tilted’ ground (identified here as ‘uncertain deformation – constrained’), and ‘slightly’ or ‘slightly to moderately tilted’ ground (identified as ‘uncertain deformation – poorly constrained’), follow the concepts developed in Section 12 of Kerr et al (2003) and also explained by Van Dissen et al. 2004.

This report differs from the Kerr et al. (2003) guidelines in that tilted ground (not specifically addressed in the guidelines) is assigned here to the uncertain deformation category. The word ‘uncertain’ is used to maintain a clear linkage to the Kerr et al. (2003) guidelines. It is not so much the location of the deformation that is uncertain, but rather an uncertainty in the effects of future deformation events. Areas classified as ‘uncertain deformation zone’ have in the past been subject to some deformation, and thus are identified here as warranting further evaluation should any significant land-use development be proposed,

Permanent elevation change hazard

Permanent, differential, changes in ground elevations as a result of progressive slow creep. This relatively minor hazard could, for example, compromise gravity drainage systems.

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Earthquake shaking hazard

Strong ground motions (up-down and sideways shaking) associated with a large earthquake on the Ostler Fault Zone. Ground liquefaction is also a potential hazard in areas underlain by wet sandy deposits. Earthquake shaking hazards affect those areas subject to the ground deformation hazards, as well as affecting the surrounding region (inland South Canterbury & inland North Otago).

5.5 Fault avoidance zones

A fault hazard/risk zonation was undertaken as part of the hydro-electricity development (Mansergh & Read 1973; Macfarlane 1981; see Appendix). That assessment took account of how the fault zone may change or evolve during future earthquakes, and thus the moderate and high risk zones were quite broad.

The present report utilises the guidelines of Kerr et al. (2003) for the purposes of avoiding the most serious hazards of fault-related ground deformation within the Ostler Fault Zone. As described in the guidelines, a fault avoidance zone comprises the mapped zones of previous ground surface deformation and, in addition, a buffer zone at least 20 m wide either side of each deformation zone.

Fault scarps, broad fault scarps and monoclinal flexures (i.e. well-defined and distributed deformation) are considered to represent a significant ground deformation hazard, and as such present threats to the safety of occupants of any dwelling sited on these classes of ground, were there to be a sudden deformation event. Lesser hazards relate to moderately tilted ground, slightly to moderately tilted and slightly tilted ground, and thus are assigned to an ‘uncertain’ ground deformation hazard zone.

A fault avoidance zone needs to take into account the uncertainty in location of the deformation features. Following the field surveys in 2005, and a similar survey of the Haybarn Fault in 2007/08 (Barrell 2008), I concluded that, by visual examination on the ground, I could pinpoint the tops and bases of a sharply expressed fault scarp to no better than about ± 5 m. Boundaries of features such as flexures, anticlines or the limits of tilted ground, could be pinpointed to, at best, ± 10 m, and more commonly ± 20 m to ± 50 m. In the ‘Twizel area’, mapped using aerial photos, the location uncertainty is considered to be about ± 50 m. This uncertainty was accommodated via a 50 m wide uncertainty band mapped either side of faults and monoclines in the ‘Twizel area’ (see Fig. 3; ‘extended’ features).

Hazard avoidance zonation is difficult in the vicinity of the Ruataniwha and Y faults, where the deformation is both complex and variable over short distances. The example shown in Photo 3 of a small fault scarp 14 m out in front of the main scarp of the Y Fault highlights that the locus of deformation is not sharply defined. It was noted that, in general, the bases of the fault scarps and flexures were more clearly defined than their crests. Taking all of these factors into account, it is proposed that hazard avoidance zones include a 50 m buffer placed along the bases of the well-defined and distributed deformation zone features, along with a 100 m wide avoidance zone on the crests of these deformation zones (Fig. 4). These are suggested to provide a robust margin of safety, cognisant of the many uncertainties, and noting that location uncertainty is already accommodated in the mapping based on aerial

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photos (see previous paragraph).

It is emphasised that the recommended widths of the hazard avoidance zones may be able to be reduced if detailed site specific investigations (i.e. earthquake-geology trench excavations) were to determine exact locations (e.g. ± 1 m) for the boundaries of previous deformation, and the number and ages of previous fault rupture events. Such investigations, of course, are costly, and would need to be in keeping with the overall economics of the proposed development. There remains a risk that such investigations may be inconclusive, or may provide an unfavourable outcome for the viability of the development.

Areas classified as ‘uncertain deformation zone’ have in the past been subject to some deformation, and thus there is a potential hazard that may require evaluation with regard to any proposed future developments.

5.6 Implications for planning and land-use

The MfE guidelines define five ‘Building Importance Categories’ (BIC), as listed in Table 2. The guidelines make a distinction between previously subdivided and/or developed sites, and undeveloped ‘greenfield’ sites, and allow for different conditions to apply to these two types of sites (Tables 3 & 4).

Table 2: Building Importance Categories and representative examples.

Building Description Examples Importance Category Temporary structures with • Structures with a floor area of <30m2 1 low hazard to life and other • Farm buildings, fences property • Towers in rural situations Timber-framed residential 2a • Timber framed single-story dwellings construction • Timber framed houses with area >300 m2 • Houses outside the scope of NZS 3604 “Timber Framed Buildings” Normal structures and • Multi-occupancy residential, commercial, and 2b structures not in other industrial buildings accommodating <5000 categories people and <10,000 m2 • Public assembly buildings, theatres and cinemas <1000 m2 • Car parking buildings

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• Emergency medical and other emergency facilities not designated as critical post disaster facilities Important structures that • Airport terminals, principal railway stations, may contain people in schools crowds or contents of high • Structures accommodating >5000 people 3 2 value to the community or • Public assembly buildings >1000 m pose risks to people in • Covered malls >10,000 m2 crowds • Museums and art galleries >1000 m2 • Municipal buildings • Grandstands >10,000 people • Chemical storage facilities >500m2 • Major infrastructure facilities Critical structures with • Air traffic control installations 4 special post disaster • Designated civilian emergency centres, functions medical emergency facilities, emergency vehicle garages, fire and police stations

The Ostler Fault Zone is assigned to Recurrence Interval Class II (Van Dissen et al. 2003; Kerr et al. 2003). Table 3 sets out the relationships between recurrence interval and BIC.

Table 3: Relationships between recurrence interval and Building Importance Category.

Average Building Importance Category (BIC) limitations Recurrence recurrence (allowable buildings) interval interval of class surface Previously subdivided or ‘Greenfield’ sites rupture developed sites BIC 1 I ≤2000 years temporary buildings only BIC 1& 2a BIC 1 >2000 years to II temporary & residential timber- temporary buildings only ≤3500 years framed buildings only BIC 1, 2a, & 2b BIC 1& 2a >3500 years to III temporary, residential timber- temporary & residential timber- 5000 years ≤ framed & normal structures framed buildings only BIC 1, 2a, & 2b >5000 years to IV temporary, residential timber- ≤10,000 years BIC 1, 2a, 2b & 3 framed & normal structures temporary, residential timber- BIC 1, 2a, 2b & 3 framed, normal & important temporary, residential timber- >10,000 years structures framed, normal & important V to (but not critical post-disaster structures ≤20,000 years facilities) (but not critical post-disaster facilities) >20,000 years BIC 1, 2a, 2b, 3 & 4 VI to critical post-disaster facilities cannot be built across an active ≤125,000 years fault with a recurrence interval ≤20,000 years Note: Faults with average recurrence intervals >125,000 years are not considered active

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Recurrence interval class, fault complexity and BIC are the three key parameters that, when brought together, enable a risk-based approach to be taken when making planning decisions about development of land on, or close to, active faults. Understanding the interrelationships between these parameters is important when developing consistent, risk-based policies and methods for guiding development of land that may be affected by surface rupture faulting.

Determining an appropriate Resource Consent Category for different combinations of recurrence interval, fault complexity and BIC, and development status, is a complex task. As the significance of what is at risk increases, the Resource Consent Category becomes more restrictive, and the range of matters that a Council may wish to consider increases.

Table 4 gives an example of resource consent categories, following the approach of Van Dissen et al. 2004, which could be applicable for the Ostler Fault Zone near Twizel. Based on fault complexity (Fig 3) it includes permitted activities for previously subdivided or developed sites, and greenfield sites, under various combinations of Building Importance Category. In applying this approach, the fault avoidance zonation (Fig. 4) identifies set-back distances that should be added to the well-defined and distributed categories of fault complexity. Note that there is no set-back from the uncertain deformation zone. It is important that Table 4 outlines a suggested approach. Ultimately, it is a matter for local authorities to decide upon and implement this type of categorisation.

Surface fault rupture is a hazard of relatively limited geographic extent, compared to strong ground shaking, and in many cases, it is possible to avoid building over fault deformation zones. If avoidance of fault deformation hazard is not practicable at a site, then planning/design measures may be able to mitigate/accommodate the surface rupture deformation anticipated at the site.

5.7 Hazard assessment

Fault avoidance zones

The degree of activity of the Ostler Fault Zone, as encapsulated by its recurrence interval for surface rupture earthquakes, coupled with available evidence suggesting that the time elapsed since the most recent surface rupture is similar to its calculated recurrence interval, means that there is no room for complacency in regard to its earthquake-related hazards. I recommend that the MfE guidelines be adopted in respect to the fault avoidance zones defined in this report (Fig.4), for planning purposes. As noted earlier, further work could be undertaken to better delineate the fault locations, and thereby may allow some reduction in width of the avoidance zones.

Uncertain ground deformation zones

As noted in Table 1, the ‘moderately tilted’ (uncertain deformation – constrained), ‘slightly to moderately tilted’ and the ‘slightly tilted’ (uncertain deformation – poorly constrained) classes of ground are assessed as having a possible threat to building integrity, but a reduced threat to life safety from ground deformation (which is the focus of the Kerr et al. (2003) guidelines).

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Table 4: An example of resource consent categories in relation to fault complexity classes for the Ostler Fault Zone (based on Van Dissen et al. 2004). Refer to Figure 4 for fault avoidance zonation (FAZ) in relation to well-defined & distributed deformation. The FAZ area, which includes set-back buffers, would apply to all categories except ‘permitted’.

OSTLER FAULT ZONE (based on Fault Recurrence Interval Class II, >2000 to ≤3500 years)

Previously subdivided or developed sites Building 1 2a 2b 3 4 Importance Category Fault complexity Resource consent category Well-defined & Permitted Permitted* Non- Non- Prohibited distributed Complying Complying Uncertain – Permitted Permitted Discretionary Non- Non- constrained Complying Complying Uncertain - Permitted Permitted Discretionary Non- Non- poorly constrained Complying Complying

Greenfield sites Building 1 2a 2b 3 4 Importance Category Fault complexity Resource consent category Well-defined & Permitted Non- Non- Non- Prohibited distributed Complying Complying Complying Uncertain – Permitted Discretionary Discretionary Non- Non- constrained Complying Complying Uncertain - Permitted Controlled Discretionary Non- Non- poorly constrained Complying Complying * Indicates that the resource consent category is permitted, but could be controlled or discretionary given that the fault location is well-defined. Italics: Italics indicate that the resource consent category could be more flexible. For example, where non-complying is indicated, discretionary may be considered more suitable by the Council, or vice versa.

The area of ‘uncertain deformation – constrained’ on the Balmoral outwash terrace west and north of Mt Ostler includes some complicated topography, such as younger steep-sided gullies eroded into the tilted and uplifted outwash terrace. In addition, there is a monoclinal flexure and a fault scarp in the southern part of this area, neither of which can be traced north into the southernmost of the gullies. It is possible that these features extend farther than is shown in Figs. 2 and 3. I recommend that more detailed investigation and assessment of active fault hazards be undertaken in the ‘uncertain deformation – constrained’ area, should any land subdivision or dwelling construction be proposed there.

Barrell (2005), in reference to that part of the assessment area on the Mt John and Tekapo outwash plains, recommended that if possible, construction of dwellings be avoided in areas mapped as ‘moderately tilted’ and ‘slightly to moderately tilted’ ground. In fact, all the areas of moderately tilted ground lie within the fault avoidance buffer zones. Barrell (2005)

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recommended that areas of ‘slightly tilted’ ground present a sufficiently low hazard to life safety that they are potentially suitable for the construction of single-story timber-framed (BIC 2a) dwellings.

Further work, such as more detailed surveying, may allow better estimates of the amounts of and extents of ‘uncertain’ deformation. It is possible that further analyses, in combination with structural design solutions, may be able to show that areas of ‘slight to moderate’, or even moderate’ tilt may be deemed acceptable for certain types of building. If any development were to proceed, it would be prudent to encourage engineering designs for houses that can accommodate adverse effects of slight tilt, should any develop as a result of a future fault rupture. Construction on piles which could be re-levelled is an example of an appropriate mitigation measure.

Areas not subject to fault deformation hazard

Areas identified as ‘not deformed’ in Figure 4, after taking account of any fault avoidance zone buffers, are considered to be free from fault rupture or ground deformation hazards. From a fault deformation hazard avoidance perspective, these areas are considered suitable for the construction of any type of dwelling that conforms to New Zealand building codes.

Permanent elevation change hazard

The precise levelling surveys identified contemporary ‘slow creep’ (progressive slight arching of the ground at a rate of up to 1 mm/year) in the vicinity of the Y and Ruataniwha Faults (Blick et al. 1989; Van Dissen et al. 1993; see Appendix). However, the reasons for the observed creep are unclear, and one cannot rule out that creep deformation could migrate into, or develop in, adjacent areas. Ultimately, this area lies within a major zone of deformation (i.e. the Ostler Fault Zone), and there may still be much to discover about its long-term behaviour.

Any dwellings in a creep deformation zone are likely to undergo progressive tilting in the building's life, although based on reported rates within the Ostler Fault Zone the effect would likely be of the order of perhaps a few millimetres difference in height from one side of the building to the other, accumulating over many years. This would have the potential to affect the plumb of the building, though would be unlikely to compromise the building’s structural integrity, and would be not normally pose a personal injury hazard. However, it would be prudent to have this assumption of structural integrity confirmed by a structural engineer.

Earthquake shaking hazard mitigation

In the event of a future ground surface-rupturing earthquake on the Ostler Fault Zone, all areas in the general vicinity of the fault (i.e. southwestern parts of the Mackenzie basin, including Twizel and Omarama townships), will be subjected to severe ground shaking. Severe earthquake ground shaking is a common potential hazard in many parts of New Zealand. The primary mitigation is to ensure that all constructions conform to the relevant New Zealand building codes. More general mitigation measures include locating buildings on relatively flat ground with good foundation conditions, securing of water tanks, heavy furniture, etc.

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Other hazards

Close to active faults, steep slopes such as hill sides or river terrace faces are potentially subject to slope instability hazards, especially in the event of a major earthquake on the fault. It is prudent to avoid situating dwellings either directly on, or at the crest or foot, of steep slopes. A set-back distance equal to the height of the slope is a useful rule-of-thumb for defining a slope instability hazard avoidance zone at the top and base of the slope.

Another hazard that may apply to the gully floors near Mt Ostler is flash flooding in the event of intense rainstorms. The existence of the gullies indicates that, in the past, there has been sufficient hydrological power to erode and form these gullies. That they are commonly dry, and lack permanent streams, may belie their potential for powerful flash floods. Potential flood hazards also exist in the vicinity of the Twizel River and its tributaries.

Concluding remarks

The Ostler Fault Zone is a major active fault, with an estimated recurrence interval of 3,000 ± 1,000 years for surface-rupturing earthquakes, and the most recent known such earthquake about 3,600 years ago. While this does not mean that an earthquake is imminent, it leaves no room for complacency. It should be assumed that significant and abrupt metre-scale ground breakage and warping will occur within the Ostler Fault Zone during the next rupture, and that it is possible that such an event may occur during the tenure of any current land development within the Ostler Fault Zone. Minimising or avoiding the adverse effects of a ground-rupturing earthquake should be a priority issue in land-use planning and development in the Twizel area.

Ultimately, the mitigation of fault-related hazards includes options such as avoiding hazardous locations, or structural designs that reduce the adverse effects of fault deformation.

The approaches described in this report should be applicable elsewhere in the general region, either on the Ostler Fault Zone or on other active fault or fold systems.

6. CONCLUSIONS

1. The Ostler Fault Zone is a major active fault system in the southwestern part of the Mackenzie basin, inland South Canterbury. Named components of the Ostler Fault Zone west of Twizel include the Ruataniwha, Y and Haybarn faults.

2. The Ostler Fault Zone is assessed as having an average recurrence interval for surface-rupture earthquakes of 3,000 ± 1,000 years. On this basis it is a Recurrence Interval Class II fault according to MfE active fault guidelines.

3. A large, ground surface rupturing earthquake on the Ostler Fault Zone would result in localised severe ground deformation at the fault scarps, lesser amounts of deformation in the immediate vicinity and severe earthquake shaking in the wider region.

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4. Within the assessment area, the surface expression of past ground deformation varies in form and position over short distances. This hinders the ability to pinpoint the areas of greatest hazards. This variability has been addressed by mapping the fault zone in terms of how well defined is the surface deformation, using the fault complexity classification in the MfE active fault guidelines. In this way, the fault zone has been mapped and classified into areas of well-defined fault deformation, distributed fault deformation and uncertain ground deformation.

5. Well-defined and distributed deformation areas are classified as Fault Avoidance Zones. In addition, the fault avoidance zones include a 50 m set-back from the downthrown side of fault scarps or monoclinal flexures, and a 100 m set-back from their crests.

6. Areas classified as ‘uncertain deformation – constrained’ are recommended as requiring more detailed assessment should any development be proposed. It is likely that areas of ‘uncertain deformation – poorly constrained’ may be deemed acceptable for the construction of some types of buildings.

7. Further more detailed site investigations may provide a technical basis for reducing the width of hazard avoidance areas. More detailed assessments may be able to show that some areas subject to less severe ground deformation hazards are suitable for certain categories of buildings.

8. The three key elements of the MfE active fault guidelines; recurrence interval, fault complexity and Building Importance Category; when brought together, enable a risk- based approach to be taken when making planning decisions about development of land on, or close to active faults. This approach has the potential to greatly assist the local authorities in the mitigation of rupture hazard, and assist good and defensible land use planning decisions.

ACKNOWLEDGEMENTS

I thank Helen Grant (Environment Canterbury) for technical discussions and the development and refinement of the project brief. The report has benefited from in-house review at GNS Science by Russ Van Dissen and Stuart Read.

REFERENCES

Amos, C.B.; Burbank, D.W.; Nobes, D.C.; Read, S.A.L. 2007: Geomorphic constraints on listric thrust faulting: Implications for active deformation in the Mackenzie Basin, South Island, New Zealand. Journal of Geophysical Research 112: B03S11.

Amos, C.B.; Burbank, D.W.; Read, S.A.L. in press: Along-strike growth of the Ostler fault, New Zealand, and consequences for drainage deflection above a non-propagating thrust. Tectonics, doi:10.1029/2009TC002613, in press.

Barrell, D.J.A. 2005: Geological hazard assessment, Ruataniwha Farm Ltd (Lot 4, DP 75206

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& Lot 7 DP 342137), Twizel, South Canterbury. GNS Science Client Report 2005/171, prepared for Ruataniwha Farm Ltd. 20 p.

Barrell, D.J.A. 2008: Geological hazard assessment of at Lots 5, 6, 7, 9 & 10 DP 353485 & Lot 2 DP 378382: Manuka Terrace and Ben Ohau Road, Twizel. GNS Science Consultancy Report 2007/388, prepared for Rob Brook Forest Ltd. 25 p.

Blick, G.H.; Read, S.A.L.; Hall, P.T 1989: Deformation monitoring of the Ostler Fault Zone. Tectonophysics 167: 329-339.

Campbell F.M.; Kaiser, A.; Horstmeyer H.; Green, A.G.; Ghisetti, F.; Gorman, A.R.; Finnemore, M.; Nobes, D.C. 2010: Processing and preliminary interpretation of noisy high- resolution seismic reflection/refraction data across the active Ostler Fault zone, South Island, New Zealand. Journal of Applied Geophysics 70: 332–342.

Cox, S.C.; Barrell, D.J.A. (compilers) 2007: Geology of the Aoraki area. Institute of Geological and Nuclear Sciences 1:250 000 Geological Map 15. Lower Hutt, New Zealand, Institute of Geological and Nuclear Sciences Limited.

Davis, K.; Burbank, D.W.; Fisher, D.; Wallace, S.; Nobes, D. 2005: Thrust-fault growth and segment linkage in the active Ostler fault zone, New Zealand. Journal of Structural Geology 27: 1528-1546.

Ghisetti, F.C.; Gorman, A.R.; Sibson, R.H. 2007: Surface breakthrough of a basement fault by repeated seismic slip episodes: The Ostler Fault, South Island, New Zealand. Tectonics 26: TC6004.

Kelson, K.I.; Kang, K.H.; Page, W.D.; Lee, C-T.; Cluff, L.S. 2001: Representative styles of deformation along the Chelungpu Fault from the 1999 Chi-Chi (Taiwan) earthquake: geomorphic characteristics and responses of man-made structures. Bulletin of the Seismological Society of America 91: 930-952.

Kerr, J.; Nathan, S.; Van Dissen, R.; Webb, P.; Brunsdon, D.; King, A. 2003: Planning for development of land on or close to active faults: A guideline to assist resource management planners in New Zealand. Ministry for the Environment, July 2003. ME Number: 483; also identified as Institute of Geological and Nuclear Sciences Client Report 2002/124. Available for download at www.mfe.govt.nz.

Macfarlane, D.F. 1981: Ohau A Power Project- Engineering geological construction report. NZ Geological Survey Report EG354, 81p.

Mansergh, G.D.; Read, S.A.L. 1973: The Ostler Fault Zone. S109. Ground movement near the Ohau A powerhouse site. NZ Geological Survey Report EG151, 12p.

McClymont, A.F.; Green, A.G.; Villamor, P.; Horstmeyer, H.; Grass, C.; Nobes, D.C. 2008: Characterization of the shallow structures of active fault zones using 3-D ground-penetrating radar data. Journal of Geophysical Research 113: B10315.

Read, S.A.L. 1984: The Ostler Fault Zone. In P.R. Wood (compiler): Guidebook to the South

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Island Scientific Excursions (Part 2: Greymouth to Christchurch). International Symposium on Recent Crustal Movements of the Pacific Region (Wellington, NZ, February 1984). Royal Society of New Zealand Miscellaneous Series 9: 121-134.

Schaefer, J.M.; Denton, G.H.; Barrell, D.J.A.; Ivy-Ochs, S.; Kubik, P.W.; Andersen, B.G.; Phillips, F.M.; Lowell, T.V.; Schluechter, C. 2006: Near-synchronous interhemispheric termination of the last glacial maximum in mid-latitudes. Science 312(5779): 1510-1513.

Van Dissen, R.J.; Hull, A.G.; Read, S.A.L. 1993: Timing of some large Holocene earthquakes on the Ostler Fault, New Zealand. p. 381-386 In: Proceedings of the Eighth International Symposium on Recent Crustal Movements (CRCM '93), Kobe, December 6-11, 1993.

Van Dissen, R.J.; Berryman, K.R.; Webb, T.H.; Stirling, M.W.; Villamor, P.; Wood, P.R.; Nathan, S.; Nicol, A.; Begg, J.G.; Barrell, D.J.A.; McVerry, G.H.; Langridge, R.M.; Litchfield, N.J.; Pace, B. 2003: An interim classification of New Zealand's active faults for the mitigation of surface rupture hazard. 8p. In: Proceedings of the 2003 Pacific Conference on Earthquake Engineering, 13-15 February 2003, Christchurch, New Zealand. NZ Society for Earthquake Engineering.

Van Dissen, R.; Heron, D. 2003: Earthquake fault trace survey, Kapiti Coast District. Institute of Geological and Nuclear Sciences Client Report 2003/77. Prepared for Kapiti Coast District Council; available for download at www.kapiticoast.govt.nz.

Van Dissen, R.; Heron, D.; Hinton, S.; Guerin, A. 2004: Mapping active faults and mitigating surface rupture hazard in the Kapiti Coast District, New Zealand. Proceedings of the 2004 Annual NZ Society for Earthquake Engineering (NZSEE) Technical Conference. Paper 21; available for download at www.nzsee.org.nz.

Van Dissen, R.J.; Heron, D.W.; Kerr, J.E.; Guerin, A.; Muspratt, M.; Hinton, S. 2006: Active faulting in the Kapiti Coast district, New Zealand: mitigating surface rupture hazard and integrating hazard information in community planning. p. 425-434 IN: Earthquakes and urban development: New Zealand Geotechnical Society 2006 Symposium, Nelson, February 2006. Wellington: Institution of Professional Engineers. Proceedings of Technical Groups/ Institution of Professional Engineers New Zealand 31.

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FIGURES

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APPENDIX A – DEFORMATION ANALYSES

Topographic profiles of Ohau River terraces across Ostler Fault

Ostler Fault Zone “creep” deformation 1966–1989

Previous zonation of Ostler Fault hazard/risk (Macfarlane 1981)

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Topographic profiles of Ohau River terraces across Ostler Fault (Fig 5.15 of Read, 1984)

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Ostler Fault Zone “creep” deformation 1966–1989 (Fig. 2 of Van Dissen et al. 1993)

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Ostler Fault Zone - hazard/risk zones (digitised from Sheet 35 of 39 from Macfarlane 1981)

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Ostler Fault Zone - risk zone descriptions (Table 4 from Macfarlane 1981)

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APPENDIX B – GIS LAYERS & REPORT PDF

The GIS layers referred to in this report and contained on the accompanying CD consist of the following shapefiles:

Figure 2

• Fault_mapping_2010.shp

• Ground_classification_2010.shp

Figure 3

• Fault_complexity_zones_2010.shp

Figure 4

• FAZ_50m_buffers.shp

• FAZ_100m_buffers.shp

• Fault_avoidance_zones_2010.shp

All the data have been compiled at a detailed scale of approximately 1:10,000. The geographic coordinate system for the data is New Zealand Map Grid 1949. A layer (.lyr) file are included for each shapefile so that the GIS layers can be displayed using the symbols and colours that appear on the figures in this report.

A PDF of the report is also on the CD.

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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