BIBLIOGRAPHIC REFERENCE

Hancox, G. T.; Langridge, R. M.; Perrin, N. D.; Vandergoes, M.; Archibald, G. 2013. Recent mapping and radiocarbon dating of three giant landslides in northern Fiordland, New Zealand, GNS Science Report 2012/45. 52 p.

G. T. Hancox, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand R. M. Langridge, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand N. D. Perrin, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand M. Vandergoes, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand G. Archibald, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand

© Institute of Geological and Nuclear Sciences Limited, 2013

ISSN 1177-2425 ISBN 978-1-972192-33-7

CONTENTS ABSTRACT ...... IV KEYWORDS ...... V 1.0 INTRODUCTION ...... 1

1.1 BACKGROUND ...... 1 1.2 GEOLOGICAL AND GEOMORPHIC SETTING OF FIORDLAND ...... 3 1.3 LANDSLIDING IN FIORDLAND ...... 3 1.4 RECENT LANDSLIDE STUDIES IN FIORDLAND ...... 8 2.0 LANDSLIDE MAPPING AND DATING 2012 ...... 9

2.1 FIELD WORK PROGRAMME ...... 9 2.2 GEOMORPHOLOGY AND GEOLOGY OF LANDSLIDES ...... 9 2.2.1 Lake Adelaide Landslide ...... 9 2.2.2 John O’Groats Landslide ...... 15 2.2.3 Lake Gunn Landslide ...... 20 3.0 SAMPLING FOR LANDSLIDE DATING ...... 25

3.1 LAKE ADELAIDE LANDSLIDE SAMPLING ...... 26 3.2 JOHN O’GROATS LANDSLIDE SAMPLING ...... 27 3.3 LAKE GUNN LANDSLIDE SAMPLING ...... 28 4.0 RADIOCARBON DATING RESULTS ...... 31

4.1 SAMPLE COLLECTION AND RADIOCARBON DATING ...... 33 4.1.1 Lake Adelaide Landslide ...... 34 4.1.2 John O’Groats Landslide ...... 35 4.1.3 Lake Gunn Landslide ...... 36 5.0 DISCUSSION...... 39

5.1 AGE OF THE LANDSLIDES ...... 39 5.2 COSEISMIC TRIGGERING OF LANDSLIDES ...... 39 5.3 CORRELATION BETWEEN LANDSLIDES AND ALPINE FAULT EARTHQUAKES ...... 41 5.4 FUTURE LANDSLIDE DATING ...... 42 6.0 CONCLUSIONS ...... 43 7.0 ACKNOWLEDGEMENTS ...... 45 8.0 REFERENCES ...... 45

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FIGURES

Figure 1 Geological map of Fiordland and south Westland showing the locations of very large and giant postglacial landslides in the region...... 2 Figure 2 Regional setting of the Alpine Fault (AF), Puysegur Subduction Zone (PSZ), and the Fiordland region (F)...... 2 Figure 3 Aerial view of Green Lake Landslide looking southeast across ~45 km2 of hummocky slide debris (sd) towards the 15 km-long head scarp (hs), which was formed when part of the Hunter Mountains collapsed into proto Lake Monowai (lm) about 12,000-13000 years ago...... 4 Figure 4 Aerial view of the ~400 Mm3 prehistoric rock slide/avalanche in weakly foliated diorite and tonalite which impounds Lake McIvor between North West Arm (nwa) and South West Arm (swa) of the Middle Fiord (MF) of Lake Te Anau...... 4 Figure 5 Loch Maree, a landslide-dammed lake at the head of Dusky Sound, was formed when debris from a large (~3 Mm3) rock fall (RF) dammed the Seaforth River (LD) and drowned a large area of beech forest (Figure 6)...... 6 Figure 6 Drowned beech forest in Loch Maree...... 6 Figure 7 Graph plot of age versus tree diameter data for silver beech forest in Fiordland, based on tree core data from Loch Maree and Lake Mintaro in Fiordland (Hancox and Perrin 2011)...... 7 Figure 8 Geomorphic map of Lake Adelaide Landslide (~750 Mm3) showing its extent and main geomorphic features, along with the planned and actual helicopter landing and samplings sites on 28 and 30 March 2012...... 11 Figure 9 Cross section through Lake Adelaide Landslide showing the slide debris spread across the head of the upper Moraine Creek Valley, which is estimated to be about 300-400 m thick...... 12 Figure 10 Lake Adelaide (942 m) at the head of Moraine Creek (mc) has been created by a large landslide dam (d)...... 12 Figure 11 Closer view of the landslide debris at Lake Adelaide...... 13 Figure 12 View of Lake Adelaide Landslide from the east showing the large slide debris (sd) dam at the head of Moraine Creek (MC)...... 13 Figure 13 View of Lake Adelaide Landslide looking south and showing the main mass of slide debris, with sample sites 5 and 6 visible on the debris ridge in the foreground (S5, S6)...... 14 Figure 14 Geomorphic map of John O’Groats Landslide (~1000 Mm3) showing its extent, main features, and the planned and actual samplings sites...... 15 Figure 15 Cross section through John O’Groats Landslide showing the extent and estimated thickness of the slide debris, which fills the entire valley from the headscarp to the coast...... 16 Figure 16 Aerial view of John O’Groats Landslide showing the ~3 km long source area on the ridge north of Mt Pembroke (2015 m) between peak 1689 m and Te Hau (1703 m), and main part of the 11 km2 of slide debris, which has created three small landslide-dammed lakes (SL, NL, LL)...... 17 Figure 17 Aerial view of hummocky landslide debris (sd) deposited in the lower John O’Groats Valley where it is displaced by the Alpine Fault...... 17 Figure 18 Sampling Site 1 is located ~300 m north of the southern landslide-dammed lake in a small clearing underlain by alluvial deposits at the mouth of a stream dammed by slide debris...... 18

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Figure 19 Large (~1 m diameter, or ~300 years old) beech tree growing on the remains of a dead tree and slide debris boulders ~100 m northeast of the Site 1 clearing (E1193711m, N5055425m)...... 18 Figure 20 Geomorphic map of Lake Gunn Landslide showing its extent and main features, along with planned and actual sampling sites on 31 March and 1 April 2012...... 20 Figure 21 Cross section through Lake Gunn Landslide showing the extent and estimated thickness of the slide debris, which fills the entire valley from the headscarp to the coast...... 21 Figure 22 Aerial view of Lake Gunn Landslide looking south down the Eglinton Valley...... 22 Figure 23 Aerial view of Lake Gunn Landslide showing the 2 km-wide headscarp, source area, slide debris (sd), semi-intact slide block (SB), and sampling sites S1 and S3...... 22 Figure 24 View of Lake Gunn Landslide looking north towards Lake Gunn...... 23 Figure 25 Mounds and conical hillocks of Lake Gunn Landslide debris spread across the 1 km-wide floor of the Eglinton Valley...... 23 Figure 26 Aerial view of Sites 1 and Site 1a on John O’Groats Landslide...... 27 Figure 27 Beech tree leaves and wood in organic silt (Sample JDC3) from a depth of about 1.25 m, directly on the top of boulders of landslide debris at a location about 150 m southeast of the Site 1 clearing...... 28 Figure 28 Lake Gunn Landslide sampling at Site 2 in the Eglinton River channel. The old log (lower left) buried within the slide debris at this site was sampled for radiocarbon dating (LG002)...... 29 Figure 29 Site 3 on top of the slide block above SH94. A soil auger was used at this site to sample the surficial peat deposits for 14C dating (LG004)...... 29 Figure 30 Plot of the calibrated radiocarbon ages (2σ cal. years BP, range and mean values) for samples from the Lake Adelaide, John O’Groats, and Lake Gunn landslides...... 33 Figure 31 Diagrammatic plot of the relative positions of radiocarbon samples and radiocarbon ages...... 34 Figure 32 Minimum ages of the John O’Groats (JOG), Lake Adelaide (LA), and Lake Gunn (LG) landslides plotted against the B.C.E/C.E (Common Era) ages (95% or 2σ range) of surface-rupturing earthquakes on the Alpine Fault from the Hokuri Creek (Hk) and Haast (Ha) records (from Berryman et al., 2012)...... 37

TABLES

Table 1 Summary of field activities and landslide dating sampling 28 March to 1 April 2012...... 10 Table 2 List of samples collected for landslide dating in Fiordland 28 March to 1 April 2012...... 25 Table 3 Radiocarbon dates from Fiordland landslide sites, 2012...... 32 Table 4 Ages of landslides attributed to Alpine Fault earthquakes in Fiordland and south Westland...... 41

APPENDICES APPENDIX 1: MODIFIED MERCALLI INTENSITY SCALE ...... 51

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ABSTRACT

This study of three giant landslides in northern Fiordland has involved the geological and geomorphic mapping, description, and radiocarbon dating of the Lake Adelaide rock-slide avalanche (~750 Mm3), the John O’Groats rock avalanche (~1000 Mm3), and the Lake Gunn rock block-slide avalanche (~300 Mm3). These three landslides are part of a cluster of at least 58 very large (≥1 Mm3) and giant (≥100 Mm3) postglacial age landslides which have occurred within ~50 km of the southern section of the Alpine Fault and the underlying Puysegur (Fiordland) subduction zone. Both of these active tectonic structures are potential sources of large (Mw >7) and great (Mw >8) earthquakes which, based on historical evidence of coseismic landsliding in New Zealand, are considered most likely to have triggered the numerous very large bedrock collapses in Fiordland and south Westland.

The calibrated 14C ages of organic material within or deposited on top of the landside deposits are thought to provide minimum ages (±2σ) for the three landslides dated in our study, namely: John O’Groats Landslide: 5785 ±125 years BP; Lake Adelaide Landslide: 6144 ±143 years BP; and Lake Gunn Landslide 7630 ±52 years BP. These ages are generally consistent with the age of the beech forest on the landslide deposits, multiple displacements of the landslide deposits in John O’Groats valley by the Alpine Fault over the last ~6000 years, and only moderate to minor erosional modification of the landslide head scarps, source areas, and slide debris deposits.

The ages of these landslides have been tentatively correlated against the paleoseismic record of earthquakes on the Alpine Fault over the last 8000 years at Hokuri Creek. Based on that record, the John O’Groats Landslide is correlated with the Hokuri Creek event Hk15 (3951–3761 years BCE, or 5902–5711 years BP); the Lake Adelaide Landslide with Hk16 (4237–4055 years BCE, or 6187–6005 years BP); and the Lake Gunn Landslide with Hk21 (5752–5422 years BCE, or 7702–7372 years BP). However, because the dates for the Lake Adelaide and John O’Groats landslides may be minimum ages, they could have occurred during earlier large earthquakes.

Because there is no similar record of past earthquakes on the Puysegur subduction zone, that potential earthquake source cannot be confirmed or ruled out, although its existence is believed to increase the likelihood that the three landslides studied, and others like them in the region were earthquake-triggered. Given the 14C dating and historical precedent evidence available at present, we therefore conclude that the Lake Adelaide, John O’Groats, and Lake Gunn landslides were most probably triggered by large or great earthquakes on the Alpine Fault or the subduction zone between 5500 and 7500 cal. years BP. Although other faults in the region are capable of producing large earthquakes, the Alpine Fault and the Puysegur subduction zone are believed to be the most likely sources of ≥MM9 shaking at the landslide sites.

This initial study has made significant progress in the dating of giant landslides in Fiordland by establishing the probable age of the John O’Groats, Lake Adelaide, and Lake Gunn landslides, and linking them to the Alpine Fault earthquakes that are likely to have triggered landslides of that size. However, more field work and landslide dating, especially of samples that can provide maximum landslide ages, is required before a clear understanding of postglacial large landslides triggered by Alpine Fault earthquakes, or earthquakes on the Puysegur subduction zone can be established with more certainty.

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Future landslide chronology studies would benefit from careful consideration and selection of organic material for radiocarbon dating. As shown in this study, bulk organic samples can provide a wide range of ages through the incorporation of both old and young carbon from higher in the stratigraphic sequence. Careful targeting macrofossils or microfossils for dating, such as leaves or pollen isolated from sediment, may improve the confidence in the chronology that is achieved in future landslide dating. It is also is important that, where possible, future landslide dating studies provide bounding ages, not just minimum ages, that have error margins that are narrow enough to allow direct correlation with the Alpine Fault earthquake chronology. In the current study the Lake Gunn Landslide provides a good example of how this can be done effectively.

Some suggested targets for future landslide dating include the Lake McIvor, Dart River, Lochnagar, Lake Purser, Cascade River, the Hope-Blue River Range landslides, and possibly other giant landslides in the region, especially those in close proximity to the Alpine Fault. We also believe that further studies are required to distinguish, date, and determine the effects of landslides that can be attributed to the last two Alpine Fault earthquakes in south Westland and Fiordland. Information from such studies would provide much needed insight into the likely effects and risk from the numerous landslides that are expected to be triggered in these regions by the next Alpine Fault earthquake.

KEYWORDS

Giant landslides, Fiordland, New Zealand, Lake Adelaide Landslide, Lake Gunn Landslide, John O’Groats Landslide, radiocarbon dating of landslides, Alpine Fault earthquakes, Puysegur subduction zone, earthquake-induced landslides, Green Lake Landslide.

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1.0 INTRODUCTION

1.1 BACKGROUND

Many thousands of landslides occur in New Zealand every year, most of which are triggered by heavy rainfall, with on average two or three economically-significant episodes of rainfall-induced landsliding each year, particularly in the North Island and northern South Island (Crozier et al., 1992). High-intensity rainstorms such as Cyclone Bola in 1988 (Trotter, 1988; Page et al., 1994) and the southern North Island in 2004 (Dymond et al., 2006; Hancox and Wright, 2005) triggered thousands of shallow landslides over ~12,000 km2 and ~16,000 km2 of the eastern and southern North Island respectively. Typically, most rainfall-induced landslides are small to moderate (<10– 1000 m3) soil slides and flows in steep hill country, although a few larger bedrock failures of ~10,000–400,000 m3 also occur. Landslides during extreme rainstorms often cause severe erosion of farmland and loss of pasture productivity, as well as widespread damage to roads, buildings, and structures, and occasionally fatalities, as occurred in the Bay of Plenty at Bryan’s Beach in 2004, and Ohope in 2011. During the 2004 rainstorm, a 300,000 m3 debris slide dammed and diverted the Hutt River, and SH3 in the Manawatu Gorge was closed for about 3 months while about 100,000 m3 of slide debris was cleared away (Hancox and Wright, 2005). In August 2011 another large rainfall-induced landslide in the Manawatu Gorge closed SH 3 for a year while remedial works were carried out (Hancox et al., 2012).

No very large (106-107 m3) landslides are known to have been triggered by heavy rainfall in New Zealand, although for several, especially the 5 Mm3 Abbotsford Landslide in 1979, prolonged moderate rainfall was probably a significant contributing factor (Hancox 2009). A number of very large landslides have also occurred without a discernible trigger in the Southern Alps of New Zealand in the last 25 years, such as the rock avalanches at Mt Cook in 1991, Mt Adams 1999, and Young River 2007, and many others have occurred in the last 170 years during large earthquakes (Hancox et al, 1997, 2002, 2005; Massey et al., 2008).

Strong earthquake shaking is the second most common trigger of landslides in New Zealand. Large earthquakes of M 7 or greater have triggered the largest and most hazardous landslides in New Zealand, causing more than 18 deaths since 1929, and possibly more than 100 deaths since human settlement (Crozier et al., 2009). Historical earthquakes in New Zealand have triggered at least 90 landslides with volumes of 1 Mm3 or greater. Fifty one of those landslides 3 occurred during the 1929 MS 7.8 Murchison earthquake, including two giant (>100 Mm ) slides in Tertiary mudstone on the coastal cliffs north of Westport (Hancox et al., 2002).

Besides the known very large and giant landslides there are also many prehistoric landslides of that size, including two giant rock and debris slides in Tertiary rocks in the eastern and western central North Island, for which seismic triggering is postulated (Perrin and Hancox, 1992; Read et al., 1992; Crozier et al., 1995). In the South Island, a number of very large prehistoric rock avalanches in the central Southern Alps of New Zealand are also thought to have been earthquake-induced (Whitehouse, 1983), possibly by earthquakes on the Alpine Fault (Yetton, 1998; Wells et al., 1999). The Fiordland region to the south contains possibly the highest concentration of very large and giant landslides in New Zealand, with at least 50 postglacial prehistoric landslides of that size identified in relatively close proximity to the Alpine Fault and the Fiordland (Puysegur ) subduction zone (Figure 1 and Figure 2).

The studies described in this report focus on the mapping, description, and dating of three giant landslides in Fiordland in March 2012, and briefly discusses their likely genesis and triggering mechanisms, and the possible objectives and benefits of future studies of this type.

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Figure 1 Geological map of Fiordland and south Westland showing the locations of very large and giant postglacial landslides in the region. The John O’Groats (2), Lake Adelaide (3), and Lake Gunn (14) landslides were mapped and dated in this study. Locations of the Hokuri Alpine Fault record site and the Round Top (RT) and Mt Wilberg (MW) rock avalanches (on the Inset Map) are also shown.

Figure 2 Regional setting of the Alpine Fault (AF), Puysegur Subduction Zone (PSZ), and the Fiordland region (F).

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1.2 GEOLOGICAL AND GEOMORPHIC SETTING OF FIORDLAND

The Fiordland region of southwest New Zealand is mountainous with relief of ~1200-2000 m, and steep slopes (35–65° or >). During the last glaciation, which ended about 14,000 years ago, Fiordland was extensively glaciated, creating many deep U-shaped valleys, glacial lakes, and ice-carved fiords (Figure 1). Most of Fiordland is made up of strong, sparsely- jointed, unweathered rocks, including gneiss, schist, diorite, and granite of Cambrian to Cretaceous age. Tertiary sedimentary rocks and late Quaternary alluvium occur in eastern Fiordland (Turnbull et al., 2010). Figure 1 and Figure 2 show the major tectonic features in the Fiordland region, which include the active Alpine Fault which comes onshore north of Milford Sound, and the Fiordland or Puysegur Subduction Zone (PSZ), which dips to the east and underlies the Fiordland region.

1.3 LANDSLIDING IN FIORDLAND

Because of the very steep terrain, landsliding occurs frequently in Fiordland, particularly during rainstorms and large earthquakes. Small to moderately large (103–104 m3) debris slides, rock falls, and tree slides are common during severe rainstorms, often affecting and sometimes closing the SH89 highway between Te Anau and Milford Sound.

In the last twenty five years moderate to large earthquakes in 1988 (MW 6.7), 1993 (MW 6.8),

2003 (MW 7.2), 2007 (MW 6.7), and 2009 (MW 7.8) have caused widespread, but mainly small-scale superficial landsliding over large areas of Fiordland (Van Dissen et al., 1994, Hancox et al., 1997, 2002, 2003; Forsyth et al., 2006; Hancox et al., 2010). The scars of these and other historical landslides are widespread on many steep slopes across Fiordland. Despite being widespread, however, these historical landslides have caused little or no damage to buildings and structures, but have resulted in moderate to major effects on roads, in some cases closing the Te Anau to Milford Sound highway for several days at a time (e.g. November 2012). A large landslide triggered by the 2003 Fiordland earthquake blocked the Wilmot Pass road for several days in August 2003 (Hancox et al 2003).

Besides the evidence of recent landsliding in the landscape, Fiordland also contains the remains of more than 50 very large landslides, including at least 15 giant (≥100 Mm3) prehistoric landslides of postglacial age (Hancox and Perrin, 1994, 2009) which occur in clusters across the region (Figure 1). Of these features, Green Lake Landslide, which occurred about 12,000–13,000 years ago about 30 km south of Manapouri, is the largest landslide (rock slide) in New Zealand (~27 km3), and possibly the largest landslide of its type on earth (Figure 3).

Most of the very large and giant landslides are rock and debris slides and avalanches which have formed on steep mountain ridges and oversteepened sides of glacially-eroded u- shaped valleys sometime after the glaciers retreated at the end of the last glaciation. The mechanisms proposed for these large slope failures generally involve a combination of oversteepening and erosional undercutting of slopes, and progressive rock mass failure following deglaciation due to the development of slide planes along major rock defects (e.g., faults, joint and foliation surfaces). For example, Hancox and Perrin (2009) found that glacial undercutting of a fault zone dipping into the deeply eroded (former) Lake Monowai valley was the main preconditioning factor for the Green Lake Landslide (Figure 3). A similar failure mechanism may also be responsible for the Lake McIvor Landslide (~400 Mm3) on the west side of Lake Te Anau (Figure 4).

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Figure 3 Aerial view of Green Lake Landslide looking southeast across ~45 km2 of hummocky slide debris (sd) towards the 15 km-long head scarp (hs), which was formed when part of the Hunter Mountains collapsed into proto Lake Monowai (lm) about 12,000-13000 years ago. Green Lake (gl) is the largest of several landslide ponds within the landslide deposit east of the Grebe River (gr).

Figure 4 Aerial view of the ~400 Mm3 prehistoric rock slide/avalanche in weakly foliated diorite and tonalite which impounds Lake McIvor between North West Arm (nwa) and South West Arm (swa) of the Middle Fiord (MF) of Lake Te Anau. The hummocky slide debris (sd), which contains large slide blocks and small lakes spread over ~5 km2, was derived from the ridge to the left (north).

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Large earthquakes on the Alpine Fault and the Fiordland subduction zone have been suggested by a number of authors (e.g. Hancox et al., 2003; Korup 2004, 2005; Hancox and Perrin, 2009) as the most likely trigger for very large and giant landslides. This inference is supported by historical evidence in New Zealand which shows that very large landslides (~1– 50 Mm3) are mainly triggered by strong earthquake shaking of MM 9 or greater (Hancox et al., 2002). Slope failures of this size have also occurred in alpine areas without an obvious trigger, such as the 1991 rock avalanche from Mt Cook (Mc Saveney et al., 1992), the Mt Adams rock avalanche in 1999 (Hancox et al., 2005), the Young River rock slide in 2007 (Massey et al., 2008), and most recently the rock avalanche from Mt Haast in January 2013.

Studies by Hancox et al. (1997, 2002) show, however, that in New Zealand extremely large and giant slope failures involving masses of 50–100 Mm3 or greater have only occurred during large earthquakes, for example: Murchison 1929 (Little Wanganui, ~210 Mm3, White Cliffs, ~120 Mm3); Arthurs Pass 1929 (Falling Mountain (~72 Mm3); Napier 1931 (Old Man’s Bluff, Mohaka ~72 Mm3). Some moderate and large earthquakes in Fiordland over the last 50 years have caused widespread superficial landsliding over very large areas (Hancox et al., 2003, 2010), but none has triggered failures similar to the very large and giant landslides that are scattered across Fiordland in relatively close proximity to the Alpine Fault (Figure 1).

One possible exception in the historical earthquake record is the poorly known 1826 Fiordland earthquake and tsunami which caused widespread coastal damage on the south and southwest coast of the South Island. Sealers in the area at the time (R Taylor, c.1854) reported that “…the coast south of Cascade Point was most shattered, scarcely beyond recognition. Large masses of the mountains had fallen, and in many places trees could be seen under the water” (Downes et al., 2005). This report suggests that the 1826 earthquake triggered large landslides and rock falls along a large length of the Fiordland coast, possibly over ~180 km between Cascade Point (near Jackson Bay) and Dusky Sound, or perhaps further south. Recent studies have shown that the earthquake caused ~1 m uplift of the coast in Cascada Bay at the mouth of Doubtful Sound, and tsunami damage in Southland (Norris et al., 2001; Downes et al., 2005). Some landslide damaged areas in south Westland which have undergone major reforestation have also been attributed to the 1826 earthquake (Cullen et al., 2003, Wells et al., 1999).

All of the available evidence indicates that the 1826 earthquake was a very large event. Based on the reports of landsliding, it was much larger, or had a different mechanism, than the 2003 and 2009 earthquakes. Downes et al. (2005) found the effects of the 1826 earthquake (coastal uplift, extensive landsliding, and a local tsunami) to be consistent with a large Fiordland subduction zone event. Their geodetic modelling suggested that 1 m uplift at Cascada Bay required a subduction earthquake larger than M 7.9, possibly ~M 8 – M 8.5.

Other information pointing to significant landsliding in Fiordland around 1826 comes from recent 14C dating and coring of beech trees on rock fall deposits by GNS Science (Hancox and Perrin, 2011). The 3 Mm3 landslide-dam that impounds Loch Maree ~50 km southwest of Manapouri created a drowned forest (Figure 5 and Figure 6) which has been 14C dated (R32198/1) at 1845 ±30 years AD. Coring of the largest (520 mm diameter) beech trees growing on boulders in the landslide dam are about 170–180 years old (Figure 7). These ages are consistent with the Loch Maree landslide occurring during the 1826 earthquake.

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Figure 5 Loch Maree, a landslide-dammed lake at the head of Dusky Sound, was formed when debris from a large (~3 Mm3) rock fall (RF) dammed the Seaforth River (LD) and drowned a large area of beech forest (Figure 6).

Figure 6 Drowned beech forest in Loch Maree. Radiocarbon dating of the dead trees in the lake, and coring of beech trees growing on large boulders in the landslide dam (see Figure 7), suggest that the rock fall occurred about 170-180 years ago, possibly during the 1826 Fiordland earthquake.

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Figure 7 Graph plot of age versus tree diameter data for silver beech forest in Fiordland, based on tree core data from Loch Maree and Lake Mintaro in Fiordland (Hancox and Perrin 2011). Data on trees greater than 1 m diameter are from the Hope Fault, north Canterbury (Langridge et al., 2007).

In the Milford Sound area, tree core data indicate that the 0.7 Mm3 rock fall deposit which impounds Lake Mintaro in the Clinton Valley was also triggered by the 1826 earthquake. In addition, several other large landslides in the Milford area may also have been caused by the 1826 earthquake, or possibly the last Alpine Fault earthquake in 1717 AD. One likely possibility which has yet to be dated is the large (~30 Mm3) rock fall and fan which dams Lake Purser ~25 km southwest of Loch Maree. The large rock fall which dams the Arthur River to form Lake Ada on the Milford Track, 5 km southwest of Milford Sound was recently 14C dated at around 900 years BP (pers. comm. Jesse Dykstra).

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1.4 RECENT LANDSLIDE STUDIES IN FIORDLAND

The work described in this report was undertaken as a pilot study for a GNS Science project entitled “Fiordland landslides and their association with Alpine Fault rupture and subduction earthquakes” using funding from the GNS Science 2011-2012 Strategic Development Fund (SDF-L16). The project was aimed at mapping and dating clusters of very large prehistoric landslides in close proximity to the Alpine Fault. Dating of postglacial prehistoric landslides in Fiordland has not previously been prioritised within the GNS landslide research team because of difficult access and high costs to the programme.

Recently, a long record of 24 paleo-earthquakes spanning the last 8000 years on the Alpine Fault at Hokuri Creek (Figure 1) in Fiordland has been developed by Berryman et al. (2012). The ability to correlate landscape effects such as very large bedrock landslides to known faulting events, and the need to understand the widespread landscape impact of large earthquakes, means it is timely that GNS puts more effort into dating postglacial prehistoric landslides. Large earthquakes on the Puysegur subduction zone in Fiordland in 2003 and 2009 and their well-documented landslides (Hancox et al., 2003, 2010) also means that giant and very-large prehistoric landslides may hold the key to understanding the maximum size of earthquakes that the Puysegur subduction zone is capable of generating and their probable environmental effects.

This report describes three giant postglacial landslides in Fiordland, namely: John O’Groats, ~1000 Mm3; Lake Adelaide, 750 Mm3; and Lake Gunn, ~300 Mm3 (Figure 1), and presents the results of geomorphic field mapping and radiocarbon (14C) dating of those landslides in March 2012. The genesis and likely triggering mechanisms of the landslides are then briefly discussed, along with possible objectives and benefits of future studies of this type.

The 2012 study was conceived primarily to initiate the landslide dating project and refine techniques for use in a more comprehensive study of the nature and genesis of giant landslides in Fiordland and other areas along the Alpine Fault in the South Island. The landslides selected for dating in 2012 (John O’Groats, Lake Adelaide, and Lake Gunn) were chosen mainly because they could be relatively easily accessed from Milford Sound and the Milford–Te Anau highway (SH94), and they all offered good prospects for 14C and other late Quaternary dating techniques. The details of the field work carried out and the results of the study are discussed next.

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2.0 LANDSLIDE MAPPING AND DATING 2012

2.1 FIELD WORK PROGRAMME

Field work for the study was carried out between 28 March and 1 April 2012 from a field base at Knobs Flat Motel on the Te Anau-Milford road (SH94) 40 km south of Milford Sound. Milford Helicopters and pilot Jeff Shanks were commissioned to transport the 5-man field team plus a considerable amount of sampling gear to selected landing sites on the Lake Adelaide and John O’Groats landslides in an AS350 B2 Squirrel helicopter. The Lake Gunn Landslide is located only ~5 km north of Knobs Flat Motel and could be easily be accessed from SH94.

In preparation for the field work, geomorphic maps of the landslides were prepared from aerial photos and data collected on helicopter inspections by G Hancox and N Perrin in 2008 and 2009, and revised during the field mapping. The updated landslide maps (Figure 8, Figure 13, and Figure 19) show the extent and geomorphic features of the landslides, along with the helicopter landing sites and sampling locations. Details of the logistics and programme of landslide mapping and sampling carried out from 28 March to 1 April 2012 are provided in a GNS Immediate Report prepared soon after the field work was completed (Hancox et al., 2012). A summary of the programme of field work and sampling carried out is presented in Table 1. A complete list of all samples collected and full location details is presented in Table 2.

2.2 GEOMORPHOLOGY AND GEOLOGY OF LANDSLIDES

2.2.1 Lake Adelaide Landslide

Lake Adelaide Landslide is a giant (~750 Mm3) rock slide (Cruden and Varnes 1996) that collapsed eastward from the ridge between Apirana Peak (1946 m) and Darran Pass (1533 m), damming the glaciated head of Moraine Creek to form Lake Adelaide. Landslide debris comprising mainly large angular boulders, with many the size of houses, is spread across an area of ~4.5 km2 in the upper Moraine Creek Valley (Figure 8). The main features and extent of the landslide are shown on a geomorphic map and cross section (Figure 8 and Figure 9 respectively). The topographic setting and morphology of the landslide are further illustrated by a number of aerial photos (Figure 10 to Figure 13).

Bedrock in the area is Cretaceous age Darran Leucogabbro, which locally comprises dark grey leucogabbro, with dykes of light-coloured pegmatite and quartz diorite. The rock mass is massive and very strong, and in places foliated (Turnbull et al. 2010). Foliation is well developed in the landslide source area, where it strikes northeast and dips ~35-40° southeast (Figure 8).

The main feature of the Lake Adelaide Landslide is the extensive deposit of bouldery debris which impounds Lake Adelaide. Although there are no direct data to indicate the depth of the slide debris in the head of Moraine Creek, the deposit is estimated from a long-valley profile and cross section to be at least 300-400 m thick to the west of the lake below the inferred source area (Figure 9). The average thickness of slide debris is estimated to be at least 200 m.

Lake Adelaide does not have a surface outlet. Instead, water from the lake flows underground through the coarse slide debris and emerges as springs in several ponds at the base of the landslide dam in upper Moraine Creek (Figure 12).

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Table 1 Summary of field activities and landslide dating sampling 28 March to 1 April 2012.

Date Landslide Field work and sampling activities Thursday Lake Field party of four (GH, RL, NP, GA)(1) picked up at Knobs Flat in Milford 28/03/2012 Adelaide Helicopters squirrel and flown into Lake Adelaide (942 m). Aerial photos taken of landslide and surrounding area. Potential landing and sampling sites evaluated and selected. Landed at Site 1(2) – a small flat, tussock-covered area with several shallow ponds at the south end of slide debris. At Site1 (NZTM E1206812m, N5036304 m, altitude 973 m) two peat samples for 14C dating (LA001, LA002) in test pit at depth of ~1.1 m, near the top of the debris. The party was then moved by helicopter to Site 5(2) at the north end of the slide debris where two cosmogenic Be10 samples (LA0005, LA006, alt. ~1010 m) were collected from a large boulder on the ridge of slide debris. Later in the afternoon on the return trip to Knobs Flat aerial photos were taken of the Lake Gunn Landslide and potential sampling sites within the landslide debris were evaluated from the helicopter. Friday John Field party of five (GH, RL, NP, GA, and MV) drove to Milford Sound and were 29/03/2012 O’Groats flown to John O’Groats Landslide. Landed at Site 1 (RL ~475 m) after taking aerial photos and looking at possible sampling sites (14C sampling was believed to be not feasible at other sites). Several probes (P) with peat corer to top of slide debris for core sample: P#1 (JGC1) brown sandy silt at 3.7 m; P#6 (JGC2) brown fine sandy silt and gravel at 2.7–3.7 m. About 100 m northeast of Site1 wood samples (JG001) were collected from the centre of a large dead beach tree for 14C dating. Saturday am - at Lake The field party of five (GH, RL, NP, GA, and MV) to Lake Adelaide (am): Peat 30/03/2012 Adelaide core sample at Site 1 (LAC001) from a depth of ~1.7 m on top of slide debris. Coring of beech trees at Site 6 was not possible because the trees were too large (1.3 m diameter, or ~450 years old). pm – at John At John O’Groats Landslide Site 1a, ~100 m southeast of the Site 1 clearing O’Groats (E1193509m, N5055251m) 3 auger samples (JGC3, JGC4, JGC5) of dark grey- black sandy-silty peat with fragments of wood and beech leaves from depths of 1.0–1.25 mm directly on top of slide debris. Several large silver beech trees were noted around the Site 1 area, the largest of which had a diameter of ~2.1 m, indicating an approximate of 700 years, while other 1–1.5 m diameter trees could be ~350–500 years old (Figure 7). Sunday Lake Gunn Ground inspection (by GH, RL, NP, GA) of Lake Gunn Landslide debris from 31/03/2012 SH94, looking at exposures in Plato Creek, along SH 94 road cuts, and the Eglinton River channel. Tube sample of grey silt debris matrix taken for OSL dating (LG001, at Site 1 (alt ~500 m). In the afternoon RL, NP, and GA traversed the Eglinton River channel, and at Site 2 (alt. ~465 m) collected two samples of old wood for 14C dating from the distal part of the landslide debris exposed in the river bank(3). Monday Lake Gunn Two party members (RL and GA) climbed the DOC track up the spur near Site 1 1/04/2012 to Site 3 (NZTM~E1210406m, ~N5015090m, RL~720 m) on the semi-intact slide block. At Site 3(3) a soil auger sample was collected from the base of surficial peat at a depth of ~0.7 m, below which was ~2 m of light blue coloured fine sandy silt to silty sand.

Notes (1) A full list of samples collected is presented in Table 2. (2) Field party members are identified by author’s initials. (3) Landslide sampling sites shown on Figure 8 (Lake Adelaide LS), Figure 14 (John O’Groats LS), and Figure 20 (Lake Gunn LS).

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Figure 8 Geomorphic map of Lake Adelaide Landslide (~750 Mm3) showing its extent and main geomorphic features, along with the planned and actual helicopter landing and samplings sites on 28 and 30 March 2012. Potential sampling Sites 2, 3, and 4 were not used because of unsuitable terrain and lack of datable materials. The landslide debris covers an area of about 4.5 km2 (main and initial failures combined).

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Figure 9 Cross section through Lake Adelaide Landslide showing the slide debris spread across the head of the upper Moraine Creek Valley, which is estimated to be about 300-400 m thick. The landslide source area was on the 1600–1900 m high ridge to the west near Darran Pass, where foliation planes dipping 35-40 into the valley were undercut by glacial erosion, making the slope vulnerable to collapse.

Figure 10 Lake Adelaide (942 m) at the head of Moraine Creek (mc) has been created by a large landslide dam (d). Rock slide debris covering an area of ~5 km2 was derived from a source area (sa) high on the ridge between Darran Pass and Apirana Peak (AP). Sample Site 1 (S1) is located near the south edge of the debris. The ridge-line scar and slide surface of an older failure (OF) is apparent north of Apirana Peak.

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Figure 11 Closer view of the landslide debris at Lake Adelaide. Sampling Site 1 is near the pond in the foreground (S1). Sites 5 and 6 (S5, S6) are on the distant debris ridge about 1.5–2 km to the north.

Figure 12 View of Lake Adelaide Landslide from the east showing the large slide debris (sd) dam at the head of Moraine Creek (MC). Water from the lake emerges in springs in a pond (p) at the toe of the debris. The head scarp and source area (sa) are visible on the ridge above the debris dam (upper right). Sample sites are also shown (S1, S5, S6).

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Figure 13 View of Lake Adelaide Landslide looking south and showing the main mass of slide debris, with sample sites 5 and 6 visible on the debris ridge in the foreground (S5, S6). To the right the ‘trough’ at the foot of the source area (sa) is filled with more recent, unvegetated (grey) slide debris (rd) and scree deposits. Variable vegetation on the landslide debris suggests that at least two or three failure phases have occurred.

Geomorphic and geological evidence indicates that the slope failure occurred on an extensive east-dipping 35-40° foliation surface and steep break-out joints in the source area (Figure 8). The foliation surface in the west side of the de-glaciated head of Moraine Creek appears to have been undercut where the former Moraine Creek glacier turned to flow east towards the Hollyford Valley, making the slope below Darran Pass more susceptible to failure (see Figure 8 and Figure 9).

Based on its geological controls and relatively long run out, the Lake Adelaide Landslide is classed as a rock-slide-avalanche (in the terminology of Cruden and Varnes, 1996). Vegetation patterns on the slide debris suggest that at least two or three major slope movements have occurred (Figure 8 and Figure 13). The main failure responsible for the thick mass of slide debris (~3.5 km2) which now impounds Lake Adelaide was derived from the ridge between Darran Pass and Apirana Peak 2 km to the north (Figure 8).

Unvegetated debris near the foot of the source area suggests that one or more smaller collapses from the head scarp occurred sometime after the main failure (Figure 13). It is also likely that a much earlier failure occurred on the ridge immediately north of Apirana Peak, evidence for which is provided by the deep notch in the ridge line ~500 m northeast of the peak, below which is an extensive ice-grooved foliation surface (Figure 10).

Hummocky older moraine deposits and possibly landslide debris from the earlier failure are preserved in the lower valley below the more recent main landslide mass. Deposits of more recent (possibly 18-19th century) moraine are present in the head of Moraine Creek 1.5–2 km southeast of Apirana Peak (Figure 8 and Figure 12).

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2.2.2 John O’Groats Landslide

John O’Groats Landslide is a giant landslide that occurred in John O’Groats Valley on the west coast 7 km north of the entrance of Milford Sound where the Alpine Fault goes offshore (Figure 1). The landslide involved the collapse of a 3 km long segment of the ridge 1.4 km north of Mt Pembroke (2015 m), including the western side of Te Hau (1703 m), into John O’Groats Valley. The failure created a huge deposit of boulders and finer debris up to 300- 400 m thick (estimated from a long-valley section, Figure 15) and covering an area of ~11 km2. Based on an estimated average debris thickness of ~100 m the volume of landslide debris is approximately 1000 Mm3. The main features of John O’Groats Landslide are illustrated by a geomorphic map (Figure 14), a cross section (Figure 15), and a number of aerial and ground photos (Figure 16 to Figure 19).

Figure 14 Geomorphic map of John O’Groats Landslide (~1000 Mm3) showing its extent, main features, and the planned and actual samplings sites. Potential sampling Sites 2 and 3 were not used because of unsuitable terrain and lack of datable materials. Landslide debris, which is believed to have reached the coast 9 km from the source area, covers an area of ~11 km2. The bouldery slide debris overruns and is displaced by the Alpine Fault in the lower valley. A large area of more recent (post landslide) alluvium has been deposited behind the fault scarp.

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Figure 15 Cross section through John O’Groats Landslide showing the extent and estimated thickness of the slide debris, which fills the entire valley from the headscarp to the coast. The Alpine Fault displaces the landslide deposit, with recent alluvium ponded against the downthrown side of the fault scarp.

The geomorphic map (Figure 14) and cross section of the landslide (Figure 15) show that the debris deposit fills the entire John O’Groats Valley, extending 9 km from the foot of the source area to the river mouth. In the lower valley, where the landslide deposit is displaced by the Alpine Fault, there is a distinct lateral bend (offset) in the river channel, and an extensive area of recent swampy alluvium ponded on the downthrown side of the fault scarp (Figure 14). The extent and lateral margins of the landslide debris shown in Figure 14 have been mapped approximately from a combination of Google Earth images, 20 m topographic contours, and recent aerial photos and inspections. In the lower valley to the west of the Alpine Fault there is clear evidence of super-elevation up the slope on the convex northern side of valley (Figure 16).

The source area of the landslide is a 3 km-long notch in the ridge and a very steep 40–50 degree ~800 m high head scarp on the western side of the 1703 m Te Hau peak (Figure 14 and Figure 16). Based on the height of the ridge to the south, the low point (1267 m) in the head scarp ridge may have had a pre-failure elevation of about 1600-1700 m (Figure 15). According to the Fiordland geological QMap (Turnbull et al., 2010), bedrock in the landslide source area belongs to the Cretaceous age Mouat Pluton, which comprises massive biotite/hornblende tonalite and granodiorite, with minor quartz diorite and granite. In the lower part of the source area the bedrock is Jagged Gneiss – mylonitic gneiss, also of Cretaceous age. These rocks are separated by an unfavourably-oriented northwest (downslope) dipping (~45°) fault, on which the slope movement is inferred to have probably occurred (Figure 15).

The landslide deposit is now covered by mature beech forest, but it is evident that the debris has a chaotic hummocky character and contains a number of debris ridges and a few small landslide ponds (Figure 14). An extensive ridge of debris in the upper slide mass may be a remnant of the original ridge line which collapsed, while several smaller ridges in the lower valley, including one near the river mouth, may represent flow pulses, or possibly separate phases of the slope failure. Three small landslide-dammed lakes were formed by drainage blockages by debris; two of these lakes are on the main tributaries in the valley head (Northern and Southern Lakes), and the third (Lower Lake) is in the main valley, from which the John O’Groats River now flows (Figure 9).

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Figure 16 Aerial view of John O’Groats Landslide showing the ~3 km long source area on the ridge north of Mt Pembroke (2015 m) between peak 1689 m and Te Hau (1703 m), and main part of the 11 km2 of slide debris, which has created three small landslide-dammed lakes (SL, NL, LL). Site 1, where 14C samples were collected, is located in a small clearing ~300 m north of the Southern Lake (SL).

Figure 17 Aerial view of hummocky landslide debris (sd) deposited in the lower John O’Groats Valley where it is displaced by the Alpine Fault. Recent alluvium (ral) is ponded in the downthrown (D) swampy area behind the fault scarp. The slide debris has risen up the convex northern side of the valley (sd*).

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Figure 18 Sampling Site 1 is located ~300 m north of the southern landslide-dammed lake in a small clearing underlain by alluvial deposits at the mouth of a stream dammed by slide debris. Carbonaceous material deposited on slide debris was obtained in auger holes at Sites 1 and 1a for 14C dating. A small pond on landslide debris ~500 east of Site 1 offers good potential for future 14C sampling.

Figure 19 Large (~1 m diameter, or ~300 years old) beech tree growing on the remains of a dead tree and slide debris boulders ~100 m northeast of the Site 1 clearing (E1193711m, N5055425m). A wood sample was collected from the centre of the dead tree for 14C dating (Wood Sample JG001).

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Debris blockages of small side streams are potential sites where alluvium and organic material may be preserved on the top of the bouldery slide debris. One such site on the south side of the slide mass (Site 1) 300 m north of the Southern Lake was one of our planned sampling sites (Figure 14, Figure 16, and Figure 18). Site 1 subsequently became the focus of the sampling efforts on the John O’Groats Landslide in 2012. Figure 18 shows the southern landslide-dammed lake and the Site 1 clearing from the air. Several peat core and sediment cores probes were put down around the site to collect carbonaceous material deposited directly on the top of the landslide debris.

Consideration was given to using dendrochronology (diameter of trees growing on slide debris) as a rough guide to the age of the landslide (Figure 19). The maximum diameter of beech trees growing around Site 1 varied from ~1 m to 2.1 m, indicating a probable forest age of ~300–700 years, based on an extrapolated diameter/age curve established for Mintaro Hut (Figure 7). This indicated that John O’Groats Landslide was at least 700 years old. The 14C dates obtained from carbonaceous material deposited directly on top of the landslide debris at Site 1 and Site 1a (Figure 18) were expected to return considerably older dates that would provide a more precise minimum age of the landslide.

Based on its probable failure mode and the nature and extremely long run out of the bouldery debris, the John O’Groats Landslide is classed as a rock avalanche. It is probably the largest known landslide of this type in New Zealand. Because of its size and close proximity to the Alpine Fault and the Fordland subduction zone, the John O’Groats landslide is most likely to have been triggered by a large-magnitude (M ≥8) earthquake, possibly on the Al[pine Fault (Hancox and Perrin 2009). Studies of historical earthquake-induced landslides in New Zealand have demonstrated that giant landslides of ~100 Mm3 or greater in size are invariably triggered by large magnitude earthquakes of magnitude 7.5 or greater. Two giant landslides were triggered by the 1929 Mw 7.8 Murchison earthquake (Hancox et al 1997, 2002).

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2.2.3 Lake Gunn Landslide Lake Gunn Landslide is a giant landslide formed by the collapse of a 2 km long segment of a high ridge on the east side of the Eglinton Valley in the Livingstone Mountains, and is crossed by the Milford Sound road (SH 94) about 70 km north of Te Anau (Figure 1 and Figure 20). The extensive landslide debris deposit, which covers an area of ~4.25 km2 and extends ~2.5 km to the western side of the Eglinton Valley, is responsible for the formation of Lake Gunn. The main features of the Lake Gunn Landslide are illustrated by a geomorphic map and cross section (Figure 20 and Figure 21 respectively), along with a number of aerial photos (Figure 22 to Figure 25).

Figure 20 Geomorphic map of Lake Gunn Landslide showing its extent and main features, along with planned and actual sampling sites on 31 March and 1 April 2012. Potential sampling sites identified from aerial photos were unsuitable because of the terrain or lack of datable materials. Much of the slide mass above SH94, on which Site 3 is located, appears to be a semi-intact slide block (sb).

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Figure 21 Cross section through Lake Gunn Landslide showing the extent and estimated thickness of the slide debris, which fills the entire valley from the headscarp to the coast. Based on a surface area of ~4.25 km2 and estimated average debris thickness of 50–100 m the volume of the landslide debris is estimated to be about 300 Mm3. The Eglinton River has cut a narrow winding channel through beech-forest covered mounds of landslide debris up to 50-100 m high, providing good river bank exposures for inspection and sampling of the debris. Road cut exposures through the debris along SH94 also offer potential sampling sites (Figure 20). The source of Lake Gunn Landslide is indicated by a 2 km-wide arcuate bite out of the ridge on the east side of the Eglinton Valley between Cascade Creek and Plato Creek (Figure 20). The source area is bounded by a steep (35°) 600–700 m high head scarp which extends up to about 1500 m, around 1000 m above the valley floor (Figure 21). The bedrock in the landslide source area is Permian age Brook Street Volcanics Group rocks, which locally comprise volcanic breccia, sandstone and siltstone. Triassic volcanic sandstone, minor siltstone and breccia are mapped along the head scarp ridge (Turnbull 2000). The rock types in the headscarp are separated by an old (inactive) northeast-trending fault, which dips to the northwest and may have provided the failure surface for the landslide movement. The main mass of landslide debris directly upslope (southeast) of SH94 appears to be a very large semi-intact slide block (area ~0.95 km2), which has moved some distance downslope, but seems from its morphology is little disrupted or deformed (Figure 20, Figure 21, Figure 22, and Figure 23). Based on present-day geomorphology of the area it is inferred that the high point on the slide block (811 m) was formerly a 1500-1600 m high peak on the western ridge of the Livingstone Mountains, which has moved laterally about 1.6 km northwest into the Eglinton Valley, with a vertical fall of between 700 and 800 m (Figure 21). Northwest of SH94 the landslide debris is spread out over ~2.4 km2 of the valley floor (Figure 16 and Figure 17). Based on its probable failure mode and long runout, the Lake Gunn Landslide is probably best classed as a rock block-slide and debris avalanche. Although the Lake Gunn Landslide would undoubtedly have dammed the Eglinton Valley to form Lake Gunn, the Eglinton River has eroded a channel which winds its way between large mounds and hillocks of slide debris. The outlet end of Lake Gunn is currently being constricted and infilled by large active debris fans built by Cascade Creek on the east side, and on the northwest side of the valley the stream draining southeast from Melita Peak (Figure 20 and Figure 22).

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Figure 22 Aerial view of Lake Gunn Landslide looking south down the Eglinton Valley. The main features of the landslide are its steep source area, a semi-intact slide block (sb) above SH94, and hummocky slide debris (sd) in the valley, through which the Eglinton River has cut a winding channel. In the foreground alluvial fans (f) are slowly constricting and infilling Lake Gunn. S1, S2, S3 are sampling sites.

Figure 23 Aerial view of Lake Gunn Landslide showing the 2 km-wide headscarp, source area, slide debris (sd), semi-intact slide block (SB), and sampling sites S1 and S3. Spot heights (m) indicate the vertical scale of the landslide.

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Figure 24 View of Lake Gunn Landslide looking north towards Lake Gunn. The Eglinton River (ER) winds its way through mounds and conical hills of slide debris (sd) west of SH94, with the slide-block (SB) clearly apparent below the recently-scarred head scarp (hs). The sampling sites are also shown (S1, S2, S3).

Figure 25 Mounds and conical hillocks of Lake Gunn Landslide debris spread across the 1 km-wide floor of the Eglinton Valley. Sampling Site 1 (S1) located in a road cut on SH94, and Site 2 (S2) on a bend in the Eglington River channel are both visible in this view.

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3.0 SAMPLING FOR LANDSLIDE DATING This Section of the report provides details of the sampling carried out on the three landslides. A list of samples collected and locations is presented in Table 2. The sampling sites are shown on the landslide geomorphic maps (Figure 8, Figure 14, and Figure 20) and various photos presented above. Table 2 List of samples collected for landslide dating in Fiordland 28 March to 1 April 2012.

Sample Site NZTM Grid Reference (m) Sample Type/ Comments Number(1) / Date No. and approx. Altitude (m) (2) dating method 1. Lake Adelaide Landslide

LA001 – 28/3 Site 1 E1206812, N5036304, 973 m Peaty soil, 14C Samples from spade-dug test pit at depth 14C ID: LA-1(3) ~1.1 m, directly on top of slide debris. LA002 – 28/3 Site 1 E1206812, N5036304, 973 m Peaty soil, 14C LA003 – 28/3 Site 1 E1206773, N5036358, 980 m Beech tree disc Annual ring counts on discs will provide LA004 – 28/3 Site 1 E1206773, N5036358, 980 m Beech tree disc age/diameter data for Lake Adelaide site. LA005 – 28/3 Site 5 E1207308, N5038214, 1010 m QD dyke, 10Be Samples of quartz diorite (?) dykes taken LA006 – 28/3 Site 5 E1207308, N5038214, 1010 m QD dyke, 10Be from large boulder of older slide debris. LAC001 – 30/3 Site 1 E1206815, N5036307, 973 m Peaty soil, 14C Core sample ~1.7 m on top of debris. 14C IDs: LA-C1a,e,h LATC1 – 30/3 Site 6 E1207548, N5037574, 1020 m Beech tree core Beech tree cores (53, 29 cm diameter) LATC2 – 30/3 Site 6 E1207548, N5037574, 1020 m Beech tree core taken for age/diameter data at site. 2. John O’Groats Landslide JGC1 – 29/3 Site 1 E1193610, N5055350, alt 450 m Sandy silt, 14C Core sample at 3.7m (on rock bottom). 14C ID: JOG-C1d JGC2 – 29/3 Site 1 E1193618, N5055378, alt 450 m Sandy silt, gr,14C Core taken 2.7-3.7 m (on rock bottom). JG001– 29/3 Site 1 E1193711, N5055425, alt 455 m Dead tree, 14C Wood from dead tree on boulder. 14C ID: JOG-1 JG002– 29/3 Site 1 E1193610, N5055285, alt 450 m Old wood, 14C Old wood samples from stream bed (002); JG003– 29/3 Site 1 E1193610, N5055285, alt 450 m Old wood, 14C stream bank 110 cm (003), 70 cm (004) 14C ID: JOG-3 below top of stream bank (general level of the Site 1 clearing surface). JG004– 29/3 Site 1 E1193610, N5055285, alt 450 m Old wood, 14C JGC3– 30/3 Site 1 E1193509, N5055251, alt 450 m Gr/black silt, 14C Auger samples of grey black sandy silt with 14C IDs: JOG-C3a,b leaves and wood fragments from depths of JGC4– 30/3 Site 1 E1193509, N5055251, alt 450 m Gr/black silt, 14C 1-1.25 m on top of boulders (boggy area in 14C ID: JOG-C4a bush 100 m SE of Site 1). JGC5– 30/3 Site 1 E1193452, N5055240, alt 450 m Gr/black silt, 14C 3. Lake Gunn Landslide LG001– 31/3 Site 1 E1210078, N5015605, alt 500 m Grey silt, OSL Lens of grey silt in slide debris in road cut. LG002– 31/3 Site 2 E1210133, N5016234, alt 465 m Old wood, 14C Sample of old wood from Eglington River 14C IDs: LG-2,7 bank exposure. Grey lake silts deformed by LG003– 31/3 Site 2 E1210133, N5016234, alt 465 m Old lake silt slide sampled (not for dating). LG004– 1/4 Site 3 E1210406, N5015090, alt 720 m Peaty soil, 14C Auger sample of peaty soil at a depth of ~1 14C ID: LGS-2 m in small boggy and mossy clearing on top of the semi-intact slide block.

Notes 1. Sample numbers are those assigned in the field. 2. Site and sample grid references and altitudes are based on GPS waypoints or NZTopo50 maps. 3. Sample IDs used for 14C analysis results (see Table 3).

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Overall, 22 samples were collected for dating purposes. Of these, 14 samples are potentially for radiocarbon dating (3 Lake Adelaide, 9 John O’Groats, 2 Lake Gunn), two from Lake Adelaide are potentially for 10Be cosmogenic dating, and one from Lake Gunn may be suitable for OSL luminescence dating. Four tree discs and tree core samples obtained from Lake Adelaide are believed to be too young to date the landslide, but will provide data for the age/diameter curve for silver beech trees in Fiordland (Figure 7).

3.1 LAKE ADELAIDE LANDSLIDE SAMPLING

The main sampling effort on Lake Adelaide Landslide was at Site 1, which is in a small flat tussock-covered, swampy area with shallow ponds located between ridges of large boulders near the southern edge of the landslide debris mass (Figure 8). The area immediately surrounding the site was closely examined on the ground, and test pit and auger sites for 14C sampling were selected in a tussock-covered area close to one of the larger ponds (Figure 11). Foot travel over the ridges of slide debris was generally difficult and slow because of the large boulders. Aerial inspection showed that three other potential sampling sites (Sites 2, 3, and 4) were likely to be unsuitable for 14C dating because of the absence of swampy ground or peaty soils in those areas.

At Site1 (NZTM E1206812m, N5036304 m, altitude 973 m) two peat samples for 14C dating (LA001, LA002) were obtained on 28/3/2012 from a test pit at a depth of ~1.1 m, near the top of the underlying bouldery slide debris. At a nearby location (E1206815 m; N5036307 m), on 30/3/2012 a peat core sample (LAC001) for 14C dating was recovered from a depth of ~1.7 m, directly on top of bouldery slide debris.

Site 5 was visited during the afternoon of 28/3/2012 where two cosmogenic Be10 samples (LA0005, LA006) of light-coloured quartz diorite (?) dykes were collected from a north-facing side of a large (~2.5 m) boulder of leucogabbro on the debris crest (see Figure 8 and Figure 13). The exposure angles to the horizon at this site ranged from 25° to 30° in the east-northeast to west- northwest sky directions. These samples have not yet been processed for dating.

Two tree-disc samples (LA003, LA004) were also obtained from 250 mm diameter wind- damaged silver beech tree growing on a large boulder in the slide debris ~100 m west of the test pit at Site 1. Because our aerial inspection had shown the presence of many large trees on the debris these samples were unlikely to indicate the age of the landslide, but were collected to provide data points on the diameter/age curve for silver beech in Fiordland (Figure 7).

At Site 6 (E1207548 m; N5037574 m) on the high debris ridge along the northeast edge of the slide mass (Figure 8 and Figure 12) three cores were taken on 30/3/2012 of stunted old silver beech trees growing on large boulders. The maximum tree diameter at this site was ~1.2 to 1.3 m (~400 years old), which was too large for our 400 mm tree-corer. Cores of two smaller trees were taken (LATC1 diameter 290 mm; LATC2, diameter 530 mm). Although the tree cores samples are not expected to indicate the age of the largest beech tree growing on slide debris, the age data obtained from Site 6 may provide data to construct a diameter/age curve for the Lake Adelaide area.

Based on existing tree core data from Mintaro Hut and Loch Maree (Figure 7), a sub-alpine silver beech tree with a diameter of 1.2–1.3 m indicates an age of approximately 450 years. Allowing for a forest establishment lag, this suggests a minimum age for the landslide of at least 450-500 years. However, the landslide is believed to be considerably older than that, as will be shown by the dates obtained for the 14C samples obtained at Site 1 which are discussed in the next section.

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3.2 JOHN O’GROATS LANDSLIDE SAMPLING

Three potential landslide sites (Sites 1, 2, and 3) were identified on the John O’Groats Landslide prior to going into the field (Figure 14). When these sites were evaluated from the helicopter sampling appeared to be feasible only at Site 1. Deep water, floating weeds, and lack of landing space was a problem at Site 2, and Site 3 near the head scarp was also unsuitable as it had been contaminated by recent rock fall and scree deposits from the headscarp. A small pond on the main area of slide debris about 500 m east of Site 1 appeared to offer good conditions for sampling, but would require specialised lake-bed coring equipment. The best location for 14C sampling was found to be at Site 1, which is located in a grassy clearing at the mouth of a small stream dammed by slide debris about 300 m north of the Southern Lake (Figure 26).

Figure 26 Aerial view of Sites 1 and Site 1a on John O’Groats Landslide.

At Site1 (NZTM E1193610 m; N5055350 m, alt. ~450 m) on 29/3/2013 a series of probes were put down to the top of slide debris through alluvium deposited by the stream which has been blocked by the landslide mass. The depth to refusal on slide debris boulders varied from ~2–4 m across the fan at the mouth of the stream, where fine gravel has been deposited. At probe site #1 a core sample of brown sandy silt (JGC1) was taken at a depth of 3.7 m. At probe site #6 a core (JGC2) was taken from a depth 2.7 to 3.7 m (brown fine sandy silt and gravel). The two cores from Site 1 are dominated by brown silty sand and fine gravel, which appears to be ‘rock dust’, washed out of the stream catchment during rain sometime after the landslide occurred. At Site 1a (E1193509 m, N5055251 m) about 150 m southeast of Site 1 (Figure 26) on 30/3/2012 three auger samples (JGC3, JGC4, JGC5) of dark grey- black sandy-silty peat containing small fragments of wood and beech leaves were collected from depths of 1.0–1.25 m directly on top of slide debris boulders (Figure 27). These samples contain little sand and gravel and appeared to have good potential for 14C dating the age of John O’Groats Landslide.

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Figure 27 Beech tree leaves and wood in organic silt (Sample JDC3) from a depth of about 1.25 m, directly on the top of boulders of landslide debris at a location about 150 m southeast of the Site 1 clearing.

About 100 m northeast of the Site 1 clearing wood samples (JG001) were collected (29/3/12) for 14C dating from the centre of a dead tree enveloped by a large (~1 m diameter) beech growing on slide debris boulders (Figure 19). Old wood samples were also collected at Site 1 from the banks of the small stream at depths of 0.7 to 1.1 m below the ground surface (JG002, JG003, JG004). Beech trees from 1m to 2 m in diameter were found growing on slide debris boulders ~150 m southeast of Site 1. The largest of trees had a diameter of 2.1 m, indicating a potential age of ~700 years, while other 1–1.5 m diameter trees may be ~350–500 years old (Figure 7). Although the beech tree age estimates indicate that the minimum age of the John O’Groats Landslide is at least 700 years, its true age is likely to be much older than that. This will be discussed later.

3.3 LAKE GUNN LANDSLIDE SAMPLING

A number of potential landslide dating sites were initially identified at Lake Gunn Landslide (Figure 20). These sites were assessed from the air for their sampling potential on 28/3/2013. Valuable information on geomorphic features in the landslide area was also provided by the manager of Knobs Flat Motels, ‘PC’ Taylor. He told us about a network of DOC tracks in the area and the existence of a potential sampling site in a swampy clearing on top of the slide mass above SH94 (Site 3), which could be reached via a track up the spur near Site 1 (Figure 20).

The last two days of the field trip were spent inspecting the Lake Gunn landslide and searching for suitable sampling sites. On 31/3/2012 exposures of slide debris along SH94 between Plato Creek and Cascade Creek, and along the Eglinton River channel were inspected on the ground. At Site 1 (NZTM E1210078m, N5015605m, alt ~500 m) in a road cut exposure a tube sample of grey silt in the debris was taken for luminescence (OSL) dating (LG001).

Later in the day slide debris exposures in the banks of the Eglinton River channel were inspected downstream as far as Site 2 (Figure 22). At Site 2 (NZTM E1210133m, N5016234m, alt ~465 m) blocks of lake sediments (soft grey silt LG003) showed evidence of deformation, possibly through contact with the debris avalanche deposit. Several samples of old wood were collected at Site 2, including an entire log (Figure 28) were located and sampled for 14C dating (LG002).

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Figure 28 Lake Gunn Landslide sampling at Site 2 in the Eglinton River channel. The old log (lower left) buried within the slide debris at this site was sampled for radiocarbon dating (LG002).

On 1/4/2012 Site 3 (NZTM~E1210406m, ~N5015090m, RL~720 m) in a small clearing on top of the semi-intact slide block above SH 94 was visited (Figure 20 and Figure 23). A soil auger was used at Site 3 to locate the base of surficial peat at a depth of about 0.7 to 1 m (Figure 29). An auger sample of peaty soil was collected at a depth of ~1 m for 14C dating (LG004). Below this level was about 2 m of light blue coloured fine sandy silt to silty sand. It is hoped that the organic material from above the contact between the peat and clastic materials will provide additional datable material to test the age or timing of the debris avalanche.

Figure 29 Site 3 on top of the slide block above SH94. A soil auger was used at this site to sample the surficial peat deposits for 14C dating (LG004).

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4.0 RADIOCARBON DATING RESULTS

After returning from the field the soil, peat and wood samples collected on the three landslides (Table 2) were sorted and processed. Thirteen samples were submitted to the GNS Science Rafter Radiocarbon Laboratory for radiocarbon (14C) analysis and dating using the latest AMS technology. Treatment of all samples submitted for 14C analysis involved a process of physical examination, cleaning, labelling, description, drying and chemical cleansing to remove sample contaminants.

A summary of the 14C analysis and dates provided by the Rafter Radiocarbon Laboratory for samples of organic material from the Lake Adelaide, John O’Groats, and Lake Gunn landslides that were submitted following the 2012 fieldwork is presented in Table 3. Figure 30 is a graphic representation of the 14C dates for these landslides, along with the known ages of some other large landslides in Fiordland.

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Table 3 Radiocarbon dates from Fiordland landslide sites, 2012.

Sample NZA Lab Δ13 Radiocarbon age Calibrated radiocarbon age Landslide Type of material dated and significance I.D. No. C (‰) (yr BP) 1σ cal yr BP 2σ cal yr BP Lake Adelaide core site – hand-dug pit. Peaty silt to organic silt. Bulk date on material from depth Lake Adelaide LA-1` 50341 -27.8 1248 ± 21 1069-1168* 1055-1178 of 0.8 m. LA-C1a 50343 -30.1 3702 ± 25 3901-4063 3870-4082 Lake Adelaide core site. Wood fragments sampled from depth of c.1.21 m in core. Lake Adelaide core site. Peaty, Raupo root traces at a depth of 1.58-1.60 m. Thought to be in LA-C1e 50342 -27.3 490 ± 30 494-517 470-535 situ material but some doubt due to the presence of roots. Lake Adelaide core site. Olive grey fine, silty sand. Pseudo-bedded fabric seen from iron- LA-C1h 50340 -28.7 5441 ± 51 6028-6279*** 6001-6287 staining. Small organic fragments at 1.67-1.70 m depth separated for dating. Forest site above John O’Groats clearing. Dry dead wood sampled from underneath a growing John O’Groats JOG-1 50344 -27.6 539 ± 20 513-533 505-542 tree, i.e. a former ‘nursery’ tree. John O’Groats clearing. Old wood sample from within stream bank cut into clearing. 3 cm thick JOG-3 50345 -26.1 146 ± 19 3-252*** 0-259** branch of wood. John O’Groats auger site 3. Grey-black sandy silt containing leaves and wood fragments on top JOG-C3a 50372 -30.8 179 ± 19 0-274*** 0-278*** of boulders. Small organic sample submitted from 1.52 m depth. John O’Groats auger site 3 in clearing. Black brown silty peat with twigs and Raupo roots JOG-C3b 50373 -25.7 454 ± 19 474-503* 453-509 (separated) depth 1.55 m in auger hole. John O’Groats auger site 4. Black-brown hairy peat containing leaves and wood fragments on JOG-C4a 50553 -25.7 359 ± 18 323-444** 315-451 top of boulders. ‘Hairs’ at a depth of c. 1.5 m separated for dating. John O’Groats core site 1 next to stream. Light yellow brown silt layer at depth of 3.55-3.65 m in JOG-C1d 50346 -28.6 5112 ± 49 5744-5890 5660-5911 core. 1 cm horizon containing organic flecks separated for dating. Closed basin site in range above Eglinton highway. Peat sampled from 0.9 m below surface of Lake Gunn LGS-2 50337 -29.5 4982 ± 28 5600-5704** 5593-5722 swampy clearing. Sample is directly above peat/ silt contact Eglinton River site. Wood sampled near the outside of log found within shattered gravels at the LG-2 50339 -27.7 6846 ± 33 7593-7666 7578-7681 toe of the landslide, exposed in the Eglinton River below highway. Eglinton River site. Small wood sample from within a large block of deformed lake sediments at LG-7 50338 -25.5 6993 ± 33 7697-7818* 7676-7912* the toe of the landslide, exposed near sample LG-2.

Notes Conventional radiocarbon (14C) age before present (AD 1950) calculated as defined in Stuiver and Polach (1977) using Libby half-life of 5568 years, and normalized to δ13C of –25 ‰. Quoted error is ± 1σ. Calibrated radiocarbon age: Calendar years before present (AD 1950) using calibration program Winscal5.0 (© Inst Geological & Nuclear Sciences) and Southern Hemisphere Atmospheric data from McCormac et al. (2004). A lab error multiplier of 1 has been applied to NZA samples. Age ranges listed are minimum and maximum values of the calibrated age range based on a radiocarbon age error of ±2σ. Explanation of calibrated ages: * comprises 2 calibrated peak distributions; ** comprises 3 calibrated peak distributions;*** comprises 4 or more calibrated peak distributions.

Figure 30 Plot of the calibrated radiocarbon ages (2σ cal. years BP, range and mean values) for samples from the Lake Adelaide, John O’Groats, and Lake Gunn landslides. For comparison the known ages of Green Lake Landslide1, Daleys Flat rock avalanche3, the Polnoon Landslide5, and several other landslides in Fiordland2 are also shown.

4.1 SAMPLE COLLECTION AND RADIOCARBON DATING

In this Section we discuss the radiocarbon dating samples collected on deposits of the three landslides that were investigated and the resulting age data for the landslide events. The data mainly comes from radiocarbon dating of organic material; however, in some cases we have collected tree rings and slabs, OSL samples and Cosmogenic dating samples (Table 2). All of the dates are presented (in the text and Table 3) as radiocarbon dates, with calibrated (cal.) years BP (before present, or AD1950) at the 2 sigma level rounded to the half-decadal level.

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4.1.1 Lake Adelaide Landslide

At Site 1, a 1.1 m deep pit was dug into the peat bog in order to test its depth and sample material immediately above the slide debris (Table 1). The stratigraphy of the pit comprised peat and organic silt. A small bulk organic sample from the silty peat (sample LA-1; depth 0.8 m) was submitted for radiocarbon dating, yielding an age of 1248 ± 21 years (yr) BP (Table 2 and Table 3).

Subsequently, a single core was extracted from the bog using a Livingstone corer to a depth of 1.7 m, at which depth the corer brought up small rock fragments. This is assumed to be the upper surface (or reworked surface) of the landslide deposit. The stratigraphy within the core ranged from peat to organic silt and included silty sand and medium sand horizons. Three samples were obtained from the core to provide age control on the development of the bog and the landslide deposit (samples LA-C1a, -C1e and -C1h). Two of these dates (LA- C1a; depth c. 1.2 m and LA-C1h; 1.68 m) were in stratigraphic order with the date from the pit. These yielded radiocarbon ages of 3702 ± 25 yr BP and 5441 ± 51 yr BP, respectively (Table 3). Sample LA-C1e (depth c. 1.59 m) yielded an age of 490 ± 30 yr BP. This sample comprised peaty Raupo roots that are likely to have grown down from above, i.e. they are not in-situ chronologically in the stratigraphy (see Figure 31). Sample LA-C1h, from the fine- grained sediments and above stony material at the base of the core, with a calibrated age of 6000-6290 cal. (calibrated) yr BP (6144 ±143 yr), represents a minimum age for the Lake Adelaide landslide deposit (Figure 30).

In addition to the sampling at Site 1, further sampling was undertaken at sites 1, 5 and 6. Near Site 1 and at Site 6 tree cores and sections collected from moderate to large sized older trees. At the time, we considered it possible that these trees could be used to date the landslide deposit. That is, we entertained the idea that either the slide deposit could be quite young based on a primary tree cover, and/or that trees growing at this elevation could be quite slow growing and therefore old. None of the age data from these tree cores and sections was older than c. 500-700 years, and therefore not comparable to the older results from the Site 1 core stratigraphy.

Figure 31 Diagrammatic plot of the relative positions of radiocarbon samples and radiocarbon ages. Note that sample LA-C1e was contaminated with Raupo roots and returned a relatively young age given its stratigraphic position.

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4.1.2 John O’Groats Landslide

In order to assess the age of the John O’Groats landslide deposit we undertook four distinct sampling strategies to date organic material. The first strategy was to collect relict wood from within the root mass of a tree growing within the forest on top of the landslide deposit above the clearing at Site 1 (sample JOG-1; Figure 19). This wood was considered to have originated from a fallen tree and yielded a radiocarbon age of 539 ± 20 yr BP (c. 500-540 cal. yr BP). This date is consistent with the age of the current forest growing across the landslide debris based on tree diameter sizes and almost certainly dates an older generation of forest (Figure 7).

The second strategy was that several wood samples were collected from stream banks within the Site 1 clearing, exposed by local stream incision. Sample JOG-3 came from a branch with attached bark and yielded a 14C age of 146 ± 19 yr BP.

The third strategy was used at the southern end of the clearing, where we extracted samples from several auger holes into peaty hollows that were perched on top of reworked slide debris, exposed nearby. Two samples (JOG-C3a and –C3b) were collected from depths of c. 1.46 and 1.52 m in auger hole 3 (Figure 30). These samples yielded ages of 179 ± 19 yr BP and 454 ± 19 yr BP. A single sample from auger hole 4 (JOG-4a) yielded an age of 359 ± 18 yr BP.

These five samples from near surface locations all yielded young ages of < c. 540 cal. yr BP. This data contrasts with the result to follow, and suggests that these dates could relate to the most recent changes in the forest and landscape (deposition of alluvium) at the Site 1 locality.

Two cores were collected with a Livingstone corer to sample the sedimentary fill within the Site 1 clearing. Both cores reached depths of c. 3.7 m or more into the stratigraphy, which is comprised of fine-grained sediments ranging from fine sandy silt to gritty sands. Only one organic sample collected from these cores (JOG-C1d; depth 3.6 m) was large enough to date. This sample yielded an age of 5112 ± 49 yr BP (c. 5660-5910 cal. yr BP, or 5785 ±125 cal. yr BP). This date is much older than the five other samples described above. As sample JOG-C1d comes from deeper in the sediment fill of the clearing, which has formed directly on top of the landslide deposit, it probably provides a useful minimum age for the John O’Groats landslide event.

Apart from the relative age estimates based on the diameter of large trees growing in the clearing and on the landslide debris, no other dating techniques have currently been tested on the John O’Groats landslide deposit. Future dating studies of the landslide pond 500 m east of the Site 1 clearing (Figure 18), and of the antiquity of the John O’Groats swamp along the Alpine Fault (Figure 17) could provide confirmation that the landslide event is of at least mid-Holocene age, as suggested by sample JOG-C1d.

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4.1.3 Lake Gunn Landslide

At Site 1, the landslide deposit and some cover deposits and soils are exposed in a SH94 road cut (Figure 23). No organic material was located in this road cut that could be used to date the relationship between the landslide debris and the cover deposits. However, in one place a fine grained sediment (grey silt) was observed filling in hummocky or channel topography above the landslide deposit in outcrop. In this case, we collected a tube of sediment for Optically Stimulated Luminescence (OSL) dating. The sample (LG001) was submitted to the Victoria University Luminescence (VLUM) laboratory and returned ages of 49.1 ± 10.0 kyr and 25.8 ± 3.4 kyr, on a feldspar and fine quartz fraction, respectively. Both dates have been described as probable over-estimates of the real age of the event in the report prepared by the VLUM lab. The relevance of these dates will be discussed below with regards to radiocarbon dates from sites 2 and 3.

Site 2 was located along a bend in the Eglinton River cut into the northern toe of the landslide deposit (Figure 25). In this area the river has exposed the landslide toe and its relationship with the pre-existing geology of the valley floor. Outcrops revealed a chaotic relationship between landslide debris and large tilted, deformed or included blocks of laminated very fine-grained sediments, which in places included chunks of wood and even logs. The relations imply that the valley had been occupied by a (glacial) lake, or possibly a fan-dammed lake backing up from Knobs Flat 8 km downstream, and a fringing forest, when the landslide occurred.

Two wood samples, one of which was collected from a large log incorporated within the mixed debris were selected for radiocarbon dating (Table 3). Samples LG-2 and LG-7 yielded ages of 6846 ± 33 and 6993 ± 33 yr BP (collectively c. 7580-7910 cal. yr BP). Based on the geologic relationship of these samples, the timing of the Lake Gunn landslide is well characterised by the slightly younger LG-2 sample, i.e. 7580-7680 cal. yr BP (or 7630 ±50 cal. yr BP).

Site 3 was located during helicopter reconnaissance of the Lake Gunn area. Site 3 is a swampy forest clearing high up within the upper part of the landslide deposit (on the semi- intact slide block) above SH94. The site was accessed by foot. Two holes were augered into the centre of the swamp with the aim of sampling the stratigraphy of the swamp and collecting material to date. Samples from both auger probes were characterised by a dark brown peat that occurred to a depth of c. 0.9 m. Under the peat layer the auger extracted fairly homogeneous blue-grey silt to silty clay for a further 2 m. A sample of peat was taken from immediately above the contact with the blue-grey silts for radiocarbon dating. Sample LGS-2 yielded an age of 4982 ± 28 yr BP (collectively c. 5595-5722 cal. yr BP).

At Site 3 we expect that it must have taken some time for a stable peat-forming environment to have formed on the semi-intact landslide block 3 following a long period of fine deposition off the upper part of the landslide deposit into the small basin. Therefore, the date from this peat represents a minimum age for the Lake Gunn Landslide. As such, it is consistent with the two ages from Site 2, that probably characterise the true age of the slide quite well (i.e. 7580-7680 cal. yr BP).

The relationships of the ages of the Lake Adelaide, John O’Groats, and Lake Gunn landslides ages to Alpine Fault earthquakes recorded at Hokuri Creek (Berryman et al., 2012) are shown in Figure 32 and discussed in Section 5.

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Figure 32 Minimum ages of the John O’Groats (JOG), Lake Adelaide (LA), and Lake Gunn (LG) landslides plotted against the B.C.E/C.E (Common Era) ages (95% or 2σ range) of surface-rupturing earthquakes on the Alpine Fault from the Hokuri Creek (Hk) and Haast (Ha) records (from Berryman et al., 2012). The landslides correlate well with Alpine Fault events, Hk15 (JOG), Hk16 (LA), and Hk21 (LG).

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5.0 DISCUSSION

5.1 AGE OF THE LANDSLIDES

The oldest calibrated radiocarbon ages obtained from organic material deposited directly on top of the John O’Groats and Lake Adelaide landslide debris and from wood fragments entrained in the Lake Gunn landside deposit are believed to provide reasonable minimum ages for the initial major landslides events that occurred at the three sites. The calibrated 95% (2σ) 14C age ranges obtained for the landslides are: John O’Groats LS, 5660–5910 years BP; Lake Adelaide LS, 6000–6290 years BP; and Lake Gunn LS, 7580–7680 years BP (Table 3, Figure 30 and Figure 32). We believe that these ages are consistent with: (1) the age of the beech forest established on the landslide deposits (older than ~300–700 years); (2) multiple displacements of the landslide deposits in John O’Groats valley by the Alpine Fault over the last ~6000 years; (3) moderate to minor erosional modification of the landslide head scarp, source areas, and deposits. The younger 14C ages which we obtained (Figure 30) provide some age control on the deposits overlying the landslide debris but do not indicate the ages of the initial slopes failures. This issue is often encountered in dating prehistoric landslides, but we believe that the oldest dates we obtained for organic material deposited directly on top of the slide debris provide the best estimate of the age of the three giant landslides dated in this study.

5.2 COSEISMIC TRIGGERING OF LANDSLIDES

A number of the very large (≥ 106 m3) and giant (≥108 m3) landslides in Fiordland and the western range front of the Southern Alps (e.g., Green Lake, John O’Groats, Lake Adelaide, Lochnagar, Cascade, Round Top, Daleys Flat and Mt Wilberg) are postulated to have been triggered by Alpine Fault earthquakes (Whitehouse, 1983; Hancox and Perrin, 1994; Wright, 1998; Hancox et. al., 2003; Korup, 2004, 2005; Chevalier et al., 2009; Hancox and Perrin, 2009; Hancox et al, 2010, Wood et al., 2011; Barth, 2013). More recently, however, evidence of tsunamis, coastal uplift, and large landslides from Cascade Point to Doubtful Sound associated with a great (≥M 8) earthquake in Fiordland in AD 1826, which has been attributed to the Puysegur subduction zone (Norris et al., 2001; Downes et al., 2005; Wells and Goff, 2007; Hancox et al., 2010). This evidence suggests that the Puysegur subduction zone is also capable of generating intensity MM9 or greater shaking across Fiordland and south Westland.

Although the most recent large earthquakes on the subduction zone in 2003 (Mw 7.2) and 2009 (Mw 7.6), have generated mainly smaller superficial landslides over large areas of Fiordland, as discussed earlier there is good reason to believe that the 1826 earthquake caused widespread landsliding, and probably a number of very large landslides such as the ~3 Mm3 landslide that dams Loch Maree at the head of Dusky Sound (Hancox et al., 2010). Given its reported effects and probable magnitude, the 1826 earthquake may have resulted from rupture of the whole of the Fiordland subduction zone as far north as Milford Sound (pers. comm. Martin Reyners 2009).

Studies of historical coseismic landsliding in New Zealand (Hancox et al., 1997, 2002) have shown that landslides with volumes of 108 m3 or greater have occurred only during earthquakes of M >7.5 and shaking of intensity MM9 or greater (Appendix 1). It is possible, therefore, that either an Alpine Fault or a Puysegur subduction zone earthquake could have triggered the Green Lake Landslide and other giant landslides in Fiordland and south

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Westland (Figure 1). Despite the presence of other sources (active faults), the Alpine Fault and the Puysegur subduction zone are regarded as the most likely earthquake sources capable of generating very large and widespread landslides over this region (Howarth et al., 2012; Barth, 2013). The strong causal link between large earthquakes and very large landslides was also recently demonstrated by the May 2008 Wenchuan M 8 earthquake in China, which triggered more than 40 landslides ranging from 1 to 27 Mm3 causing at least 20,000 fatalities (Yin et al., 2009).

It is often difficult to determine whether or not a prehistoric landslide was earthquake- triggered, especially in the case of random large landslides of 106 to 107 m3, which can occur without an obvious trigger (e.g., 1991 Mt Cook and 2013 Mt Haast rock avalanches), or smaller landslides of 105 m3 or less, which are commonly triggered by severe rainstorms (Trotter, 1998; Page et al., 1994; Hancox and Wright, 2005; Hancox and Thomson, 2013). However, giant or mega landslides of 100 Mm3 or greater such as the Lake Adelaide, John O’Groats, and Lake Gunn landslides are much more likely to have been triggered coseismically. This is particularly true in Fiordland where more than 50 very large and giant landslides of postglacial age occur in clusters in relatively close proximity to known earthquake sources such as the Alpine Fault and the Puysegur subduction zone. For these reasons we believe it is more likely than not that the Lake Adelaide, John O’Groats, and Lake Gunn landslides were triggered by large earthquakes on either of these potential sources.

Comparison of the calibrated radiocarbon ages for the three landslides studied (Table 3) with the paleoseisological record of Alpine Fault earthquakes provides one of the more convincing ways of determining if the landslides were, or could have been coseismically triggered. Unfortunately no similar record has been compiled for prehistoric earthquakes on the Puysegur subduction zone, so that source cannot be confirmed or ruled out. However, by comparing the calibrated 14C ages of the landslides dated with the Alpine Fault earthquakes recorded by Berryman et al. (2012) in the continuous c. 8000-year earthquake record from Hokuri Creek, 20 km north of John O’Groats Landslide, we believe we are able to match the radiocarbon landslide ages with specific Alpine Fault earthquakes (Figure 32). The ages and the probable triggering Alpine Fault earthquakes for these landslides dated in this study and several other very large landslides in Fiordland and south Westland are summarised in Table 4.

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Table 4 Ages of landslides attributed to Alpine Fault earthquakes in Fiordland and south Westland.

Alpine Fault Landslide1 Calibrated 14C age Earthquake3 References4 (Volume Mm3) (years BP and BCE/CE ages)2 (Years BCE/CE) Mt Wilberg rock avalanche (33 Mm3) ~730 BP (1220 AD/CE) Ha2 (1215–1372 CE) Chevalier et al., 2009 Lake Ada rock fall (32 Mm3) ~900 BP (~1050 AD/CE) Hk1–HK 2 (Figure 32) Dykstra, pers. comm. 2012 Daleys Flat rock avalanche (18 Mm3) 975–1225 BP (725–975 CE) Hk1 (749–929 CE) Wood et al., 2011 Round Top rock avalanche (45 Mm3) 1020 BP (880–980 CE) Hk1 (749–929 CE) Wright, 1998 Cascade rock avalanche (750 Mm3) 1290 BP (556–778 CE) Hk2 (642–727 CE) Barth, 2013 Cleddau rock fall ~4600 BP (~2650 BCE) Hk12 (2861–2661 BCE) Dykstra, pers. comm. 2012 Polnoon Burn Landslide (~140 Mm3) 5223–5379 BP (3273–3429 BCE) Hk 13 or Hk 14 ? Hancox and Salt 19905 John O’Groats rock avalanche (1000 Mm3) 5660–5910 BP (3710–3960 BCE) Hk15 (3951–3761 BCE) This study Lake Adelaide rock slide (750 Mm3) 6000–6290 BP (4050–4340 BCE) Hk16 (4237–4055 BCE) This study Lake Gunn block slide/avalanche (300 Mm3) 7580–7680 BP (5630–5730 BCE) Hk21 (5752–5422 BCE) This study Bowen Falls rock fall ~8900 BP (~6950 BCE) Pre 8000 years BP Dykstra, pers. comm. 2012 Green Lake Landslide (27 km3) ~12,000–13,000 BP Pre 8000 years BP Hancox and Perrin 2009

Notes 1. Landslide locations are shown in Figure 1; (2) Ages (95% or 2 σ ranges) are calibrated radiocarbon years BP and equivalent CE/BCE ages for comparison with Alpine Fault earthquakes. (3) The Alpine Fault earthquake records listed here are from Berryman et al., 2012, for Haast (Ha) and Hokuri Creek (Hk) events. (4) References are for reported landslide ages. (5) From dating of peat and twigs in lake sediments from time of lake formation (NZ4930 - calibrated radiocarbon age of 5301 ±78 yrs BP).

As shown in Table 4 the three landslides dated in this study correlate well with three Alpine Fault events. The John O’Groats Landslide (5660–5910 BP/ 3710–3960 BCE) is correlated with the Hokuri Creek event Hk15 (3951–3761 BCE); the Lake Adelaide Landslide (6000– 6290 BP/4050–4340 BCE), with the Hk16 event (4237–4055 BCE); and the Lake Gunn Landslide (7580–7680 BP/5630–5730 BCE) with the Hk21 event (5752–5422 BCE) at Hokuri Creek.

The 14C age obtained for the Lake Gunn Landslide, from wood fragments within the slide debris, is likely to be the true age of the landslide event. However, the 14C dates obtained for the Lake Adelaide and John O’Groats landslides may be minimum ages, so correlation of these landslides with a specific Alpine Fault event is less certain. Because of the error in the age ranges for the landslides and the Alpine Fault event dates, the frequency of Alpine Fault events (~330 years), and the possibility that two of the ages are minimum values, the correlations between the landslide ages and specific Alpine Fault events could vary by plus one or two faulting events.

5.3 CORRELATION BETWEEN LANDSLIDES AND ALPINE FAULT EARTHQUAKES

Based on the data available at present, the landslide ages used for these correlations are the best available minimum ages for the initial failure of the landslides. We therefore conclude that the Lake Adelaide, John O’Groats, and Lake Gunn landslides could have been triggered by Alpine Fault earthquakes between about 5500 and 7500 years cal. years BP. However, given the large temporal errors in the ages of the landslides and the Alpine Fault earthquakes (Table 4, Figure 32) it is not possible to unequivocally attribute any of the giant landslides studied to an Alpine Fault earthquake. Consequently, we are not able to exclude the possibility that the landslides were triggered by large earthquakes on the Puysegur subduction zone, or perhaps another active (Quaternary) fault in the region (Figure 1).

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While the Alpine Fault and the Puysegur subduction zone are the largest seismogenic sources in northern Fiordland, they are not the only ones capable of producing ≥MM9 shaking at the landslide sites. Nevertheless, the Alpine Fault and the Puysegur subduction zone are the closest and most frequently active seismogenic structures in the region, and they are therefore believed to be the most likely sources of the MM9 or greater shaking required to trigger the Lake Adelaide, John O’Groats, and Lake Gunn landslides.

5.4 FUTURE LANDSLIDE DATING

This initial study has made significant progress in the dating of giant landslides in Fiordland, establishing the probable age of the initial failures of the John O’Groats, Lake Adelaide, and Lake Gunn landslides, and linking them to the Alpine Fault earthquakes that are most likely to have triggered landslides of that size. The study contributes important data to the list of landslides in Fiordland and south Westland that are attributed to Alpine Fault earthquakes (Table 4). However, it is clear from that list, and the many known but as yet undated very large and giant landslides in that region (Figure 1) that the project has only just begun. Much more field work and landslide dating is required before a clear pattern of postglacial large landslides that have occurred during Alpine Fault earthquakes, or earthquakes on the Puysegur subduction zone can be established with more certainty.

Perhaps the most apparent exclusion from the results of landslide studies to date is the failure to recognise and date very large and giant landslides that can be attributed to the last two Alpine Fault earthquakes. In part this may be due to the need for more detailed landslide mapping along and in close proximity to the Alpine Fault in south Westland and Fiordland. This is now possible with considerable ease and accuracy using Google Earth, Topo50 topographic maps, and where available Lidar imagery. Some suggested targets for future landslide dating in Fiordland and south Westland include the Lake Mc Ivor, Lochnagar, Dart River, Lake Purser, Cascade River (Hope-Blue River Range, lower and upper Cascade valley), and possibly some other known very large and giant landslides in the region.

It may also be useful to attempt to 14C date landslides using the lowermost organic samples from the bottoms of landslides ponds and lakes. If suitable datable samples can be obtained this technique might be a better method of dating the emplacement age of the landslide debris. One possible site where this dating method could be trialled is at the upper landslide pond on the John O’Groats Landslide, which would allow the dates to be compared with those obtained in this study. Further deep sampling and dating of carbonaceous material deposited directly on top of the Lake Adelaide slide debris would also provide greater certainty of the true age of that landslide. This could perhaps be done during the 2013-2014 summer field season in conjunction with the Alpine Fault studies that are planned to be carried out in the lower John O’Groats Valley.

Future landslide studies would also benefit from careful consideration and selection of organic material for radiocarbon dating. As shown in this study, bulk organic samples can provide a wide range of ages through the incorporation of both old and young carbon from higher in the stratigraphic sequence. Careful targeting macrofossils or microfossils for dating, such as leaves or pollen isolated from sediment (Howarth et al., 2013; Vandergoes and Prior, 2003), may improve the confidence in the chronology that is achieved in future landslide dating.

We also believe that it is important for future landslide dating studies to provide bounding ages where possible, not just minimum ages, that have error margins that are narrow enough to allow direct correlation with the Alpine Fault earthquake chronology. In the current study the Lake Gunn Landslide provides a good example of how this can be done effectively.

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6.0 CONCLUSIONS

This study has involved the geological and geomorphic mapping, description, and radiocarbon dating of the Lake Adelaide rock-slide avalanche (~750 Mm3), the John O’Groats rock avalanche (~1000 Mm3), and the Lake Gunn rock block-slide avalanche (~300 Mm3) in northern Fiordland. These three landslides are part of a cluster of at least 58 very large (≥1 Mm3) and giant (≥100 m3) postglacial age landslides which have occurred within ~50 km of the southern section of the Alpine Fault and the underlying Puysegur (Fiordland) subduction zone. Both of these active tectonic structures are potential sources of large (Mw

>7) and great (Mw >8) earthquakes which, based on historical evidence of coseismic landsliding in New Zealand, are considered most likely to have triggered the numerous very large bedrock collapses in Fiordland and south Westland.

The calibrated 14C ages of organic material within or deposited on top of the landside deposits are thought to provide minimum ages (±2σ) for the three landslides dated in our study, namely: John O’Groats Landslide: 5785 ±125 years BP; Lake Adelaide Landslide: 6144 ±143 years BP; and Lake Gunn Landslide 7630 ±52 years BP. These ages are generally consistent with the age of the beech forest on the landslide deposits, multiple displacements of the landslide deposits in John O’Groats valley by the Alpine Fault over the last ~6000 years, and only moderate to minor erosional modification of the landslide head scarps, source areas, and slide debris deposits.

The ages of these landslides have been tentatively correlated against the paleoseismic record of earthquakes on the Alpine Fault over the last 8000 years at Hokuri Creek. Based on that record, the John O’Groats Landslide is correlated with the Hokuri Creek event Hk15 (3951–3761 years BCE, or 5902–5711 years BP); the Lake Adelaide Landslide with Hk16 (4237–4055 years BCE, or 6187–6005 years BP); and the Lake Gunn Landslide with Hk21 (5752–5422 years BCE, or 7702–7372 years BP). However, because the dates for the Lake Adelaide and John O’Groats landslides may be minimum ages, they could have occurred during earlier large earthquakes.

Because there is no similar record of past earthquakes on the Puysegur subduction zone, that potential earthquake source cannot be confirmed or ruled out, although its existence is believed to increase the likelihood that the three landslides studied, and others like them in the region were earthquake-triggered. Given the 14C dating and historical precedent evidence available at present, we therefore conclude that the Lake Adelaide, John O’Groats, and Lake Gunn landslides were most probably triggered by large or great earthquakes on the Alpine Fault or the subduction zone between 5500 and 7500 cal. years BP. Although other faults in the region are capable of producing large earthquakes, the Alpine Fault and the Puysegur subduction zone are believed to be the most likely sources of ≥MM9 shaking at the landslide sites.

This initial study has made significant progress in the dating of giant landslides in Fiordland by establishing the probable age of the John O’Groats, Lake Adelaide, and Lake Gunn landslides, and linking them to the Alpine Fault earthquakes that are likely to have triggered landslides of that size. However, more field work and landslide dating, especially of samples that can provide maximum landslide ages, is required before a clear understanding of postglacial large landslides triggered by Alpine Fault earthquakes, or earthquakes on the Puysegur subduction zone can be established with more certainty.

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Some suggested targets for future landslide dating include the Lake McIvor, Dart River, Lochnagar, Lake Purser, Cascade River, the Hope-Blue River Range landslides, and possibly other giant landslides in the region, especially those in close proximity to the Alpine Fault. We also believe that further studies are required to distinguish, date, and determine the effects of landslides that can be attributed to the last two Alpine Fault earthquakes in south Westland and Fiordland. Information from such studies would provide much needed insight into the likely effects and risk from the numerous landslides that are expected to be triggered in these regions by the next Alpine Fault earthquake.

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7.0 ACKNOWLEDGEMENTS

This research was made possible partly by 2011-2012 SDF-L16 funding (field work), and partly through the GNS Science landslide research projects of the Natural Hazard Research Platform (NHRP). The authors wish to acknowledge the encouragement from Pilar Villamor in undertaking this research, and also the Department of Conservation (DOC) for providing permits for us to work in the selected field areas. We also thank GNS colleagues Mike Page and Jamie Howarth for their helpful reviews of the draft report.

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APPENDICES

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APPENDIX 1: MODIFIED MERCALLI INTENSITY SCALE

LANDSLIDE CRITERIA IN THE MODIFIED MERCALLI (MM) INTENSITY SCALE

1. Landslide and environmental Effects (MM5 to (MM10) – New Zealand 2008

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2. Relationship of MM Intensity to Peak Ground Acceleration (PGA) and earthquake-induced landslide opportunity (after Hancox et al. 2002).

The graph above shows the relationship of MM Intensity to peak ground acceleration (PGA) range based on the mean and mean plus one standard deviation correlations of Murphy and O'Brien (1977) landslide opportunity on New Zealand (from Hancox et al. 2002). The overlap in the PGA values for different MM intensities reflects the scatter in PGA/MM data. The blue squares indicate PGA /MMIs and return periods modelled for the Plateau Hut area (Table 2). The EIL Opportunity classes define the relative likelihood of earthquake-induced landslides occurring in areas of different shaking (PGA/MM Intensity) based on ground damage effects established for New Zealand. Five classes of relative EIL opportunity are recognised, as follows: 1. Very Low (≤ MM5-6): Very small rock and soil falls on the most susceptible slopes. 2. Low (MM6-7): Small landslides, soil and rock falls may occur on more susceptible slopes (particularly road cuts and other excavations), along with minor liquefaction effects (sand boils) in susceptible soils. 3. Moderate (MM7-8): Significant small to moderate landslides are likely, and liquefaction effects (sand boils) expected in susceptible areas. Noticeable damage to roads. 4. High (MM8-9): Widespread small-scale landsliding expected, with a few moderate to very large slides, and some small landslide-dammed lakes; many sand boils and localised lateral spreads likely. Severe damage to roads, with many failures of steep high cuts and road-edge fills. 5. Very high (≥MM9): Widespread landslide damage expected. Many large to extremely large landslides; sand boils are widespread on alluvium, and lateral spreading common along river banks; landslide-dammed lakes are often formed in susceptible terrain. Extensive very severe damage to roads - failures of steep high cuts and road-edge fills. References Dowrick, D.J., Hancox, G.T., Perrin, N.D., Dellow, G.D. 2008. The Modified Mercalli Intensity Scale – Revisions arising from New Zealand experience. Bulletin of the New Zealand Society for Earthquake Engineering, 41(3) 193–205. Hancox, G.T., Perrin, N.D., Dellow, G.D., 1997. Earthquake-induced landslides in New Zealand and implications for MM intensity and seismic hazard assessment. GNS Client Report 43601B. Hancox, G.T., Perrin, N.D., Dellow, G.D., 2002. Recent studies of historical earthquake-induced landsliding, ground damage, and MM intensity in New Zealand. Bulletin of the New Zealand Society for Earthquake Engineering, 35(2) 59–95.

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