Landslides (2009) 6:253–262DOI 10.1007/s10346-009-0156-5Received: 19 November 2008Accepted: 8 May 2009Published online: 30 May 2009© Springer-Verlag 2009

Guillaume Chevalier . Tim Davies . Mauri McSaveney

The prehistoric Mt Wilberg rock avalanche, Westland,New Zealand

Abstract The Mt Wilberg rock avalanche inWestland, New Zealandoccurred before 1300 AD andmay have occurred as a consequence ofan Alpine fault earthquake in ca. 1220 AD or earlier. Its ∼40×106 m3

deposit may have briefly obstructed the Wanganui River, but onlyabout 25% of its surface morphology still survives, on terracesisolated from river erosion. The landslide appears to have movedinitially as a block, in a direction controlled by a strong rock mass atthe base of the source area, before disintegrating and spreadingacross terraces, fans, and floodplains. Rock avalanche deposits inWestland have relatively short expected lifetimes in the ruggedterrain and high rainfall of the area; hence, the hazard from suchevents is under-represented by their current remnants.

Keywords Rock avalanche . Earthquake trigger .

Emplacement mechanics . Dating . Coseismic landslideprobability . New Zealand . Westland

IntroductionMajor earthquakes in mountainous landscapes are expected tocause large coseismic rock avalanches (e.g., Keefer 1984, 1994)because the intense ground accelerations can initiate deep-seatedfailure surfaces, or exceed the shear resistance on existing ones.Hazards to human facilities and infrastructure from large rockavalanches are potentially severe, due to the large mass, highvelocity and long runout of such events; they can also block riverscausing consequential dambreak and sedimentation hazards.

The western range front of the Southern Alps, New Zealand(Fig. 1) is known to have been affected by several great (M∼8)earthquakes on the interplate Alpine fault in the last millennium(approximately 1717 AD, 1620 AD, 1420 AD, and 1200 AD: Yetton1998; Sutherland et al. 2006); however, previously, only onesubstantial landslide along the range front had been studied—the40×106 m3 Round Top deposit (Wright 1998). We here report onanother deposit of similar size at the range front, the Mt Wilbergrock avalanche beside the Wanganui River (Figs. 1, 2, and 3),mentioned but not studied in Wright (1998) and Korup (2004).Like the Round Top deposit, this appears to pre-date the last twoAlpine fault earthquakes, in ∼1720 AD and ∼1620 AD, leaving theconundrum that no large landslide deposit has yet been founddating from the last Alpine Fault earthquake even though it hasbeen estimated to have been a Great Earthquake (Yetton 1998). Thedegrees to which the Mt Wilberg deposit has been eroded byWanganui River and the 1999 Mt Adams deposit has been erodedby Poerua River help explain the apparent paucity of rockavalanche deposits in Westland.

Regional settingThe geological and geomorphological setting of the westernSouthern Alps is well described by, for example, Korup (2004,

2005) and Cox and Barrell (2007). Briefly, 60–80% of thetranspressional motion between the Pacific and Australian Plateslocalizes on the Alpine fault, with strike-slip motion of 23±2 mm/year and dip slip of 12±4 mm/year normal to the strike direction(Norris and Cooper 2001, De Mets et al. 1994; Yetton 1998;Sutherland et al. 2006). The plate boundary is locked above 11 kmdepth, except for fault ruptures that cause major to greatearthquakes, for which there is evidence of about eight in the lasttwo millennia (P. Almond, Lincoln University, New Zealand, perscomm 2007), each causing up to 8 m of horizontal and 3 m ofvertical offset on the fault trace. High grade (k-feldsparamphibolite) schists are exhumed near the fault zone, with anarrow zone of mylonite and curly schist immediately southeast ofthe fault. The mylonite locally contains pseudotachylite. Thedegree of P–T metamorphism reduces to the south-east, towardsthe main divide of the Southern Alps where weakly metamor-phosed greywacke sandstones (zeolite-facies metamorphism) areexposed (Fig. 4; Cox and Barrell 2007). West of the fault arebasement rocks of the Australasian Plate, comprising GreenlandGroup sandstone, granitic hills and ranges, and other lithologies ofCretaceous and Tertiary ages, overlain in many places by massivePleistocene moraines. At intervals of 10–20 km, large rivers exitNW-trending mountain valleys to flow on extensive braidplains,10–20 km across the foreland between deeply eroded moraineridges to the sea. There is soil sequence evidence that episodic,widespread aggradation of the braid and flood plains has occurred,at least in the last 1,400 years (Berryman et al. 2001; Davies andKorup 2007), possibly in response to major earthquakes. Rainfallvaries between ∼5 ma−1 at the range front to ca. 10–15 ma−1 about10 km west of the main divide (Henderson and Thompson 1999);snow rarely falls on the foreland but can fall at any time of yearabove ∼1,500 m, and significant glaciers remain above ∼2,500 m.

There is some evidence that the mountain hypsometry is indynamic equilibrium with tectonic uplift; this is not so clear west ofthe fault (Davies and Korup 2007), but since sea level stabilizedabout 6,000 years ago, sufficient fluvially transported sediment hascrossed the fault and traversed the foreland that foreland storagehas reached long-term dynamic equilibrium.

Local settingMt Wilberg forms a NE-trending ridge on the west side of theWanganui River at the range front of the Southern Alps, Westland(Figs. 1 and 5). To the WNW, this ridge is bounded by the valley ofHarald Creek, which has formed along a trace of the Alpine fault.To the south-east, the ridge is flanked by the Wanganui River. TheMt Wilberg rock avalanche fell from the north side of the ridgefrom elevations between 150 and 500 m asl.

Harald Creek flows partly along the trace of the Alpine fault andhas formed a large alluvial/debris flow fan which abuts against the

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Fig. 1 Location and topographic mapsof the Mt Wilberg rock avalanche; gridsquares = 1 km

Fig. 2 DEM view of the Mt Wilbergrock avalanche from the north-west (novertical exaggeration)

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Fig. 3 Mt. Wilberg rock-avalanchesource area and deposit viewed fromthe north

Fig. 4 Geology and section (lower) ofthe western Southern Alps in thevicinity of the Wanganui River (FromCox & Barrell, 2007; Grid = 5 kmsquare)

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west side of the rock avalanche deposit. The elongate rockavalanche source lies immediately south-east of the inferredlocation of the Alpine fault and has its long axis approximatelyparallel to the likely fault trend. The deposit is partly located on topof a terrace similar to those on the opposite side of the valley andfurther upstream, and partly on recent outwash and the flood plainof the Wanganui River. The river obviously has eroded part of thedeposit, both in and adjacent to the present river channel and tothe north-west, where it has also trimmed the fan deposit of HaraldCreek. Alpine fault lithologies (curly schist, mylonites, ultracata-clasites, and fault gouge) are exposed in Harald Creek, and the faultevidently underlies the deposit of the rock avalanche. Present inthe bed of Harald Creek are occasional massive boulders of theultracataclasite that are sparsely jointed and very resistant toabrasion; many contain veins of pseudotachylite.

Source areaThe MtWilberg rock avalanche source is a bowl-shaped depressionlow on the north-east-trending ridge of Mt Wilberg (Figs. 1 and 6).The source is about 450 m high with a maximum width of 500 m,and an apparent depleted volume of about 33×106 m3. Very littlerock is exposed in the source area, and the dense vegetation makesaccess extremely difficult. Near the top of the source area are signsof ground slumping.

The orientation of the long axis of the source area isapproximately parallel to the Alpine fault and the Mt Wilbergridge and is at a high angle (about 45°) to the expected fall line of thelandslide (Figs. 6 and 7: note that the river channel migratedsignificantly between 1948 and 1984). Evidently, initial motion of therock mass was towards the north-east, apparently constrained by afeature that is now exposed by the landslide as a rock ridge. Not untilthe mass had translated some distance did it begin to spreadlaterally. One explanation appears to be that the failure surfacedaylighted in a manner controlled by the presence of resistant rock,now revealed as the ridge at the base of the source area. We believethat the initial motion was that of an essentially intact block guidedby the resistant rock forming a lateral boundary low on the slope.

When the block had traveled some distance, it began to disintegrateand so could spread laterally; lack of obvious debris in the expectedfall-line due north of the source indicates that collapse andspreading were not dominant before the source area was evacuated.

The bowl-shaped source area indicates a relatively deep-seatedfailure surface, perhaps locally as much as some hundreds of metersbelow the original ground surface; such deep-seated failures aretypical of many coseismic landslides and allow the failure volume tobe much greater than would occur from a typical shallow aseismiclandslide of the same surface area (e.g., Hovius et al. 1997).

An unusual feature of the landslide is the relatively lowelevation of the edifice from which the landslide fell; the top of thesource area is at about 500 m asl, which is unusually low for such alarge-volume landslide. Presumably, this is because the ridge-top atthis elevation was close to the Alpine fault. Large coseismiclandslides more commonly fall from much higher elevations anddeeper in the mountains because there are many more potentialsources at high elevation within the ranges than at the range front.

Deposit morphology and volumeMuch of the surviving surface of the landslide deposit has sufferederosion, has been modified for agriculture and a building platform,or is obscured by dense forest. Nevertheless, the overall form of thesurviving surfaces is evident from stereophotography (Fig. 7); fromthe narrow base of the source area the debris spread within anincluded angle of about 90° over the Harald Creek fan and alluvialflats to the north; it was initially confined to the east by the spur of

Fig. 5 Detail of the geology of the Mt Wilberg rock avalanche site (From Cox &Barrell, 2007: Grid = 5 km square)

Fig. 6 Vertical aerial photograph (SN C8340 of 12 February 1984) showing sourcearea and deposit (full lines) and other features noted in text. Note that the river,bridge and highway locations are as of 1984 and do not necessarily correspond tothose of Figures 4 and 5 which are as mapped in 1991

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Mt Wilberg and spread more to the east once past this spur. Itsnorthern part is emplaced over a terrace, and exposures on theriver-cut face show the rock avalanche debris there to be about10-m deep (Fig. 8). Towards the north, the deposit steps down ca.5 m (Fig. 9); this could be a relict of the spreading of the originalblock, but alternatively may indicate an edge of a buried terrace, orthe presence of distributed debris deformation due to post-depositional Alpine fault displacement.

Many boulders have been taken from the deposit in forming abuilding platform and to clear fields for pasturage; these arestockpiled on one area of the deposit (Fig. 10). The boulders aretypically several meters across, angular but with slightly roundededges. They are mostly of curly schist, with other lithologies alsofound close to the Alpine fault. We interpret these to be from acarapace of the deposit, which is usually much less comminutedthan the interior (Dunning 2004).

Fig. 7 Vertical aerial stereo pair of theMt Wilberg rock avalanche site(SN1531/20–21 of 2 April 1948).Arrows indicate step in deposit profilethat appears to correspond with theAlpine fault; if so, upthrow is somemetres, suggesting more than one faultuplift episode since the deposit wasemplaced. Note that the river, bridgeand highway locations are as in 1948and do not correspond to those ofFigures 4, 5 and 6

Fig. 8 Rock-avalanche debris over-lying fluvial and morainic deposits(lower) exposed on river-cut face. Totalheight of section ~ 20 m. See Fig. 6 forlocation

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We trenched the edge of the deposit where it had been trimmedlong ago by the Wanganui River, to a depth of∼3–4 m, constrainedby the water table and ingress of surface water during heavy rain.Rock avalanche debris was buried by 1–2 m of fluvial gravels(Fig. 11), confirming that the deposit’s surface extent waspreviously greater than at present.

We also investigated the contact of the rock avalanche depositwith the debris flow fan of Harald Creek, using ground-penetratingradar. This showed that the deposit underlies a part of the fan(Fig. 12a); thus, Harald Creek has aggraded against the rockavalanche deposit since its emplacement. The present steep edgeof the rock avalanche deposit appears to extend to some depthbelow the present fan surface and so the apparent landslide edgehere is the result of fan aggradation and deposit trimming byHarald Creek in the time since the rock avalanche occurred. Again,

the indication is that the extent of the rock avalanche is greaterthan is presently evident on the ground surface.

If the source volume were 33 million m3, this would bulk toabout 40 million m3 (assuming a deposit porosity of ∼20%;McSaveney 2002); if the deposit depth were say 20 m on average(assuming greater depth in the proximal region than in the moredistal exposures), then the areal extent of the deposit would be∼2 km2. The surface area of the presently exposed deposit is about0.5 km2, suggesting that about 75% of the deposit has been buriedor eroded since the landslide fell.

Exposed on the bed of the Wanganui River is an elongate clusterof large, dark boulders (Figs. 6 and 9; note that in the recent photoof Fig. 9 the river is again in a different location to those in Figs. 6and 7) of a distinctive lithology with only one known source in thecatchment, adjacent to the Alpine fault. The lithology isultracataclasite (or ultramylonite—highly indurated rock thathas been intensely comminuted by fault movement but reweldedby thermal and pressure metamorphism to become tough andhighly resistant to abrasion) with injected pseudotachylite. Thesame lithologic assemblage is found in boulders in Harald Creekclose to the Alpine fault and in the stockpiles on the depositsurface. The very large river boulders are just discernible on 1948aerial photographs, indicating that despite much lateral migration,the river bed level has been essentially stable for the last 60 years. AGPR survey (Fig. 12b) suggested that the boulder deposit extendsbeneath gravel of the modern flood plain, towards a road north ofthe true right bank of the river, indicating that the landslideinitially extended across the river, possibly to the foot of a highglacial outwash terrace along the valley wall. We conclude that theboulders were emplaced by the rock avalanche. The dashed outlineon Fig. 6 shows the estimated extent of the original landslide onthis basis. The deposit may have blocked the river briefly, but beingprobably fairly thin there, the blockage would soon have beenovertopped and scoured; since the stored water volume would havebeen small in relation to flood flows in the river, survival to thepresent day of evidence of this brief damming is highlyimprobable.

Fig. 9 Step (~5 m high) in debris-surface profile indicated by dashedlines, and cluster of ultramyloniteboulders in bed of Wanganui River.View is towards source of rockavalanche

Fig. 10 Stockpile of angular boulders removed from the landslide carapace.Boulder edges are slightly rounded by impacts, not all attributable to extractionand stockpiling

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Landslide ageSeveral indicators have been used to constrain the approximateemplacement time of the deposit.

. Dendrochronology: a number of the larger trees on the depositwere cored and annual growth rings counted (Table 1); tree agessuggest that the deposit was emplaced shortly before ca. 1380 AD(if an allowance of ∼100 years is added for establishment ofpodocarp seedlings, Wardle 1980). The stump of a very largerecently cut Matai tree (Prumnopitys taxifolia) was also sampledfor ring-counting; at least 700 rings were identified but thecenter of the tree was not preserved. This indicates that this treestarted growing substantially before 1300 AD and implies thatthe deposit was emplaced before the ca. 1420 AD Alpine faultearthquake; thus, if it were coseismic, it would have to date to anearthquake about 1200 AD, or earlier. The presence of apodocarp tree substantially older than 700 years suggests thatthe present forest ecotype, with abundant mature emergentpodocarps, is a climax forest and may no longer contain livingtrees recording the time of formation of the deposit.

. A distinct step in the deposit, close to the line of the Alpine fault,may reflect coseismic uplift of the eastern side of the depositsubsequent to its deposition (Fig. 9). If this is the case, then theamplitude of the step suggests that two or three uplift events of∼2–3 m are needed to explain it, and the deposit must predate both theca. 1717 AD and ca. 1620 AD Alpine fault events and perhaps alsothe ca.1420 AD one as well. The step is not a sharp break beneaththe forest cover; elsewhere along this fault, ruptures through thickunconsolidated materials appear as distributed deformation andnot as a sharp, discrete trace of a fault scarp. Since the faultmovement has deformed an initially irregular bouldery surface,the amount of surface displacement under the dense, liana-entangled rainforest cover is very difficult to measure with usefulprecision.

The available evidence indicates that the landslide predates theca. 1420 AD earthquake. The ring count on the Matai stump cannotbe reconciled with a younger date than 1300 AD, so we infer thatthe landslide dates from an earthquake at ca. 1220 AD or, less likelyfrom an earlier one.

Fig. 12 Ground-penetrating radar images of (a) section 1 west side of rockavalanche deposit underlapping Harald Creek fan; (b) section 2 on true right bankof Wanganui River showing alluvial gravels overlying boulders. See Figure 6 forlocations of GPR section lines

Fig. 11 Section exposed in trench atbase of river-cut face of deposit. Seefigure 6 for location

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DiscussionIn summary, the Mt Wilberg rock avalanche:

1. is deep-seated, immediately adjacent to the Alpine fault andprobably coseismic in origin;

2. was probably caused by an Alpine fault earthquake ca. 1220 AD(or earlier);

3. fell diagonally to the direct downslope direction;4. was about 30×106 m3 in source volume;5. has been much eroded by the Wanganui River so that <25% of

the original landslide surface remains.

Coseismic and aseismic landslidesSince 1991, there have been a number of large (∼106 m3) landslidesin the Southern Alps that have not been triggered by earthquakes:Mt Cook, 1991 (McSaveney 2002); Mt Fletcher (2), 1993 (McSaveney2002); Mt Adams, 1999 (Hancox et al. 2005); Mt Vampire, 2007(Cox et al. 2008); Young River, 2007 and Joe River, 2007. None ofthese appeared to have any obvious trigger; all fell in the absence ofboth tectonic seismicity and rainfall. All left the source area scars ofindistinct morphology (i.e., the source area morphology wasindistinguishable from other mountainsides in the vicinity otherthan in freshness of scar surface), the failure surfaces beingrelatively shallow (∼10–50 m deep) and of large surface area. Bycontrast, the failure surfaces of a number of known large coseismiclandslides were deep-seated (e.g., Shattered Peak; McSaveney 1978:Falling Mountain; Davies and McSaveney 2002: Hattian, Pakistan;Dunning et al. 2007: Ananeva; Delvaux et al. 2001). However, atleast one large aseismic landslide has been deep-seated (e.g.,Sattichhu, Bhutan; Dunning et al. 2006), so this is not a uniquecharacteristic only of seismic initiation; in addition, landslidesonto the Black Rapids glacier triggered by the Denali earthquake of2002 had source area depths of ∼30–50 m (Jibson et al. 2006).However, it seems reasonable to associate large, deep-seatedlandslide source areas that include ridge tops close to known activefaults with a coseismic trigger, and this has commonly been done(e.g., Acheron landslide, NZ; Smith et al. 2006: Round Top, NZ;

Wright 1998), and deposits associated with deep-seated sourceareas are dated for paleoseismic purposes (Mitchell et al. 2007).

In mechanical terms, it is reasonable to expect that strongground shaking can initiate deep-seated failure surfaces, as theupper part of a mountain edifice experiences greatly enhancedground acceleration (e.g., Geli et al. 1988; Ashford et al. 1997;Havenith et al., 2003). Buech (2008) recorded ridge top accelera-tions an order of magnitude greater than the equivalent flat-ground accelerations in natural low-magnitude earthquakes.Seismic shaking is known to cause extraordinarily intense andrapidly time-variant stresses deep inside the edifice, comparedwith the slow alterations of (mostly shallow) stresses caused byweathering, rainfall, or stress corrosion.

EmplacementAs indicated above, the flow direction (or fall line) of the MtWilberg rock avalanche was unexpected given the topography ofthe slope from which it fell; instead of traveling due north, it fell tothe north-east.

The presence of a distinct ridge across the base of the sourcearea (prominent in Fig. 3) is unusual in rock avalanches. Weassume that this originates in a resistant rock stratum parallel to(and probably associated with) the Alpine fault, above which thefailure surface daylighted; its presence as a topographic featureafter the landslide suggests that it guided the initial north-eastwardemplacement direction of the avalanche. Because there would havebeen substantial gravitational forces on the released rock in anorthward direction, immediately, the failure surface was com-plete, and these were resisted, a substantial degree of strength inthe entire failing block is implied during its initial motion towardsthe north-east, and the initial motion of the rock avalanche maytherefore have been as a coherent block. Davies et al. (2006) havedemonstrated that rock fragmentation at the base of large intactblock-slides can cause frictional resistance to motion to be verylow, so it is possible that the Mt Wilberg rock avalanche initiallyaccelerated very rapidly towards the north-east, before breaking upand flowing as a rock avalanche. While it appears likely that a rock

Table 1 Dendrochronology data

Tag Straw Specie DBH(cm)

Notes Ringcount

Chronological centercalculation

Estimated deposition datea

(cal year)674 40 Rimub 148 On scarp edge 436 N 1470 AD666 85 Rimu 85 ?? N ??665 98 Miroc 90 Elevated establishment microsite 476 Y; +40.7 years 1389 AD671 5 and

192Miro 135 Huge tree, branching wide and high. Took two cores 457 and

526N 1380 AD

667 107 Rimu 105 Near creek 357 Y; +101.32 years 1448 AD661 64 Rimu 109 Just off the landslide deposit, on the moraine edge.

Broke corer716 N 1190 AD

670 162 Rimu 106 Just off the landslide deposit, on the moraine edge 576 N 1330 AD660 103 Rimu ca. 150 DBH estimated 336 N 1570 AD663 95 Rimu 132 On fan apex ?? N ??662 55 Rimu 118 Rotten core at end ?? N ??669 54 Miroc ca. 200 DBH estimated 357 Y; +98.7 years[?] 1451 AD668 804 Rimu ca. 190 Rotten center. Broke corer. DBH estimated 366 N 1540 AD

a Following Wardle (1980), we add ∼100 years for establishment of podocarp seedlings beneath broadleaf forestb Rimu (Dacrydium cupressinum): Rimu is a slow-growing tree. It typicallyappears as an emergent frommixed broadleaf-temperate rainforest. Its life span is approximately 800 to 900 years. D. cupressinum is generally outperformed by its competitors in both shadeand treefall gaps, but has a longer life span than most other speciesc Miro (Podocarpus ferrugineus): 500 years is approaching the end of its normal life span

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avalanche sourced from massive rock will travel initially as anintact block, this is the first field evidence for such motion; it alsocasts doubt on the many analyses (e.g., Davies 1982; Campbell 1989;Campbell et al. 1995; Straub 1997; Calvetti et al. 2000; Iverson andDenlinger 2001; McDougall and Hungr 2004; Sosio et al. 2008;Staron 2008) which assume that the whole motion takes place as anon-comminuting grain flow because this requires that allcomminution in a rock avalanche occurs at the very start of themotion.

Rock avalanche frequencyWhitehouse and Griffiths (1983) and McSaveney (2002) derived anempirical frequency of ~10−6 km−2 a−1 for rock avalanches greaterin volume than 106 m3 for the eastern Southern Alps. On the basisof probability, most of these rock avalanches were assumed to becoseismic. If this same frequency were applied to the ~5,000-km2

alpine area of south Westland (Hovius et al.1997), we would expect∼5×10−3 large landslides every year (one every 200 years or so). Asnoted earlier, no rock avalanche deposit has been noted in this areathat associates with either of the last two Alpine fault earthquakes,a time period of about 600 years. While the area is of high reliefand steepness, and heavily vegetated, million-cubic-meter depositsshould still be easily identified if they were present, so there is anapparent deficit of such events (Korup 2005).

Our identification of evidence that the Mt Wilberg rockavalanche has lost at least 75% of its surface area to river erosionsince it was emplaced indicates that the erosive environment of theregion is well able to destroy rock-avalanche evidence that isaccessible to a river. The known rock-avalanche deposit at RoundTop (Wright 1998) was emplaced onto a very wide floodplain, sonearby rivers in that case had alternative courses and the depositremains substantially uneroded; while the Mt Wilberg depositremains visible only where it did not constrain the course of theWanganui River. A rock avalanche deposit in a narrow river valleycan be rapidly reworked downstream by the river—for example,the 1999 Mt Adams landslide (Hancox et al. 2005) had an initialvolume of about 107 m3 but 70% of it had been removed throughriver erosion by 2005. Hence, the survival time for visible evidenceof a large rock avalanche in the western Southern Alps is short—ofthe order of decades to centuries. By contrast, in the drier and lessvegetated eastern ranges, where there are much wider valleys,deposits of 106 m3 are still identifiable by surface morphology after3,000 years (Whitehouse and Griffiths 1983), indicating a muchlonger average survival time.

Thus, the paucity of large rock avalanche deposits in thewestern Southern Alps cannot be interpreted as indicating that thehazard from such events is low. Recent identification of supra-glacially transported rock avalanche debris in some major glacialdeposits (Tovar et al. 2009; Larsen et al. 2005; Cox et al. 2008)implies that rock avalanches are responsible for much glacialmoraine material and are rather frequent. Smaller rock avalanches,of about 104–105 m3 in volume, are reported several times each yearin the western Southern Alps (www.geonet.org.nz/landslides).

Rock avalanche hazardIdentification of two large, possibly coseismic landslides along thewestern range front of the Southern Alps suggests that othersmight occur in association with earthquakes on the Alpine fault.The current annual probability of a major Alpine fault earthquake

is 1–2% (Rhoades and Van Dissen 2003); the interval since the lastrupture in ca. 1717 AD is the longest such interval in the lastmillennium (Yetton 1998), so it is reasonable to expect a very largemagnitude earthquake (MW 7.9–8.0) in the near future. It followsthat identification of potential rock avalanche sources likely toimpact present or future developments is a matter of somerelevance to emergency response planning for Westland.

ConclusionsThe Mt Wilberg rock avalanche was likely triggered by strongground acceleration above the hanging wall during a rupture of theAlpine fault which immediately underlies the source area. Ages oftrees on the deposit preclude failure after ca. 1300 AD, and so it isinferred to have fallen around ca. 1220 AD or in an earlier Alpinefault earthquake. Its initial volume was about 33×106 m3; it mayhave moved initially as an intact block towards the north-east, thendisintegrated and spread over terraces and floodplain, probablyreaching to the foot of a high terrace on the opposite valley sideand briefly blocking the Wanganui River.

The original surface morphology of the Mt Wilberg rockavalanche is preserved only where it has been protected from rivererosion. Where rock avalanche deposits in the western SouthernAlps can be reworked by rivers, they remain recognizable for onlydecades to a few centuries, so the probability of such hazardoccurrences is not accurately determinable from known deposits.

AcknowledgmentsWe thank John Sullivan, landowner, for the access and advice and forthe Matai sample and digger; Mason Trust, University of Canterburyfor funding for GGC; Andrew Wells of Hawea for dendrochronolo-gical expertise and assistance; and Sandy Hammond of LincolnUniversity for laboratory facilities and assistance. This research wassupported by the New Zealand Foundation for Research, Science andTechnology, through the Public Good Science Fund.

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G. ChevalierDepartamento de Ingeniería del Terreno, Universitat Politècnica de Catalunya,Cartográfica y Geofísica - E.T.S. Ingenieros de Caminos C. y P.Campus Nord,Módulo D2, 301c/ Jordi Girona, 1-308034,Barcelona, Spaine-mail: guiinnz@gmail.com

T. Davies ())Department of Geological Sciences, University of Canterbury,Private Bag 4800,Christchurch, New Zealande-mail: tim.davies@canterbury.ac.nz

M. McSaveneyGNS Science,P.O. Box 30368Lower Hutt, New Zealand

Original Paper

Landslides 6 • (2009)262