Nabukelevu volcano (Mt. Washington), Kadavu – a source of hitherto unknown volcanic hazard in Fiji

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Journal of Volcanology and Geothermal Research 131 (2004) 371^396 www.elsevier.com/locate/jvolgeores

Nabukelevu volcano (Mt. Washington), Kadavu ^ a source of hitherto unknown volcanic hazard in Fiji Shane J. Cronin a; , Marie A. Ferland b;1 , James P. Terry c a

Institute of Natural Resources, Massey University, Private Bag 11 222, Palmerston North, New Zealand b Marine Sciences Programme, University of the South Paci¢c, Suva, Fiji c Geography Department, University of the South Paci¢c, Suva, Fiji Received 9 May 2003; accepted 17 November 2003

Abstract Nabukelevu volcano (805 m) is a small (ca. 3.4 km3 ) hornblende/biotite^andesite dome^breccia complex. It is the youngest in a Plio^Pleistocene series of volcanoes related to a presently inactive subduction zone in southern Fiji. We present new evidence of up to four Holocene eruption episodes from this volcano, with onshore evidence of the latest activity post-1686 : 40 years BP, and offshore evidence of tephra falls between 2250 : 70 and 780 : 50 years BP. Scoriaceous pyroclastic flow deposits of one eruptive episode contain pottery fragments, presumably entrained from habitation areas during emplacement. Like many composite edifices in moist climates, Nabukelevu is prone to failure, the propensity in this case exacerbated by up to three edifice-cutting fault zones. The fault-induced weak and saturated zones have been the focus of repeated edifice failure through late Holocene debris avalanches of between 10^100 million m3 . Many of these avalanches entered the sea, and these or additional submarine failures of the lower island flanks have led to emplacement of at least one major late Holocene submarine mass-flow deposit with distinctive mineralogy in the Suva Basin to the north. Two of the debris avalanches dated at post-2350 : 140 and post1750 : 60 years BP apparently inundated local habitation areas, and the deposits incorporate pottery and human remains. A widespread local legend describing catastrophic events on Nabukelevu corresponds in content with geologic findings to provide additional evidence of a late Holocene eruptive and debris avalanche disaster on Kadavu during the latter part of the last ca. 2000 years of human occupation, possibly as recent as between AD 1630 and 1680. The present hazardscape of the Nabukelevu area includes common landslides induced by frequent earthquake swarms and cyclones. Larger edifice failures, possibly related to volcanism or fault movement, have the potential to create local tsunami, which under favourable conditions could reach areas near Fiji’s capital, Suva, 110 km to the north. @ 2003 Elsevier B.V. All rights reserved. Keywords: Kadavu; Fiji; volcanic hazards; Holocene; debris avalanche; block-and-ash £ow; submarine mass £ows

1

Present address: Central Washington University, Ellensburg, WA 98926-7439, USA. * Corresponding author. Present address: GEOMAR Forschungszentrum, Abt. Vulkanologie und Petrologie, Wischhofstrasse 1^3, D-24148 Kiel, Germany. Fax: +49-430-600-1400. E-mail address: [email protected] (S.J. Cronin).

0377-0273 / 03 / $ ^ see front matter @ 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0377-0273(03)00414-1

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1. Introduction The prominent Nabukelevu volcano (highest point designated on topographic maps as Mt. Washington, 805 m asl) dominates the island of Kadavu (Fig. 1) and is the youngest and westernmost of a series of high-Si andesite and dacite volcanoes that form the Kadavu Island Group of southern Fiji (Woodrow, 1980; Verbeeten, 1996). Volcanism along the Kadavu Group appears to have been related to northward subduction of the Australian Plate beneath the Fiji Platform along a trench-like feature known as the Hunter Fracture Zone (Brocher and Holmes, 1985) or the Conway^Kadavu Lineament (Pelletier et al., 1998). Kadavu was considered to range in age from Pliocene in the east to late Pleistocene in the west, with a single K^Ar age on lava from Nabukelevu at 0.48 : 0.92 Ma (Whelan et al., 1985). Like many volcanoes in the Southwest Paci¢c, Kadavu has been inactive during the period of European contact. Hence it has never been seriously considered as a source of volcanic hazard. Nunn (1999) postulated that volcanism on Nabukelevu continued into the late Holocene, based on its morphology and a common oral tradition recorded from Kadavu (e.g. Beauclerc, 1909). We report here on new ¢eld studies and radiocarbon dates that con¢rm that volcanism and debris avalanches have occurred since human occupation of Kadavu. The match between our geologic evidence and the culture history of the island demonstrates the power of oral traditions for interpreting recent geological events in the Southwest Paci¢c. Based on a reconstructed late Holocene geologic record and historically reported events we outline the geologic hazardscape of western Kadavu, and how the interaction of volcanism, cli-

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mate, and neotectonics a¡ect the stability of the Nabukelevu edi¢ce. Volcanism, landslides and debris avalanches from the edi¢ce not only a¡ect the immediate surrounds, but may also pose a hazard to locations as far as Suva, Fiji’s capital and the hub of commerce in the Southwest Paci¢c region, located 110 km to the north (Fig. 1).

2. Tectonic setting and geologic background of Kadavu The Kadavu Island Group (Fig. 1) is an emergent portion of the eastern Hunter Ridge (Verbeeten, 1996) that lies along and northward of the Conway^Kadavu Lineament (Pelletier et al., 1998). The island group consists of at least 12 overlapping high-Si andesite and dacite volcanoes (Woodrow, 1980), with products K^Ar dated between 3.4 and 0.48 Ma (Whelan et al., 1985). Since ca. 3 Ma, volcanism in other areas of Fiji has been dominated by intraplate alkali basalts (Gill and Whelan, 1989) resulting from opening of the Lau Basin between Fiji and Tonga and separation of Fiji from a subduction in£uence (Colley and Hindle, 1984; Whelan et al., 1985). Hence, in this context the Kadavu Island Group is considered tectonically, petrologically and geochemically unusual (e.g. Gill and Whelan, 1989). The calc^alkaline volcanism of Kadavu is attributed to northward subduction of the Australian Plate along the Hunter Fracture Zone (sometimes termed the Kadavu Trench; Colley and Hindle, 1984; Rodda and Kroenke, 1984). Past subduction is evidenced by a deep trough along the Hunter Fracture Zone (Launay, 1982; Brocher and Holmes, 1985), the truncation of magnetic anomalies against the trough (Malaho¡ et al., 1982), and the typical subduction geochemical signature of the Kadavu lavas (Verbeeten, 1996).

Fig. 1. (A) Map of the geography and tectonic setting of Fiji with interpreted rates and directions of plate movement (after Pelletier et al., 1998). (B) Topographic sketch map of the Nabukelevu province with village locations. (C) General bathymetry of the Kadavu^Suva Basin area of southern Fiji as adapted from the 1:250 000 bathymetric charts of the region held by the Fiji Mineral Resources Department. Note continuation of the Hunter Ridge SW of Kadavu and the edge of the Kadavu Trench to the SE of the island group. The numbered crosses represent sampling localities on the 1998 Koyo-Maru cruise. Sediments from sites 8^10 are described in the text, with core samples from sites 8 and 9 shown in Fig. 9.

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Present seismicity suggests that plate convergence at the Hunter Fracture Zone has ceased (Hamburger et al., 1990), and the pattern of activity now indicates extensional or strike-slip deformation of the upper plate of the former subduction zone. Johnson and Molnar (1972) document fault plane solutions of left-lateral strike-slip faulting on a NE-trending fault zone

in the Kadavu area, although some earthquakes have a thrust focal mechanism on a NW-striking fault plane (Sykes et al., 1969). In addition, Pelletier et al. (1998) consider that 20^30 mm/yr of convergence or left-lateral strike-slip still occurs between the Australian Plate and the Fiji Platform along the Conway^Kadavu Lineament/ Hunter Fracture Zone.

Fig. 2. Geological sketch map of Nabukelevu province, combining details of Woodrow (1980), with results from this study. Locations of samples in Table 1 are labelled.

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3. Geology of Nabukelevu volcano and environs 3.1. General The eastern boundary of Nabukelevu volcanic province is bounded by a prominent NE-striking normal fault with a westward downthrow of at least 100 m (Fig. 2). Similar faults are mapped in other portions of the province, including at least two that bisect the Nabukelevu edi¢ce. Many of the faults show evidence for combined normal and left-lateral strike slip, causing distinct notches in the southern coastline (Fig. 2) and o¡setting some ridges and stream channels. Hot springs are located along some of these faults at the shoreline. The main Nabukelevu edi¢ce (805 m asl) is not a classic-shaped stratovolcano, but a steep (slope angles of 55‡ in places), £at-topped complex of biotite^andesite and dacite lava domes, stubby lava £ows and coarse volcaniclastic breccias, at least partly underlain by hyaloclastite deposits. Its basal diameter is between 3 and 3.5 km, and it has a bulk volume above sea level of ca. 3.4 km3 . To the southwest of the main edi¢ce lie older and/or contemporaneous lavas, breccias and

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domes, while to the west and northeast, the edi¢ce is surrounded by £at-lying dacite lavas (Table 1). O¡ the western coast, Nagigia Island is a raised coral platform, apparently uplifted after 205 ka, but since 120 ka it has subsided by ca. 2 m (Nunn and Omura, 1999). Subsidence of Nagigia is consistent with a general downthrow on faults west of the centre of Nabukelevu edi¢ce. South of the main edi¢ce, and along strike of the central-edi¢ce fault, lies a lava dome (Cikobia Point) of similar biotite^andesite composition (Table 1). This is joined to the main island by a coral and volcanic pebbly sand tombolo. Daviqele Bay, to the south of the edi¢ce, is mostly 6 10 m in depth, with an irregular £oor, comprising 5^6-m-high hummocks of partly coralcovered, lava boulders surrounded by sands. A barrier reef protecting this bay lies farther to the southeast, o¡shore of another lava dome and tombolo. North of the edi¢ce sea depth increases rapidly into the Suva Basin, reaching 2100 m at 15 km o¡shore. A submarine topographic high, termed the Kadavu Rise, continues southwest of Kadavu and a major submarine edi¢ce (summit 6 200 m depth) lies 15 km south of Nabukelevu. The Kadavu rise continues SW as part of the Hunter Ridge/Hunter Fracture Zone system.

Fig. 3. Photograph of the SE £anks of Nabukelevu edi¢ce, view from Daviqele village. Forming the skyline to the right is a lava coulee on the upper eastern £anks (cf. Fig. 2), the upper part of the Daviqele debris avalanche scarp (with parallel sub-vertical surface ridges) occurs immediately left of the coulee.

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Table 1 Geochemistry of Nabukelevu and related eruptives Samplea Long E Lat S Sample type

AV54 178‡1.26P 19‡8.98P Lava PreNabukelevu lavas

Wt% SiO2 TiO2 Al2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 LOI Total mg kg31 As Ba Ce Cr Cu Ga La Nb Ni Pb Rb Sc Sr Th U V Y Zn Zr Rock type Modal % Plagioclase Clinopyroxene Spinel Hornblende Biotite Quartz

AV48 177‡58.37P 19‡8.98P Lava Satellite plug related to Nabukelevu

AV14 177‡58.20P 19‡6.84P Lava Early edi¢ce lavas

AV50 177‡59.06P 19‡7.88P Loose boulder Mid-edi¢ce lava or breccia

64.69 0.55 16.28 3.27 0.07 2.69 4.42 5.03 2.18 0.38 1.42 101.33

64.89 0.59 15.99 3.31 0.07 2.60 4.17 4.83 1.98 0.39 1.71 100.88

58.28 1.00 16.27 4.64 0.08 3.80 7.13 4.73 1.69 0.55 0.96 99.65

61.84 0.87 15.45 3.60 0.07 3.29 5.42 4.50 1.98 0.54 1.34 99.30

371 71.83* 109 44* 19* 33.33* 12.1 114 3.60* 23 14 1580 3.94* 1.05* 158 13 57* 174

417 74.27* 51 41* 18* 34.48* 11.3 55 5.62* 31 9 1377 5.55* 1.91* 108 14 51* 172

414

409

21

34

9.4 24

9.9 38

andesite 7 5 3 5 2

K10 177‡57.74P 19‡7.94P Scoria block Nabukelevuira pyroclastic £ow

63.95 0.55 16.05 3.69 0.07 2.66 4.28 4.28 2.03 0.33 1.99 99.88

K4 177‡57.69P 19‡7.64P Scoria lapilli Nabukelevuira pyroclastic £ow

64.54 0.51 16.14 3.40 0.07 2.27 3.79 4.33 1.84 0.24 2.87 100.00

34 8 909

32 11 914

78 13

94 13

147

180

18 390 89 29 18 17 46 10 36 10 32 10 897 5.5 2.4 90 15 43 146

4 387 74 32 23 16 34 12 39 13 28 10 794 3.6 2.8 72 21 41 153

andesite

dacite

dacite

dacite

dacite

10 1 1 3 4

25 2 5 3 5

15 3 2 10 3 noted

17 2 1 14 5 noted

16 2 1 16 4 noted

Major elements (%) and trace elements (mg kg31 ) measured by XRF (except * values measured by ICP^MS). Samples with AV pre¢xes are from Verbeeten (1996), K-pre¢x samples are from this study. a Location numbers can be found on Fig. 2.

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Table 2 Radiocarbon dates collected during this study Laboratory number

Measured age (years BP)

Sample detail

Calibrated age rangea

Location

Wk-7550b

6 200

n.d.

19‡6.83PS 177‡58.37PE

Wk-11 587c

243 : 39

Charcoal £akes within BAF deposition unit of the NW £ank BAF deposit fan (Fig. 2) exposed in fresh landslide scarp at ca. 2 m depth. Charcoal £akes within BAF deposition unit of the NW £ank BAF deposit fan (Fig. 2) exposed in fresh landslide scarp at ca. 15 m depth.

Beta-135 357c 780 : 50 Wk-11 588c

1686 : 40

Wk-7548b

1750 : 60

Beta-135 358c 2250 : 70 Wk-7547b

2350 : 140

Wk-7549b

2420 : 90

Beta-135 359c 4850 : 70 Wk-7551b

12 950 : 130

Fresh planktonic foraminifera tests from 7^9 cm depth in a submarine core at site 8 (Fig. 9) Peat soil within summit swamp core at 2.10^2.15 m depth (Fig. 5) below scoriaceous tephra layer. Charcoal £akes within main Qalira debris avalanche deposit

AD AD AD AD AD

1520^1560 1630^1680 1760^1810 1930^1950 1065^1290

(0.14) (0.51) (0.30) (0.05)

AD 240^440 AD 120^430

Fresh planktonic foraminifera tests from 17^19 cm depth in a submarine core at site 8 (Fig. 9) Charcoal £akes within Daviqele debris avalanche deposit

710^325 BC

Organic soil at base of summit swamp core (Fig. 5; 3.9^4 m depth) Fresh planktonic foraminifera tests from 12^14 cm depth of a submarine core at site 9 (Fig. 9) Wood fragments from 12^14 cm depth of a submarine core at site 9 (Fig. 9)

800^350 BC

850^50 BC

3805^3535 BC 13 900^12 850 BC

19‡6.78PS 177‡58.64PE

18‡47.90PS 177‡54.30PE 19‡7.22P52PS 177‡59PE 19‡8.36PS 177‡58.43PE 18‡47.90PS 177‡54.30PE 19‡8.34PS 177‡59.53PE 19‡7.22PS 177‡59.52PE 18‡54.20PS 177‡54.00PE 18‡54.20PS 177‡54.00PE

a Calibrated ages: Wk-dates are given as 2 c ranges calculated using OxCal v.3.5, Bronk-Ramsay (2000) and calibrated using the curve of Stuiver et al. (1998); Beta-date calibrations are given as 2 c ranges following the guidelines supplied by the Beta laboratory, the Wk-11587 date given as all possible ranges within 2 c with probabilities in parentheses. b Conventional L counting determinations, University of Waikato Radiocarbon Dating Laboratory, New Zealand c AMS determinations, Beta-codes from Beta Analytic Laboratory, USA; Wk-codes from University of Waikato Radiocarbon Laboratory and Institute of Geological and Nuclear Sciences, New Zealand.

3.2. Lavas Nabukelevu lavas are generally s 10-m-thick stubby £ows or domes with well developed columnar jointing, and prominent £ow-banding textures. The summit area and upper eastern £anks of the edi¢ce are formed by a prominent dome or coulee (Figs. 2 and 3). A minimum age estimate of 2420 : 90 years BP (Wk-7549; Table 2) is derived from dating the lowermost peaty sediment within a small swamp developed on this dome near its summit. Pale-grey biotite-bearing high-silica andesites and dacites are the most common lithologies (Woodrow, 1980; Verbeeten, 1996). The lavas are porphyritic, with 30^40% phenocrysts set

within a very ¢ne-grained granular groundmass of mostly plagioclase and glass, with some rocks containing rare magnetite. Strongly oscillatory zoned plagioclase is the most common phenocryst and can be up to 20 mm long; ¢ner plagioclase crystals (1^3 mm) tend to be elongated laths and are often aligned. Brown and reddish brown, elongate hornblende crystals ( 6 1 mm, rarely to 5 mm) are rare to common in di¡erent samples. An opaque alteration rim surrounds many hornblendes, with smaller crystals commonly being completely replaced. Biotite and pale green augite crystals range from rare to common abundances between samples, the former being up to 8 mm in diameter, whereas the latter reach only 3 mm in length.

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3.3. Volcaniclastic deposits 3.3.1. Block-and-ash £ow deposits Monolithologic breccias are poorly exposed in many parts of the edi¢ce. The best exposures are incisions through a smooth-surfaced fan on the northwest sector of the edi¢ce. This fan lies below a vegetated, steep-sided lava dome (Fig. 4). The dome is roughly circular in plan with a basal diameter of 260 m. It rests upon the steeply NWsloping £ank of Nabukelevu, has a maximum height of 200 m, but only 120 m at its mid-point. The estimated volume of the dome is 5U106 m3 . The dome-fan system sits within a large scarp, which at the level of the dome measures 750 m across and 400 m deep. The greyish-brown and reddish-brown poorly sorted breccias are generally massive, but in some exposures there is a faint horizontal fabric in the form of strings of block-sized clasts. The deposits are lapilli and block-rich (50^70% by volume) and contain ¢ne blocks 6 1.5 m in diameter. They are mostly matrix-supported, with a friable coarse-medium ash matrix. Clasts are uniformly angular to sub-angular, dense (2.6 g cm33 ), glassy

(some with conchoidal fracture shapes) and poorly vesicular biotite andesite. Other phenocrysts commonly present in clasts include hornblende, augite and plagioclase (up to 8 mm long). The matrix consists of ¢ne glassy rock fragments of the same lithology and abundant, commonly broken, free crystals. Individual beds of between 2 and 8 m thick are de¢ned by both sharp and gradational contacts, rarely separated by cm-scale pockets of ¢ne ash. Weak coarse-tail reverse grading occurs in some units. Very rare, ¢ne £akes of charcoal are dispersed within the deposits in two exposures. Natural Remnant Magnetic (NRM) orientations of blocks within stacked units at two locations were determined with an FG Electronics BR-2 portable magnetometer (10 blocks sampled from each stratigraphic unit). Hoblitt and Kellog (1979) describe this method as a quick and fairly reliable alternative to laboratory studies of remnant magnetism. All clasts measured showed uniformly aligned magnetism, which we interpret as thermal remnant magnetism and indicative of emplacement above Curie point temperatures. Although similar aligned measurements have been attrib-

Fig. 4. Photograph of the NW £anks of Nabukelevu edi¢ce. A small, forested lava dome (cf. Fig. 2) occurs within a broad collapse scarp (labelled). Below this dome toward the right in this view, occurs a fan of BAF debris mantling debris-avalanche deposits and lavas. The steep northern £ank is formed from an additional collapse scarp, the minor recent landslide scars occurred during a seismic swarm in 1998.

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uted to chemical remnant magnetism through post-depositional authigenic mineral formation (McClelland, 1996), at Nabukelevu the deposits are fresh, and their sedimentary features, location with respect to a dome, and charcoal content are compatible with a hot-emplacement mechanism. We interpret these monogenetic deposits as block-and-ash £ow (BAF) deposits. The dense clasts, the location of the fan with respect to the dome and the lack of interbedded fall deposits, suggests that the BAFs were probably generated by gravitational collapse and mass-wasting of a growing lava dome, with little explosive energy. The best modern analogy is that of BAF formation on Merapi volcano (e.g. Neumann van Padang, 1933; Abdurachman et al., 2000). The minimum volume of BAF deposits on land, considering that many of the pyroclastic £ows would have entered the ocean, is at least 50U106 m3 , or in dense-rock-equivalent terms, roughly ¢ve times that of the dome remaining. As the dome formed on the steeply sloping £anks of Nabukelevu it was probably very unstable, comparable to that during the 1989^1990 eruption of Redoubt Volcano (Miller, 1994) and the 1990^ 1995 Unzen eruption (Nakada et al., 1999; Ui et al., 1999). The NW £ank BAF deposits seem to be the youngest on the edi¢ce, with skeletal sandy soils developed directly into them and no evidence of prolonged weathering nor burial by colluvium or other eruptive products. Radiocarbon determinations from charcoal £akes within the BAF deposits at two locations (Table 2) resulted in ages of 6 200 years BP (Wk-7550, at a coastal site at 6 2 m depth), and 243 : 39 years BP (Wk-11587, at ca. 300 m on the NW volcano £anks and s 15 m depth in deposits). The Wk-7550 sample, although being within an unweathered BAF deposit, is relatively shallow and may have been contaminated by modern carbon. However, the Wk11587 sample seems reliable, being located within an unweathered BAF deposit unit (with magnetically aligned clasts) at a considerable depth below surface soils. Various calibrated calendar ages are possible for Wk-11587 (Table 2), although the most probable lies between AD 1630 and 1680 (P = 0.51).

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Similar BAF deposits surround other domes on and around the Nabukelevu edi¢ce. A second, smooth surface, BAF deposit fan is mapped on the upper southeastern £anks of the edi¢ce (Fig. 2), although no charcoal was recovered from these deposits. 3.3.2. Nabukelevuira scoria unit A scoriaceous deposit is mapped within two lobes, to the east and southwest of the main Nabukelevu edi¢ce (Fig. 2). The unit is exposed at Nabukelevuira village, in new road cuttings toward Daviqele village and is found within the base of several auger and core holes west of Nabukevuira. The eastern lobe deposits are only encountered at the base of the present soil (at ca. 1 m depth) within pits and cores. In addition, scoriaceous blocks of the same lithology are found in colluvial deposits below the large dome/coulee on the eastern edi¢ce £anks. At the northern margins of the southwest lobe, the 0.25^0.3-m-thick deposit has a mantling, fall character and comprises clast-supported, white ¢ne-coarse, angular, scoria lapilli (maximum diameter of the ¢ve largest clasts, Dmax (5) = 40 mm). Bulk densities of the scoria lapilli were measured by coating particles with a spray-on silicone sealant and suspending in water (cf. Houghton and Wilson, 1989). Particle densities ranged between 1.06 and 1.39 g cm33 (mean = 1.25 g cm33 , standard deviation = 0.12 g cm33 ; n = 20). This uniformly highly vesicular character contrasts with all other eruptive products mapped on Nabukelevu. Toward the centre of the southwest lobe, the deposit has a mass-£ow character, being a massive, poorly sorted matrix-supported breccia (Fig. 5 ; K4 site). In two localities, rare reddish brown, plainware pottery fragments (20^50 mm long) occur throughout the deposit. At the central axis, within a palaeo-topographic low, the southwest lobe deposit is s 5.5 m in thickness, comprising three main depositional units (Fig. 5 ; K10 site), a lower massive ¢negrained unit, a coarser central unit and an overlying sequence of variably bedded diamictons. All units have a mass-£ow character, and rare pinkstained plainware pottery sherds (20^40 mm in

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VOLGEO 3040 9-2-04 Fig. 5. Description and stratigraphy of Nabukelevuira scoriaceous pyroclastic £ow deposit at the locations of samples K4 and K10 (see Table 1 and Fig. 2), and in a composite section from two cores through a swamp near the volcano summit (19‡7.22PS, 177‡59.52PE).

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diameter and 8^12 mm in thickness) are found within the lowest unit. In all beds the scoria is highly crystalline, containing up to 30% phenocrysts by volume in the solids portion, which comprise in order of abundance: plagioclase laths (2^12 mm long), biotite £akes (2^7 mm diameter), hornblende and augite (both 0.5^5 mm in length). NRM was measured on 10 blocks from each of the lower two units and two beds within the upper sequence (upper sequence samples included 10 lava clasts and 10 scoria clasts from each bed). Blocks from the lower two units showed aligned magnetic orientation, while those from the upper sequence had random orientations. Hot emplacement of the lower two units at the axial location is indicated by the aligned magnetic orientation of clasts, coupled with possible baking (pink surfaces) of the rare pottery sherds and the presence of jig-saw fractured blocks. Sedimentary features imply a highly concentrated pyroclastic £ow. The upper units, with random clast magnetic orientations and high contents of exotic lithologies (including apparent rip-up clasts of clayloam), appear to be debris-£ow deposits resulting from post-eruption reworking of the scoriaceous pyroclastic £ow deposit and older units higher on the volcano slopes. The eastern lobe is not well exposed and only the upper 10^20 cm of deposit, below ca. 1 m of colluvial soil could be recovered from cores and pits. These deposits are of ¢ne^medium, angular, pale brown and white scoria lapilli of the same lithology and density as in the southwest lobe. The deposits contain few ¢nes, are clast-supported and well sorted. Based on these characteristics we interpret the eastern lobe to be fall deposits associated with the eruption of a pyroclastic £ow on the southwest £anks. The uniformly vesiculated nature of the Nabukelevuira deposit and the associated fall deposits suggest an open-vent eruption with pyroclastic £ows generated by partial or full column collapse (e.g. as originally described from St Vincent ; Anderson and Flett, 1903). Similar scoria-and-ash £ow deposits are described by Nairn and Self (1978) from vulcanian-style eruptions of Ngauruhoe (New Zealand) in 1975. The degree of vesic-

381

ulation in the scoria clasts at the two volcanoes is also similar, although the Ngauruhoe deposits contain up to 20% of dense lava clasts in contrast to the Nabukelevuira unit with 6 3%. While scoria-and-ash £ows may also result from explosive destruction of a lava dome (Miller, 1994; Robertson et al., 1998; Nakada et al., 1999), such deposits also often contain a wide range in clast vesicularities. The Nabukelevuira deposit distribution indicates a source vent on or near the summit plateau. A large dome £ow or coulee underlies the present summit, and the scoriaceous deposit overlies this dome, being recovered from cores into peaty sediment in the summit area (Fig. 5 ; Summit swamp). A shallow crater-like feature immediately west of the summit dome is considered the most likely vent area. The maximum age of this eruption is loosely constrained by the entrainment of pottery within a pyroclastic £ow unit. Due to the lack of diagnostic patterns, the sherds cannot be related to the motif-based successions described from other Fijian islands (Frost, 1979; Cronin and Neall, 2000). First human settlement in the Fiji area occurred between 950 and 750 BC (Anderson and Clark, 1999). The earliest settlers are known for their distinctive geometrically patterned ‘Lapita’ style pottery (e.g. Kirch, 1997; Spriggs, 1997). Lapita sites are found in a few areas of Fiji, mostly concentrated on the western islands (Frost, 1979; Best, 1984). There have been no systematic archaeological studies on Kadavu, nor has any Lapitoid or other patterned pottery been described from the island. Hence, the plainware pottery found within the deposit indicates a maximum eruption age of 950^750 BC. A more precise maximum age of 1686 : 40 BP (Wk11588; Table 2) is indicated from dating peaty sediment directly below a scoria tephra layer correlated to the event, within a swamp core on the summit dome/coulee (Fig. 5). This corresponds to a calibrated calendar age maximum of between AD 240 and 440. 3.3.3. Other tephra falls Apart from the scoriaceous tephra fall described above, no other mappable fall deposits

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were found around Nabukelevu. The absence of distinctive fall deposits could be due to the steep slopes, high rainfall and high weathering rates in the area. Recent localised ash falls are, however, indicated by the soil pattern, with Nabukelevu province having weakly leached sandy loams developed from weakly weathered ash (Twyford and Wright, 1965), in contrast to the strongly weathered red heavy clays over most of the remainder of Kadavu. Localised distribution of minor ashfall is consistent with the mapped youthful domes and BAFs of Nabukelevu, (cf. Watanabe et al., 1999). Distal sea£oor sediments collected in surface grab samples within the Suva Basin to the north comprise calcareous oozes, which contain foreign fragments of fresh volcanic glass, biotite and hornblende (see below). The glass and ma¢c crystal contents of the oozes decrease with distance from Nabukelevu, but are still noted up to 40 km from the volcano. 3.4. Debris avalanches The characteristics of the late Holocene debris avalanche events from Nabukelevu Edi¢ce are summarised in Table 3). 3.4.1. Daviqele event Woodrow (1980) mapped a fan of landslide debris, based on its geomorphology, on the southeastern £anks of Nabukelevu, upslope of Daviqele village (cf. Fig. 2). The deposit covers approximately 1.2 km2 on land, which can be extended to 3.5 km2 based on the extent of a hummocky terrain of large coral-encrusted volcanic boulders within Daviqele Bay. It comprises a single massive debris avalanche unit, at least 10 m thick, as exposed in a quarry west of Daviqele village. A 650m-wide scarp on the edi¢ce occurs directly upslope of the deposit (Fig. 3), cutting through the westernmost part of the summit area. Deposits exposed in the scarp are 10^20-m packages of grey massive BAF units, interbedded in places with lava. Within the Daviqele quarry, £akes of charcoal and pottery sherds are mixed throughout the lower 5 m of the deposit exposure. Over 30 reddish

and pale-brown, plainware pottery sherds were extracted from a 10-m2 face, ranging between 20 and 80 mm in diameter and 3 and 10 mm in thickness. Rare bone fragments (either human or large animal) also occurred within the deposit, the largest being skull fragments up to 40 mm diameter and a 20U50-mm tubular bone fragment. These artefacts indicate that the Daviqele debris avalanche incorporated former habitation sites and inhabitants (or at least burial sites) within its path. Charcoal £akes yielded a radiocarbon age of 2350 : 140 years BP (Wk-7547; Table 2). This charcoal could either relate to the human artefacts (many of the pottery sherds are charred), or derive from collapsed BAF deposits. 3.4.2. Northwest £ank collapse An U-shaped collapse feature on the northwestern £ank of Nabukelevu is 750 m across at its widest point and up to 400 m deep (Fig. 4). This scarp is partially re¢lled by a dome and associated fan of BAF deposits (as described above). The latter units also overlie deposits interpreted to relate to the collapse feature along part of the northwest coast (Table 3). The bulk of the collapsed material apparently travelled o¡shore, given the proximity of the present coast and the lack of onshore deposits (Fig. 2). There are no direct estimates of the age of this collapse, but the subsequent scarp-¢lling products are apparently the youngest on the edi¢ce, with BAF deposits being emplaced possibly as late as AD 1630^1680 (see above). It is possible that the dome and BAF fan were emplaced immediately following the NW collapse, in a similar fashion to the 1980 St Helens events (e.g. Moore et al., 1981). 3.4.3. Talaulia collapse A large listric collapse scarp occurs on the northern £anks of Nabukelevu. This steep and mostly unvegetated scarp extends from near the summit plateau (ca. 800 m) to the sea (Fig. 6). Deposits of later landslides form small fans that bury the lower 100^150 m of the scarp and form a narrow bouldery beach. The scarp is up to 700 m wide, with a step in its headwall at about 200 m from the top. Two other smaller

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Table 3 Summary characteristics of late Holocene debris avalanche events from Nabukelevu Edi¢ce Event

Age years BP (Table 2)

Volume (106 m3 )b

H/La

Source rocks

Deposit lithology

Daviqele

6 2350 : 140 (Wk-7547)

30

0.16^0.17

BAF deposits and lavas of the southwestern summit area.

NW £ank

s 243 : 39 (Wk-11 587)

70^100

?

Lavas and undescribed coarse breccias

Talaulia

?1840s

?

Qalira

6 1750 : 60 (Wk-7548)

Main: 50^80; two others: 10 Main: 10

Massive lavas, minor breccias Weathered lava domes and breccias

Greyish brown silty matrix (40% by volume), with common free plagioclase and biotite crystals to 4 mm diameter. 95% of clasts grey and reddish grey fresh lava, 5% yellow highly weathered lava. Greyish brown silt-loam matrix (30%), greyish and pale brownish grey hard lava clasts (95%), minor soft weathered clasts. O¡shore

?

Brownish-grey silt loam matrix (40%). 60% of clasts, hard grey lavas, 40% pale brown and pale grey weathered soft lavas.

a Height of vertical drop vs. length of runout ratio, cf. Siebert (1984), ? values occur where deposit runout limits are o¡shore and undetermined. b Volume estimates calculated from scarp dimensions.

scarps (ca. 10U106 m3 removed) occur on the same £ank. No terrestrial deposits are preserved from this collapse, and the scarp may continue below sea level. Since the scarp cuts an undated portion of the edi¢ce, the failure initiation age is presently unknown. However, Jackson (1853) describes an earthquake in the 1840s, which caused a major collapse of this northern face. In Seemann’s (1862) sketch of Nabukelevu, this scarp is the prominent central feature. Hence, either the earthquake of the 1840s generated landslides on an existing scarp, or it was formed at this time. 3.4.4. Qalira collapses At least three small debris-avalanche deposits and related scarps occur on the southern extension of the Nabukelevu dome complex, at the coast and west of Qalira village (Fig. 2). The largest scarp is around 350 m wide by 240 m high. Subsequent smaller landslides, including events in the last two decades, have built up a 0.5^2-mthick debris-£ow deposit veneer over a palaeosol developed into the main debris avalanche unit. The deposit of the largest collapse (Table 3) contains both dispersed plainware pottery frag-

ments up to 5 cm diameter, and rare dispersed charcoal fragments, both apparently entrained in the landslide. The dated charcoal (Table 3) is most likely derived from human activities prior to the event, based on its coexistence with pottery fragments. 3.5. Suva Basin recent sedimentary record The Suva Basin lies between Kadavu and Beqa and Vatulele Islands to the north, and is oriented subparallel to the Kadavu group (Fig. 1C). Steep slopes lead into the southern part of the basin from Nabukelevu, averaging 17‡ to the 1500 m isobath and 10‡ between 1500 and 2100 m, which is reached by 15 km from the volcano. Surface sediment grab samples were collected along a transect between western Kadavu and Beqa reef (Fig. 1C) during a 1998 joint research cruise of the University of the South Paci¢c and the National Fisheries University of Japan, on board the Koyo-Maru. Cores of 5 cm diameter were pushed vertically into the intact sediment inside a large van Veen grab sampler. Core depth was dependent on the amount of sediment recovered in the grab sampler. Two of the sampling

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Fig. 6. Photograph of the Talaulia scarp, forming part of the northern £anks of the edi¢ce. The top of the scarp is almost 800 m asl, at least one major collapse from this area occurred in the 1840s (Jackson, 1853).

sites are of particular interest, site 8 (2130 m deep) in the centre of the basin and site 9 (2208 m deep), nearer the Kadavu slope. Most cores from the Suva Basin transect comprise clay-rich muds that contain very little sand; we call this type 1 sediment. In addition to these clay-rich muds, the cores from sites 8 and 9 contain intervals of turbidite-like deposits (Fig. 7), which consist of sandy mud that ¢nes upward into silty muds (type 2 sediments). The sandy mud is also characterised by shallow-water foraminifera, ostracods and pteropods (Collen, pers. commun., 2000), and more terrigenous sediment than type 1 sediment (including ferromagnesian minerals and volcanic glass derived from western Kadavu ; Fig. 7). Core 9 also contains wood fragments in the type 2 sediment. The inorganic fractions of the two sediment types were examined from sites 8, 9 and 10 (Fig. 1C); although site 10 contains only type 1 sediment, its location proximal to Kadavu makes it an interesting comparison. The ¢ne sand fraction at site 10 is ca. 5% by weight compared to type 1 sediment at sites 8 and 9, which contains 6 2.5%

¢ne sand. We attribute the increase in sand to proximity to the source (Kadavu). X-Ray Di¡raction (XRD) analysis on the inorganic sand fraction of type 1 sediment indicates that the four main components are quartz (25^49%), feldspar (19^34%), volcanic glass (14^37%), and ferromagnesian minerals (5^15%). In contrast, the type 2 sediment contains more volcanic glass and ferromagnesian minerals (37^68% and 10^15%, respectively) and less quartz and feldspar. The ferromagnesian minerals were examined in further detail by XRD and modal counts (300 grains) under polarising microscope (Fig. 8). For type 1 sediment, the proportion of biotite drops considerably between site 10 and the sites farther from Kadavu (8 and 9) and no magnetite was present at site 10. For the basin sites (8 and 9), the proportion of each ferromagnesian mineral is similar in the types 1 and 2 sediments, although the total amount of ferromagnesian minerals is greater for the type 2 sediments (Fig. 8). Many of the sand-sized biotite, hornblende and augite grains at site 10 (type 1 sediment) have partial glass selvedges and are associated with

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385

Fig. 7. Stratigraphy, grain-size, CaCO3 content in muds, and % of heavy minerals in the sand fraction, of cores collected in surface sediment grab samples from the Suva Basin during a 1998 cruise of the Koyo-Maru (see Fig. 1C for sampling locations). Site 9 is closest to Kadavu. Types 1 and 2 sediment are described in the text and details of radiocarbon dates are contained within Table 2.

s 30% volcanic glass fragments. Based on the type of homogenous mud sediment the dacitic glass and related mineral fragments are found within, the systematic ¢ning and decrease of the foreign grains with distance from source, we consider their most likely origin to be from dustings of distal ¢ne-grained tephra fall from Nabukele-

vu. These grains could have been farther distributed by turbidity currents, but the type 1 sediment shows no such bedding characteristics. Biotite also dominates (33^39%) the crystalline inorganic silt fraction at site 10 (type 1 sediment), while it comprises only 14^21% of the crystalline inorganic silt at the more distal sites (8 and 9). The

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Fig. 8. Average feromagnesian mineral assemblages within inorganic sand fraction of type 1 sediment at Suva Basin sites 8 (30 km from Nabukelevu), 9 (25 km) and 10 (18 km), and of type 2 sediment at sites 8 and 9 (Fig. 1C).

inorganic silt fraction of type 2 sediment contains much less biotite (3^4%) ; the remainder of the fraction is made up of kandite, quartz and feldspar with lesser chlorite, smectite and vermiculite. Biotite contents in the crystalline clay fraction of both types 1 and 2 sediment are similar throughout all three sites (13^20%), and kandite group minerals dominate (being common alteration products of glass), along with chlorite, smectite and vermiculite. These data indicate that the relatively high biotite content in the sand and silt fractions are diagnostic of sediment input from Kadavu. The Nabukelevu volcano is the only recognised locality on Kadavu with biotite^andesite dominated lavas (Woodrow, 1980; Verbeeten, 1996). The biotite appears to be sourced from both mass wasting of the volcano and tephra falls blown over the Suva Basin, the latter evidenced by the large amount of volcanic glass at site 10. To date sediments within core 8, more than 300

of the freshest planktonic foraminiferids were manually separated from 1-cm-thick intervals. Two stratigraphically consistent dates (Fig. 7 ; Table 2) were obtained from within intervals of type 1 sediment immediately below the type 2 sediment unit, and near the base of the sampled core. The upper of these dates provides a maximum age for the mass £ow generating the type 2 sediment in core-8 of 2250 : 70 years BP. Between the two dated intervals, the content of apparent air-fall derived volcanic glass and ferromagnesian minerals in the type-1 sediment is the highest. This could indicate the time range when Nabukelevu was explosively active, and ¢ts the on-land derived date for the Nabukelevuira event. Using the same planktonic foraminiferid sampling procedure in core 9, a date of 4850 : 70 years BP was obtained from within the type 2 sediment (Fig. 7 ; Table 2). This seems too old in comparison with the deposition rate estimated from dates in core 8. Dating of wood fragments from the same stratigraphic slice produced an even older age (Fig. 7 ; Table 2). Hence, it appears that the mass £ow producing the core-9 type 2 sediment sampled a large stratigraphic slice of terrigenous and marine sediment, and that neither of these dates provide an adequate age estimate of the event. The content of fresh volcanic glass, biotite and shallow water foraminifera within the type 2 sediments (turbidite-like deposits) suggests their derivation from the northern submarine £anks of the Kadavu edi¢ce, most probably in the region of youngest volcanism at Nabukelevu. Based upon the thickness of overlying type 1 sediment, and the proximity of cores 8 and 9, the turbidite-like deposits in both cores could represent the same event. That the type 2 sediment in core 9 is thickest is consistent with the site being closer to Kadavu/Nabukelevu. Given the variations of dates from material within this sediment at 12 cm in core 9, the basal sandy silt may be a composite of the deposits of several submarine mass £ows, although this was not apparent during drilling. It is possible that the turbidite-like type 2 sediments relate to a collapse or collapses that began on land and continued into the sea. They lie within runout range for subaerial debris avalanches of

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equivalent volume to the Kadavu events (cf. Siebert, 1984), and submarine debris £ows and avalanches are known to be far more mobile, even on low slopes (e.g. ElverhWi et al., 2003). However, the sediment does not resemble submarine debris avalanche deposits (e.g. Schmincke and Segeschneider, 1998), and hence may represent the outer margins of £ow or post-collapse sedimentation.

4. Oral traditions Oral traditions or legends are the main means of recording momentous historic events for the Melanesian and Polynesian peoples (Bascom, 1965; Finnegan, 1995). They have been used to interpret past volcanic and other geologic events in Papua New Guinea (Blong, 1982), Tonga (Taylor, 1995), Fiji (Cronin and Neall, 2000) and the Solomon Islands (Petterson et al., 2003). A commonly reported legend from Kadavu may provide clues to recent events on Nabukelevu. The legend describes the story of two gods, ‘Tanovo’ who presided over Ono Island (northeast Kadavu group; Fig. 1C) and ‘Tautaumolau’ who presided over the southwestern end of Kadavu (Deane, 1909). Beauclerc (1909) relates that before the described events, the main island of Kadavu was lower than it is now, covered with ordinary black loam and not high enough to block the setting sun, as viewed from Ono Island. One day Tanovo discovered that a great hill (buke levu) had been built up during the night in southwestern Kadavu, obstructing his view of the setting sun. Angered, Tanovo began over several nights to tear down the new hill, ¢lling coconut baskets with earth from its side. He carried the earth a convenient distance north of Ono and dumped it into the sea (forming an island now called Dravuni (‘ash’, Nunn, 1999). Tautaumolau eventually noticed this, and the following night he caught Tanovo just as he was about to head homeward with his baskets ¢lled with earth. To escape, Tanovo stooped down to hide in the sea. Tautaumoulau then bent down and drank the sea dry at that place. Tanovo proceeded to £ee, but his load retarded his £ight, and he cast one earth¢lled basket aside. He ran more rapidly, but the

387

violence of the motion disturbed the earth in his baskets causing more to fall out as he approached the north end of Kadavu. By the time he reached north of Dravuni, Tanovo cast the remainder of his load aside, and then tried to escape by crossing the ocean to Rewa (near Suva on SE Viti Levu), but found the water too deep. The two gods eventually fought, and the ¢nal victor appears to depend on the allegiance of the storyteller (cf. Beauclerc, 1909; Deane, 1909). The Reed and Hames (1967) version of the tale indicates the mountain of Nabukelevu was already present but otherwise follows the same theme of Tanovo taking large portions of earth from the mountain in coconut leaf baskets. In this case Tanovo’s work left a crater at the top of Nabukelevu, and the earth removed was apparently used to form the island of Ono. Aspects of this legend appear fanciful, such as the creation of Ono Island with products from Nabukelevu, but legends describing ashfall are often invoked to explain other landforms in Paci¢c oral traditions; the islands ‘created’ are often good indicators of where tephra fell (e.g. Taylor, 1995). The geologic record of recent events on Nabukelevu appears consistent with an overall legend interpretation of a dome-forming and tephra eruption, during which a £ank collapse/debris avalanche occurred and entered the sea to cause a tsunami. A crater or scarp remained where earth was removed from Nabukelevu, and an ash column was blown to the east and northeast by the prevailing high-level winds (cf. Reid and Penney, 1982). The legend is well remembered and in great detail, which may indicate it is relatively young. By comparison, similar legends and place names record individual volcanic eruptions dated between AD 460^600 and AD 120^310 on Taveuni Island of Fiji (Cronin and Neall, 2000). The Taveuni legends are not as detailed or well known, which may be a function of greater age or less widespread e¡ects. The two known Kadavu eruption possibilities include the Nabukelevuira pyroclastic £ow eruption after AD 240^440, or, more likely the most recent dome and BAF eruption within the NW debris avalanche scarp, possibly as late as sometime between AD 1630 and 1680.

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Table 4 Historical damaging earthquakes a¡ecting Kadavu and their impacts in Nabukelevu province Epicentre locationa

Date

Local time and details

Magnitude

Intensity (MM)

2^9 Nov 1998

Swarm of s 200 events recorded and felt, mainshock 16:50 h, 2 Nov 1998

ML 5.3, others mostly 64

VI^VII O¡shore, WSW of (highest in Nabukelevu, Daviqele shallow events and Qalira)

Jul^Aug 1983

Swarm of s 85 recorded events 17 July 03:50 h 19 July 18:09 h 22 July 12:59 h

20^26 Dec Swarm of ? events 1981 Mainshocks: 20 December 21:51 h 22 December 01:03 h 5 Jun Single shock, 0422 h 1976 17 Dec Single shock, 09:02 h 1975

ML 4.7 ML 5.1 ML 4.8

ML 4.5 ML 4.5 ML 3.2 Ms 5.5

17 Apr 1963

Single shock, 14:11 h

Ms 6.6

14 Sep 1953

Single shock, 12:27 h

Ms 6.75

13 Feb 1950

Single shock, 10:13 h

Ms 6.5

17 Nov 1935

Single shock, 19:40 h

Ms 6.0

29 May 1930

Single shock, 04:00 h

?

27 Apr 1927 20 Sep 1921

Single shock, 14:50 h

?

Single shock, 11:16 h

? Ms 6.7

Impact in Nabukelevu province

Reference

Widespread small-scale Prasad, written landslides ( s 100 events), commun., 1999; blocking roads with and local debris and threatening interviews buildings; cracks in during this concrete buildings in study, May Qalira and Daviqele; 1999. ground cracks in ¢ll sediment in Daviqele; small water and sand boils in Daviqele ¢ll sediment; discoloured water o¡shore of Cikobias; selfevacuation of Daviqele, Qalira, Nasau and Nabukelevuira villages V^VI O¡shore, W and A number of small-scale Hamburger and (highest WSW of Nabukelevu; landslides (4 major events Qiolevu, 1983 in Qalira) 19.08^19.17‡S, noted, but several others 177.77^177.80‡E reported), debris in Qalira village; cracks in concreteblock building in Qalira; widespread alarm ? O¡shore, WSW of No details available Everingham, Nabukelevu; 1987 19.34^19.46‡S, 177.98^178.12‡E ? 19.1‡S, 177.9‡E Felt, no further details Everingham, available 1987 III^IV Between Suva and No details available Everingham, Kadavu; 1987 18.5‡S, 178.6‡E V O¡shore, WSW of Long duration, no details Everingham, Nabukelevu; available 1987 19.36‡S, 178.36‡E IV^V Suva^Beqa area; Tsunami waves average Houtz, 1962; 18.25‡S, 178.25‡E 2 m on coastlines Everingham, 1987 Everingham, VI N of Nabukelevu; Landslide from the top 1987 18.86‡S, 179.79‡E of Nabukelevu edi¢ce; widespread alarm ? up to III O¡shore, WSW of Landslide and damage Hamburger and in Suva Nabukelevu; recalled by older people Qiolevu, 1983; 19.5‡S, 177.5‡E Everingham, 1983 ? up to IV O¡shore, N of No details available Everingham, in southern Nabukelevu; 19.0‡S, 1983 Viti Levu 178.0‡E ? East of Kadavu; No details available Everingham, 19.0‡S, 179.0‡E 1983 East of Kadavu; No details available Everingham, ? up to V 1983 in western 19.0‡S, 179.0‡E Viti Levu

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Table 4 (Continued). Epicentre locationa

Date

Local time and details

Magnitude

Intensity (MM)

14 Oct 1918

Single shock, 00:39 h

Ms 5.5

? up to VI O¡shore, N of in southern Nabukelevu; 19.0‡S, 177.5‡E Viti Levu VIII ? O¡shore, N of Nabukelevu; 19.0‡S, 178.0‡E

1840^1850 ?

?

Impact in Nabukelevu province

Reference

No details available

Everingham, 1983

Many deaths, including Jackson, 1853; landslide burial of a Everingham, cave, within which 30^40 1983 women were sheltering for the night; part of the ‘head blu¡’ (north face of Nabukelevu) was shaken down in a landslide

a Epicentre locations prior to 1950 may have errors of up to 100 km due to uneven station distribution, inaccurate reporting of arrival times and manual earthquake locating (Everingham, 1983; Hamburger et al., 1990).

5. Other hazardscape factors: earthquake, landslide and tsunami history A number of historical earthquakes have caused damage in the Nabukelevu area (Table 4), although the pre-1930 part of this record may be incomplete because this part of Fiji had little contact with the main colonial administration. Many of these earthquakes were centred o¡shore to the west and southwest of the province at about 10 km depth (Hamburger et al., 1990). Focal mechanisms for some of the events include left-lateral strike-slip motion on a NE-striking fault zone (Johnson and Molnar, 1972) and thrust motion on a NW-striking fault plane (Sykes et al., 1969). In recent history, with the deployment of portable seismometers, at least three earthquake swarms with hundreds of events have been recorded. The 1983 and 1998 swarms (Table 4) caused widespread alarm due to earthquakes and landslides that occurred continuously over weeks. In both cases the southern villages selfevacuated (Qalira and Davigele/Nasau), with people moving east to Kabariki village. Five historical events generated landslides. The most serious a¡ects, including 30^40 fatalities, resulted from a large event sometime between 1840 and 1850 (Jackson, 1853). The main physical impacts from the 1983 and 1998 earthquakes, as well as the highest earth-

quake intensities estimated from previous events, are concentrated in the southern part of Nabukelevu. In particular, the villages of Qalira, Daviqele/Nasau and parts of Nabukelevuira have suffered the most from landslides, ground cracking and building failure. Landslide locations during 1983 and 1998 (Fig. 9) are mostly on scarps of the debris avalanches described above, or concentrated along the NE-striking faults mapped through the edi¢ce (Fig. 2). There are no written records of locally generated tsunamis in the Nabukelevu province, although they may possibly be inferred from the legend described above. In addition, many older local inhabitants in the area are aware (or have been told by their elders) that when they feel earthquakes, a local tsunami may occur (Daviqele village, pers. commun. 1999). Two distal tsunamis have been documented on Kadavu. The ¢rst followed the 1953 Suva earthquake, produced up to 2 m runup on most Kadavu coastlines, and killed two people in NE Kadavu (Houtz, 1962). A second tsunami £ooded the low-lying parts of Nabukelevuira village in 1960, forcing people to relocate to the present village site on higher ground (Nunn and Omura, 1999; local inhabitants, pers. commun. 1999). This latter tsunami was probably a Chilean-sourced event, recorded as a 0.5-m-high wave in Suva on 23 May 1960 (Everingham, 1984).

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6. Discussion 6.1. Geologic hazard assessment for Nabukelevu province The main geological hazards facing Nabukelevu province are interrelated, with some hazard processes (volcanism, earthquakes, cyclones) generating other dangers (debris avalanches, tsunami, landslides). The frequency and interrelationship

of the main geologic hazards can only be approximated due to the short period of time for which records or geologic data are available. 6.1.1. Volcanic eruption-related hazards At least two late Holocene eruptions occurred from Nabukelevu and during the period of human occupation, including the Nabukelevuira event, after 1686 : 40 years BP (AD 240^440; Table 2), and an event forming a dome and BAF fan

Fig. 9. Locations of signi¢cant landslides during the 1998 and 1983 earthquake swarms (the latter from Hamburger and Qiolevu, 1983).

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on the NW £anks, possibly as recent as AD 1630^ 1680. Additional possible late Holocene eruptions include that forming the present summit dome, sometime before 2420 : 90 years BP (800^350 BC; Table 2). This may or may not have been related to an eruption forming a BAF deposit fan on the upper eastern volcano £anks. The fan collapsed as part of the Daviqele debris avalanche, hence a minimum age is 2350 : 140 years BP (850^50 BC; Table 2). Further evidence for late Holocene volcanism comes from the Suva Basin cores (Site 8) where the highest content of biotite-bearing volcanicash-derived components in type-1 sediment lies in a horizon bracketed by dates of 2250 : 70 years BP (710^325 BC) and 780 : 50 years BP (AD 1065^1290; Table 2). Late Holocene volcanism appears to have been concentrated in the upper portions of the Nabukelevu edi¢ce. Lava e¡usion has also occurred in outlying areas (e.g. Cikobia Point along a fault zone running NE through the main edi¢ce). However, this more ma¢c volcanism has deeper weathering pro¢les and thus may represent the early stages of Nabukelevu construction. The most common volcanic processes during past Nabukelevu eruptions include dome and lava £ow emplacement with associated BAFs plus explosive eruptions involving tephra falls and associated small column-collapse pyroclastic £ows. Both of these types of pyroclastic £ow are the highest threats to life. Hence, hazard zones mapped (Fig. 10A) for them are based on considerations of the mapped extent of recent eruption products, along with energy line or H/L concepts (e.g. Yamamoto et al., 1993; Sheridan and Mac|¤as, 1995), considerations of local topography, and possible variations in vent location. Hazard zone A (Fig. 10A) is based on the potential distribution of dome or lava-£ow collapse generated BAFs from the summit area of the volcano. Since BAF deposit fans around Nabukelevu are truncated by either younger deposits or the coast, only maximum H/L ratios of 0.41^0.47 can be calculated. For rockfall or collapse-generated BAF’s at Unzen between 1991 and 1995, H/L ratios were mostly between 0.3 and 0.6 (Ui et al., 1999), and at Colima between 0.33 and 0.38

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(mean 0.35; Sheridan and Mac|¤as, 1995). We use an H/L value of 0.35 from the volcano summit area, since this encompasses the locations of suspected Holocene vent areas and the ¢ve major channels that drain summit area catchments. Hazard zone B is based on the potential distribution of small column-collapse pyroclastic £ows from the summit area of the volcano. A maximum H/L of 0.25 is estimated from the distribution of the Nabukelevuira pyroclastic £ow deposit, although both the actual height of collapse and distribution limits are unknown. Sheridan and Mac|¤as (1995) estimated average H/L ratios of 0.24 for larger BAFs and pumice pyroclastic £ows on Colima, and Yamamoto et al. (1993) present ratios of s 0.1 for small volume pyroclastic £ows. A ratio of 0.2 is probably most realistic for the relatively small Nabukelevu volcano. Tephra falls and perhaps rare more-energetic pyroclastic £ows are expected to a¡ect areas within hazard zone C (Fig. 10B). Presumed Nabukelevu tephra-derived glass and mineral grains are found within basin muds (our type 1 sediment) up to 40 km north of the volcano, although they are more common at 30 km. Predominant winds blow toward the east at altitudes of s 3000 m, but lowlevel winds blow toward the NE (Reid and Penney, 1982). The legends described above indicate debris being carried and dropped as far as the Kadavu Group islands near Ono, consistent with the upper-level wind patterns. In the absence of on-land depositional evidence, the boundary of hazard zone C is drawn as an ellipse, elongated toward the east, encompassing the island of Ono and extending a maximum 70 km from Nabukelevu. Outside this zone probably only nuisance level impacts occur, with the exception of possible hazard to air tra⁄c. 6.1.2. Debris avalanche generation and hazards Nabukelevu is a steep volcanic pile interlayered with weak and saturated volcaniclastic breccias. The humid tropical climate of the area promotes rapid weathering rates and intense storms; up to 10 tropical cyclones with attendant high-intensity rainfalls (100^250 mm/day) passed over Kadavu between 1970 and 1993 (Terry, 1999). Important debris avalanche triggers on Nabukelevu may in-

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Fig. 10. Geologic hazard maps for Nabukelevu and environs. (A) Volcanic hazard zones from pyroclastic £ow and lahar related hazards, grey-shaded patches are locations of vulnerable villages. (B) Hazard zone for most likely tephra fall hazards. (C) Hazard zone for probable limits of damaging debris-avalanche induced tsunami from Nabukelevu. (D) Hazard zones for small-scale landslide hazards during local earthquakes of ML v 5.0, and during cyclones with high intensity ( s 100 mm/24 h) rainfalls.

clude: (1) cyclonic high intensity rainfall events ; (2) large or shallow local earthquakes; (3) intrusion of magma into shallow levels below the edi¢ce; and (4) volcanic eruptions. All of these events have occurred in the late Holocene history of Nabukelevu. The causes are listed in order of decreasing frequency, but in possible order of increasing collapse-generating e⁄ciency. Magma intrusions have triggered several recent cone collapses (e.g. Siebert, 1984), and these may have occurred three or four times since the mid-Holocene on Kadavu. The NW

£ank collapse was followed by emplacement of a lava dome and BAF deposit fan, and hence may have been triggered by magma intrusion, in a manner similar to that described at Augustine volcano, Alaska (Siebert et al., 1995). The other major landslide scarps appear unrelated to volcanic activity, and may have been triggered by local earthquakes, particularly by motions along the faults cutting through the edi¢ce. The Talaulia, Qalira and Daviqele event scarps are all located along or near the edi¢ce-cutting fault zone, an area probably mechanically weak-

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ened relative to other parts of the edi¢ce. High hydrothermal £uid pressures may also develop in this fractured body of rock such as observed during magma intrusions (e.g. Voight et al., 1983). Earthquakes involving large movements along this zone may be e⁄cient collapse-triggering mechanisms (e.g. Matsuda and Ariyama, 1985). Small-scale earthquakes and cyclonic storms, although more frequent, probably generate only minor collapses and landslides. This is borne out by the experiences of local inhabitants and the recorded e¡ects of recent earthquakes and cyclones (e.g. Hamburger and Qiolevu, 1983 ; Table 4 ; Terry, 1999). Often the smaller-scale landslides occur as remobilisation of portions of large debris-avalanche scarps, particularly on the Talaulia, Qalira and Daviqele scarps. This probably relates to seismically or hydrothermally weakened rocks in these areas and the location of the most severe ground motions during many earthquakes. Potentially the most destructive hazard present is that posed by large-scale edi¢ce collapse (0.03^ 0.1 km3 ). Three such events have occurred in the late Holocene at Nabukelevu. Although a future similar event could a¡ect any portion of Nabukelevu province, present topography likely prevents debris from travelling eastward of the fault scarps forming the eastern boundary of the province (cf. Fig. 2). Nabukelevu debris avalanches of this scale would undoubtedly reach the sea to generate at least locally important tsunami. For example, the model of Heinrich et al. (1998) for the entry of 0.04 and 0.08 km3 debris avalanches into the sea from Montserrat indicated wave heights of around 10 m within 3 km of the entry location, dropping to around 1^2 m at 10 km distance. A re¢ned model of this collapse (Heinrich et al., 1999) implied 15-m waves within 2 km, and s 5-m waves within 5 km of the entry point. Models of Kienle et al. (1987) also indicate that signi¢cant tsunami hazard is probably con¢ned to within 10 km of source for the scale of potential Nabukelevu collapses. Although the above model values cannot be directly applied to Nabukelevu due to di¡ering coastline geometry, bathymetry and dimensions of the collapsing mass, they suggest a methodol-

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ogy to construct collapse generated tsunami hazard zones. Heinrich et al. (1999) note that wave amplitudes decrease as a function of 1/kradial distance from source, and with depths of s 1000 m initially generated waves disperse into waves of various frequencies, some decreasing as a function of 1/radial distance. Using this decay law and probable maximum initial wave heights of 10^ 15 m (cf. Heinrich et al., 1998, 1999), tsunami waves of I0.5 m could be expected up to 100 km from Nabukelevu, a radius encompassing Kadavu, Beqa and Vatulele Islands, as well as large parts of the southern coast of Viti Levu. However, ampli¢cation of waves due to local coastal bathymetry and geometry complicates such generalised calculations. In summary, large debris avalanches could affect any part of Nabukelevu province, tsunami hazards are likely to be signi¢cant within 10 km along the Kadavu coast, and for local areas within a 100 km radius where local bathymetric and coastal geometry factors combine to amplify distal waves (Fig. 10C). 6.1.3. Earthquake and landslide hazards A more frequent hazard in the Nabukelevu province is that of moderate-sized earthquakes, during which the most important seismic e¡ect is landsliding. Frequent cyclonic storms have also generated local landsliding. Landslides are concentrated in large debris avalanche scarp areas and also along the major fault-controlled valleys. Based on these factors, we determine potential landslide hazard to be greatest for the village of Qalira, which has also su¡ered (along with Daviqele) the greatest impacts from earthquake shaking (Fig. 10D). 6.1.4. Vulnerable elements Around 2150 people live in the eight villages of Nabukelevu (Fiji Bureau of Statistics, 1998), most of them within volcanic hazard zone A or B (Fig. 10A). Most of the remaining 9500 inhabitants of Kadavu live in coastal communities around the island, leading to a high vulnerability to tsunami hazard. At a lesser, but perhaps locally important tsunami hazard are coastal communities on Beqa (1240 total population) and Vatulele Islands

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(910), plus many more along the southern coast of Viti Levu. The nearest airstrip to Nabukelevu, at Vunisea, lies well within volcanic hazard zone C and any tephra fall will a¡ect its operation. A single 4WD road in the south and walking tracks along the north and south coasts are the only land routes out of the province. There is one landing site for a large boat at Nabukelevuira village. The Nabukelevu areas economy is based on subsistence and cash-cropping agriculture. Infrastructure includes one hospital (Daviqele), three schools (Daviqele, Nabukelevuira and Talaulia), a tourist resort on Nagigia Island, plus diesel generators, radio telephones and small motor boats in most of the villages.

also focused along these fault zones and existing debris avalanche scarps. Edi¢ce failures or submarine failures from the Nabukelevu area of Kadavu have led to at least one signi¢cant turbidite deposit in the Suva Basin to the north, as well as probably causing local tsunami. The correspondence of many elements in a local oral tradition from Nabukelevu with geological evidence is striking, with both lines of evidence pointing toward a locally important late Holocene eruption and related debris avalanche. Such legends when combined with geological evidence can be an important tool for interpreting palaeohazard records in the Southwest Paci¢c.

Acknowledgements 7. Conclusions On- and o¡shore geology coupled with archaeological and ethnological evidence demonstrates that volcanism has continued at Nabukelevu into the late Holocene. In addition, Nabukelevu is one of only two Fijian centres (along with Taveuni; Cronin and Neall, 2000) that has a¡ected local settlers, and should be considered as an additional potential source of volcanic hazard within the Fiji group. The magma compositions remain consistent with a subduction source despite the apparent cessation of active subduction in this region. Volcanism appears dominated by formation of domes and minor pyroclastic £ows. The 800-m-high volcanic complex is highly unstable, being constructed from a mixture of lavas and breccias, cut by at least three active fault zones, and situated in a high rainfall environment with frequent large storms. The faults in particular appear to exacerbate the propensity of the edi¢ce to failure by creating weak shattered rock zones, promoting increased hydrothermal £uid pressures through deep fracturing and episodically being the focus of strong ground motions. Failure of these weakened zones formed a series of late Holocene debris avalanches during the period of human occupation. Landslide hazards caused by the frequent earthquake swarms in the area are

Fieldwork for this project was made possible and enjoyable by the hospitality and assistance of Ratu Maravu Soqosoqo (Pancho), Emma, Cakacaka, and people from Daviqele, Nabukelevuira, and Lomati villages. Prof. Patrick Nunn kindly engaged in many useful discussions and provided pre-publication copies of relevant manuscripts on the Nabukelevu area. We gratefully acknowledge the assistance of the captain, scientists and crew of the Koyo-Maru (National Fisheries University, Japan) and SOPAC in the collection of Suva Basin grab samples. Other logistical support in Fiji was provided by SOPAC, the Fijian National Disaster Management O⁄ce (particularly Agapusi Tuifagelele) and the District O⁄ce of Kadavu. We gratefully acknowledge support from the New Zealand Foundation for Research Science and Technology (contract MAU702), the University of the South Paci¢c URC and the Alexander von Humboldt Foundation. We thank V. Neall, S. de Silva, J. White, C. Waythomas, R. Waitt, J. Major, and K. Cashman for comments on earlier versions of this work.

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