JOURNAL OF GEOPHYSICALRESEARCH, VOL. 91, NO Bl2. PAGES 17,465-17,484,DECEMBER 10,1989

ProdigiousSubmarine Landslides on the HawaiianRidge

J. G. Moons.t D. A. Clncur,.r R. T. HolcoNae,2P. W. LIpunN,3 W. R. NonnaRRK.rANo M. E. TonnesaNr

The extensive area covered by major submarine mass wasting deposits on or near the Hawaiian Ri{ge has been delimited by systematicmapping of the Hawaiian exclusive economic zone using the side-lookingsonar system GLORIA. These surveys show that slumps and debris avalanchedeposits r a.e .*posed over about 100.000km of the ridge and adjacent seafloor from Kauai to Hawaii, covering an area more than 5 times the land area of the islands.Some of the individual debris avalanchesare more than 200 km long and about -5()00kml in volume. ranking them among the largeston Earth. The slopefailures that produce thesedeposits begin early in the history ofindividual volcanoeswhen they are small submarine seamounts.culminate near the end of subaerialshield building, and apparently continue long after dormancy. Consequently.landslide debris is an important element in the internal slructure of the volcanoes.The dynamic behaviorof the volcanoescan be modulatedby slope failure, and the structuralfeatures of the landslidesare relatedto elementsof the volcanoesincluding rift zones and fault systents.The landslidesare of two generaltypes, slumpsand debris avalanches.The slumps are slow moving, wide (up to I l0 km). and thick (about l0 km)with transverseblocky ridgesand steep toes. The debris avalanchesare fast moving. long (up to 230 km) compared 10 width. and thinner (0.0-5-2km)l they commonly have a well-detinedamphitheater at their head and hummocky terrain in the lower part. Oceanic disturbance caused by rapid emplacementof debris avalanchesmay have produced high-levelwave deposits (such as the 36-5-melevation Hulopoe Gravel on Lanai) that are found on severalislands. Most present-daysubmarine canyons were originally carved subaeriallyin the upper parts of debris avalanches.Subaerial canyon cutting was apparently promoted by the recently steepenedand stripped slopesof the landslideamphitheaters.

INrRooucrtou delta slopes, where the relatively shallow water allows high-resolutionsurveys using side-looking sonar, reflection profiling. borehole drilling, and measurementsof geotechni- New perspectiveson the extent and character of subma- cal properties IColenrun ttnd Gorrison, 1977: Prittr and rine slope failures have been obtained with the \ystematic Colemun.l982. 1984tPrior et ul.,1984u,bl. In sloperegions (EEZI mapping of the Hawaiian exclusive economic zone with lower sediment accumulalion rates, earthquakes are (Geologic using the side-looking sonar system GLORIA commonly the tavored triggering mechanism for ground Long-Range Inclined Asdic). In the fall of l9tl8. GLORIA failure over areasof a f'cw hundred squaremeters to as much extending 200 nautical miles from land were com- surveys as 80,000 kmr [e.g.. Fieltl et ul., 1982: Normark, 1974 pleted in a zone from the southeast end of the Hawaiian Kenyon,19871(see also review articlescited earlier). Ridge adjacentto the island of Hawaii northwest to beyond The larger masswasting deposits from continentalmargins the the island of Kauai. In this region which includes extend well into deep water and, including the debris flows principal Hawaiian islands, l7 well-definedmirjor landslides associatedwith many submarine slope failures, may reach (Figures Table l). and remnantsof were mapped I and 2 and severalhundred kilometersin length IJacobi, 19761'Bugge et severalothers partly covered by younger depositsalso were a1.. l98tll. Becausemuch of the deposits from these larger found. This report documents the characteristicsof these failures lie in deep water (>2000 m), less is known about irt- landslides, examines their morphologic variations. and their internal structure and composition as compared to processes modified tempts to relatethem to the that built and continentalshelf structures(see discussion by Normurk and the volcanic systemsof the Hawaiian Ridge. Gutmut'her tl9tlSl). The Storegga submarine slide on the The study of submarinemass-wasting deposits composed Norwegian continental margin exceeds5000 kmr in volume poorly bedded volcanic rocks presentsdifficult of massiveor and is one of the largestmass failures lBugge €/ d1., 19881.lt problems not encounteredin the better understooddeposits apparentlyrepresents three episodesoffailure, the secondof composed of sedimentary materials. Slumps. block slides. which may be associatedwith a tsunami deposit along the flows, such processesare widely recognized debris and other east Scottish coast [Dnx'son et al., 1988]. (see in nearly all types of continentalmargin settings reviews Slope lailures on sedimented continental margins are by Moore [1978], Embler, [|982], Kenyon ll987l. Colentun comparatively easy to document as compared to those on and Prior I19881,Lee U989D.These depositsare common rn volcanic slopes.The small-scalefailures in bedded sediment areasof high sedimentationrates and include thick accumu- appear on high-resolution reflection profiles because the lations of sediment with slope-parallelbedding. Features missing sediment section is obvious, and the mass wasting produced by sedimentfailure are well describedon modern depositsshow as lens- or wedge-shapedunits with irregular, -' hummocky surfaces and little or no coherent internal struc- u -s. a;. gic al Su rv e y. M en I o Park. c aI i fo rn i a. ture. Seismic reflection profrles can discern these deposits 'U.S. GeologicalSurvey, University of Washington.Seattle. even when buried by later sediment, and both the scar and iU.S. GeologicalSurvey. Denver. Colorado. base of sediment mass-wasting deposits can be clearly This paperis not subjectto U.S. copyright.Published in 1989by recognized. the AmericanCeophysical Union. In contrast, mass failure on the submarine slopes of Papernumber 89JB027,53. oceanicvolcanoes must be recognizedprimarily on the basi:

I 7..16-5 t7,466 MoonE.er el.: SunvlnrNELelrosLtor.s. Hewerr

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I )

,-*" oA,,-\

l t6l

1OOKM I : ! #i;:,.@ u* t ,". , ::::. 13k L

Fig. l. Bathymetric map of the southeasternHawaiian Ridge showing major mass w:rstingfeatures (heavy dashed lines) identified by number in text and'Iable l. Contour interval 200 fathoms (366 m). Biise from lJ.S. Nuvul O ce a rut g rup hic' Ollic e ll97 3).

of surfacefeatures as the basalticrock and rubbleare nearly Dana's faulting model was rejected by llitchcot'k ll90(t:. opaqueacoustically. Standard seismic reflection profiling who believed that erosion alone was sufficient to form thc systemsgenerally do not provide subsurfaceinformation on cliffs without the additional complexity of faulting. Werrr- the internalstructure of slidecomponents or of the slopes u,orth |9271, comparing estimates of marine and fluvia. wherethey originated.With relativelyslow accumulation of erosion rates. concluded that marine erosion alone *a. primarily pelagic sediment,the HawaiianRidge slopeadja- insulhcient to produce some of the larger sea cliffs (e.g.. centto the islands hasinsufficient sediment cover to Drovide those of north Molokai) and that they were probably pro- muchassistance in defininelandslide features. duced by faulting. The idea of flank faulting eventuallyled to the concept thai PnevrousWonr oN Hewe.rreNSlopE Fallunes the faults bound the headwalls of giant landslides.Steurn' The role of major landslidingin shapingHawaiian volca- and Clark [1930] and Stearns and Mucdonald |9461 sug. noeshas long been a subjectofdebate. An earlycontroversy gestedthat the 50-km-longHilina fault system on the sourh concernedthe relativeimportance of faultingversus marine flank of Kilauea volcano owes its origin to, and is at the heac erosion in producing high cliffs along the coasts of some of, a major landslide on the steep seaward slope. Macdonul,; islands.The high cliffs of northeastOahu (NuuanuPali), as [l956] later discounted landsliding, suggestingthat largc- well as thoseof eastNiihau, the Napali coastof Kauai, and volume intrusion beneath the volcano center forcibly up- northernMolokai wereinterpretedby Dana [1890,p. 290]as lifted the summit causing faulting on the volcano flank. resultingfrom "profound fracturingof the mountaindome" Moore and Krivoy [1964]revived the landslideconcept fo: and "downdrop" of the seaward part beneath the sea. Kilauea's south flank and suggestedthat not only is thc nh*, n Moont Er lr-.: SunvnntNE LANDSLIDES,Hewnlr t7,467

158 1560 T u(-. {,ri

.;,;it + 220 7- ena^ ^ ^ 1::l.:;:.:.;a.i-.f: 5 Nuuanu

I l-a;.. / 2 -: :l

MOLOKAI

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Fig. 2. Map of southeasternHawaiian Ridge showing major slides bounded by dashed lines identified by number in text and Table l; compare with Figure l. Dotted area, hummocky ground (widely spacedwhere subdued);hachured lines, scarps;thin, downslope-directedlines. submarinecanyons and their subaerialcounterparts; heavy dashed line, axis of the Hawaiian Deep: dash-dottedline. crest of the Hawaiian Arch.

': r) lr)fll'. i :rng.il ,':' Hilina fault system related to gravitational sliding but also the pre-Hawaiianseafloor and hence that the slide was as .rntl ll L.. :., :hat the east rift zone of the volcano forms the upper margin muchas l0 km thick. Episodicmovement of this landslide :'.'. .rlttttC of a large seaward moving slump. An analysis of geodetic has been incorporatedinto a model for the long-termerup- i d ;lltl. - and seismic evidence led Su'an.sonet al. U9761to support tive behaviorof Kilauea tHolcomb, 1987). .'hat lr i:". this notion of a mobile south flank of Kilauea and to add the Irregular blocky topographicwelts extendingdownslope ioncept that the south flank moves in response to forceful from giant amphitheaterson the submarinenorth flanks of trfliefil ll.:l rntrusion of magma into the rift zones. They assumed that Molokai and Oahu were interpretedas representinggiant c\. .\I( r;'': the volcano flank is mobile downward to a depth of 5-8 km landslidesby Moore U9641.Macdonald and Abbott 11970,p. l9-161'..t' .rndsuggest that the base of the mobile zone is not defined by 3651,however, questionedthe prodigioussize of these n lhL'\\rl':- e single plane or sole but that seaward displacement gradu- features,compared with previously known landslides,and ll thc'hd.:- ally decreasesdownward [see also Crossen and Endo,1982]. proposedthat the blocky seamountsare in situ volcanoes. \l ttt tlt ttlL . -: Lipman et al. |9851 assessed the deformation associated The results of three separateoceanographic investigations tnul lrrsc' n ith the 1975earthquake (M : 7 .2) on Kilauea's south flank by MathewsonU9701, Langford and Brill tl972l, and An- -trrc iblr uI' and concluded that a deep-seatedgravitational slump, partly drex,sand Bainbridge |972, p. 1121caused each of themto .' flank. related to forcible intrusion of magma into the rift zone at its reject the landslidehypothesis, with the latter two studies Jr)nccPtl(': head, could explain the geologic and geophysical observa- favoringthe in situ volcanoconcept. Later measurementsof trnlr is thc rions.They suggestedthat the sole of the creepingmass was the magnetic stratigraphy of East Molokai volcano led 1',7.468 Moonr, r,r ,cL.: Sunr4entNe LeNosr-loe,s. HAwAll

TABLE l. HawaiianSubmarine Slides

Area. Length," widrh,b No. Name Location km: km km Type'

1 North Kauai North Kauai 14.000 140 100 D (0.H.9) 2 South Kauai South Kauai 6,800 100 _50 D (0.6-1.2) 3 Kaena NE Oahu 3.900 80 45 D 4 Waianae SW Oahu 6,100 50 80 s (r.0) 5 Nuuanu NE Oahu 23,000 235 35 D (0.8-l) 6 Wailau North Molokai I 3,000? < l9-5 40 D (0.7) 7 Hana NE Maui 4,900 85 ll0 S 8 Clark SW Lanai 6,t00 ls0 30 D (0.s-l) 9 Pololu North Hawaii 3,500 130 20 D l0 South Kona West Mauna Loa 4,600 80 80 S I I Alika-l West Mauna Loa 2,300 88 t.) D (0.9-l) 12 Alika-2 West Mauna Loa 1,700 95 l5 D (2-5) 13 Ka Lae, west South Hawaii 8-50 85 l0 D 14 Ka Lae, east South Hawaii 950 75 l0 D (2) 15 Hilina South Hawaii 5,200 40 100 s 16 Papa'u South Hawaii 200 20 o SF 17 Loihi South Hawaii ,500 l5 l 0-30 L

Total 97.600

Located by number in Figure 2. "Length of Waianae and South Kilauea landslidesomits indistinct irregular topography beyond steeDtoe. 'width ot head of landslide. ' D, Debris avalanche;(number) is averagenumber of hummocks appearingin GLORIA imagesper square kilometer; S, Slump; SF, sand rubble flowl [., three unclassif'eidlandslides.

Holcomb [985] to support the notion that the north half of conducted aboard the British vessel M/V F-arnellaby the the caldera is missing and probably was carried beneaththe U.S. Geological Survey and the U. K. Institute of Oceano- sea by landsliding. graphic Sciences.The syslematic and uniform nature of the Bathymetric mapping of Papa'u seamounton the subma- GLORIA regional surveys has been particularly valuable in rine south flank of Kilauea volcano led Moore and Peck a comparative study of this type. The surface features and [965j to propose that this feature is a relatively young other sonic characteristics can be directly compared be- landslide, but Macdonald and Abbot [970, p. 317] favored tween different parts of the same landslide or fiom widely its origin as a shield volcano built on a rift zone with the spacedslides of different age. same trend as that of Hualalai volcano. Later ocean floor Early utility of the GLORIA system for investigating photography, submersible diving, and analysis of dredged submarine landslidingin the Hawaiian region came in 1986 sampfes led Fornari et ul. ll979l to conclude that Papa'u when sonar images collected off the southwest flank ol' seamount is in fact a relatively recent landslide lobe with a Mauna Loa outlined a major region of submarine slope volume of about 40 kmr. failure. These surveys delineated the Alika and Ka Lae Irregular bathymetry suggestingsubmarine mass wasting debris avalanches. coverinq an area ol' about 5000 kmr on the west flank of Mauna Loa was investigatedfiom the lLipmun er ai., 19881. U.S. Geological Survey vessel ,5. P. Lee in 1976and 1978. This work showed that a large region (including the south DescnrprroNoF SLTDES Kona slump and the Alika slide complex) between the previously enigmatic, subaerially exposed Kealakekua and Alongthe 700 km reachof the HawaiianRidge from Kauai Kahuku faults was involved in seaward sliding that reached to Hawaii,well-exposed slope failures involve roughly one- ridge(Figures 80 km offshore lNormark et al., 19791.Analysis of bathy- halfof theflanks of the l and2). Mostof these metry and newly acquired Sea MARC II side scan sonar slideshave been known only sincethe recentcompletion of images of small areas led Fornttri and Cumpbell [1987] to the GLORIA surveys,and our understandingof them is emphasize the importance of gravitational slumping and basedlargely on this data baseof restrictedscale. Finer debris flows on the flanks of the Hawaiian Ridee. blockyfacies, and relatedturbidite deposits, doubtless ex- tend beyondthe slidemargins as shownon Figure2. We hope that this preliminaryreconnaissance study will help Mr'rnoos guide more detailed surveys and sampling programs on The GLORIA system, which generatessonographic im- specific slides. Brief descriptionsof the individual land- ages of the seafloor as much as 22.5 km to each side of the slides,from northwestto southeastalong the ridge,are given ship track, can resolve features as small as a few hundred below(see also Table l). meters across[,Somers et a\.,1978]. In addition to GLORIA, The largeHawaiian slope failures can generallybe divided 3.5-kHz echo-soundingand air gun seismic reflection pro- into two structural types based on the classificationof filing were employed during the survey of the seafloor Varnes[978], but the term landslideis retainedas a general extending 200 nautical miles (370 km) off the island shores. term for all slopefailures including these submarine features. This regional mapping of the U.S. EEZ around Hawaii was The first type (example,Hilina slump)includes the broad

ffi4n:* Moone er ,lL.: SusN4nnlNr.LlNosr-lors, Hewlrt l7.469

0 North Kauai 1

-5 v

0 0 SouthKauai 2 Pololu 9

.5 v

0 0 >.1 Kaena Alika-1 v 3 1 1

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0 0 -l Waianae 4 Alika-2 12 v J 4-3 F c-4 ttJ -r Y o

0 0 -1 KaLae-E 14 'r,'/irib1 thc tlf Oceanr'- -1 l.rturer)t'thc i aluablcrr. v icaturc\ irnc 'mparctl Fc' 0 0 Clark -l Hilina ironr \\ idci. 8 n\ c\tigirtrn! rmc ln l9Sf -5 -5 c\l llank.': V lJflnC \l()n( 50 100 r50 50 100 150 rnd Klr l-.'c DISTANCE,KM DISTANCE,KM t 5(XX)knr: Fig. 3. Longitudinal profilesof landslidesas numberedin Table I and shown in Figures I and 2. Hummocky terrain is indicated by dots (widely spaced where faintly visible). and lower end of dots is known lower limit of debris avalanche.Projected trace of Hawaiian Deep axis shown by triangle. Vertical exaggerationis 7.5.

: l'rtlm Kau.,. r!)ughl! t)nc- and steeperfailures that have a steep scarp at their toe, are noes in the study area and Dalrymple, 19871. ,lr)\l o1'thc.c lClague cut by transverse faults into a few large blocks, and com- the landslidefeatures on their slopesare the omPletion (': Consequently, monly lack a well-developedamphitheater at their head. The to subsequentisland subsidence, ero- : of thcnl r. mostdegraded owing second type (example, Nuuanu debris avalanche)includes rcale. Ijtnc: sion, and sedimentation.GLORIA imagesshow that a the narrow and more far-reaching landslides that show \)ubllL-\\ c\- landslidecomplex covers 14,000km2 alongthe north and 'r!ure well-developed in their upper parts, are bro- 1. \\ c amphitheaters northeastsides of Kauai(Figures I and 2), andmuch of this ken into large blocks in their midsections,and terminate in dr rt ill hcip terrain is characterizedby acoustic returns suggestiveof an apron of distinctively hummocky terrain. The first type lft)gfanl\ !\l: hummockyor blocky material.The marginsof continuously termed slumps appear to have moved slowly and r idual lani' are and hummockyterrain extend as far as ll5 km from the island, debris avalanches ge.are gi\ ct: intermittantly. The secondtype are called and areas of more isolatedblocks and hummocksappear rapid failure. and commonly representa single episode of 20-40km fartherseaward particularly on the northwestside. lr be diride c The irregular edge of the hummocky terrain apparently 'ihcatirln .'t Slide 1: North Kauai Debris Avalanche resultsfrom accumulationof overlying sediment.A broad gencr.r. I J\ a The volcanoes on Kauai and Niihau completed their ridge,possibly constructed oflarge landslide blocks, extends 1nc featLlrc. shield-building stage about 5 Ma and are the oldest volca- 120km northeastfrom the northside ofthe islandto beyond .,, 1[g lrpttuj

iliiip' ffi'' t7,470 Moonr er eL.: SuguenrNr,LeNoslroes. Heweu

8 offshore. The horizontal nature of this postslide bench 3' 1.50 indicates that it is a sea level feature carried down by E6 regional subsidence. Hence slope failure took place when .Y the island stood at least 900 m above its present level and J .c4 was about twice as large. Therefore subaerial parts of the .9 island were involved in the failures. Likewise, much of the o canyon cutting was subaerial, but extension of the canyons 2 below the 900-m-deep bench may have resulted from sub- marine erosion processes.

120 15 E7 Slide 2: South Kauai Debris Avalanche 100 o .1 E South ofthe island of Kauai is a broad area ofblocky and J80 4a s10 hummocky terrain, about 80 km wide, that extends 110 km d60 offshoreand covers an areaof6800 km2 {Figures 1, 2, and 5). !l '2 This deposit was apparently derived from a seriesof debris 3' .5 05 E40 .8 avalanches originating from both the south flank of Kauai '11 12 o- volcano and the east flank of Niihau volcano on the west. 16 .a e .13 . 14 .. The eastern part of the deposit is apparently overlain b! lavas belonging to a younger volcanic field between Oahu 30000 and Kauai. The west part of the deposit is augmented b1 slope failures from the steep southeast flank of Niihau. Linear sediment courses, one heading offshore from the 20000 Makaweli Graben, trend down the south flank of Kauai to Area the upper limit of imagedavalanche deposits (Figure 5). The km2 axis of the landslideis marked by a broad longitudinal ridge 10000 that crossesthe axis of the Hawaiian Deep and forms a lori welt separatingtwo basrns. o" gt . Block size as seen on GLORIA images varies from pixel a att dimensions(50 x 125m) to massesmore than 5 km on a side 0 100 200 300 Most recognizableblocks are 500 m to 2 km in size, and Length,km commonly stand 50-100 m above the general surface a' in records. Fig. 4. Length versus height, width, and area for large Hawaiian shown 3.5-kHz echo The average maximum slumpsand debrisavalanches identified by number in text and Table sediment thickness on top of the blocks based on 3.5-kHz l. Height is measured from toe to sea level; lines show overall echo records is about 25 m, assuming that this postfailurc. gradient in degrees. Width of the head of the landslide and area are acousticallytransparent material has the same sound veloc- measuredbelow sea level. 'lhis ity as water. is the greatestsediment thickness seenon slide debris reviewed in this study suggestingthat this slidc is among the oldest. the axis of the Hawaiian Deep (Figures l and 3). Hummocky terrain is not recognized on much of the southeast side of this ridge, perhapsbecause of deep burial by turbidites Slide 3: Kuenu Debris Avulonche moving down the volcanoflank and pondingbehind this ridge. The Kaena debris avalanche heads in a broad amphithe- The upper submarineslopes off north Kauai are scalloped ater 40 km wide on the north side of the Kaena Ridge, thc by three amphitheaterlikeindentations, each of which prob- submarine ridge extending northwest from Oahu toward ably representsthe headof a major slopefailure (FiguresI Kauai (Figure l). The landslide moved north more than 9ti and 2). The compoundnature of the landslideis further km into the axis of the Hawaiian Deep (Figure 2) where rt suggestedby the greaterwidth of the deposits,when com- produced a low swell betweentwo deeperbasins. The upper paredwith othersof similarlength (Figure 4). The northeast- part of the slope failure deposit, below the l-km-high amphi- ern scallopfed the smallestlandslide that movednortheast. theater headwall, contains tilted fault blocks whose lon-r: The central scallop is the headwall of a major landslide axes are normal to the direction of movement. One such whichmoved north and is characterizedby themost distinc- block is l2 km long and 5 km wide and standsmore than 3(Xt tive hummockyterrain. The northwesternscallop is appar- m above its base [Brocher and ten Brink,1987, Figure 8,]. ently the amphitheaterof a third landslide,represented by Beyond the blocks, the hummocky nature of the deposit i. hummocky terrain extendingat least 140 km north- subdued and poorly resolved in the GLORIA images. The northwestfrom the island. individual hummocks are generally widely spaced, suggesl- This northwesternamphitheater (offshore from the Napali ing that the landslideis older, and more deeply sedimented, coast area) is scored by several submarinecanyons that than the neighboring Nuuanu debris avalanche to the east. extendfrom about200 to 2300m depthand can be projected The feature shows some characteristics of a slump; it is upslopeto major subaerialcanyons IShepard ond Dill, 1966]. comparatively wide relative to its length (Figure 4) and The submarinecanyons begin about 3 km offshoreand are meets the deep seafloor at a steep zone more than I km high best developed above a 900-m-deepbench about 8 km (Figure 3). The slope failure may have originated as a block Moone er aL.: Sust\4entNpLaNosLrors. Hewll t7,471 lide bench I do*n br llace * hen l! le\ el and \arts of the nuchof thc he canlonr I from sub,-

t,lockvand ndsI l0 km l.l.and5t :r ol'dehnr k ol' Kaurr n the \\c\t .rrerlaintr r\een Olhu .i.. lmentedf'r s.' u] rrow:i of Niihau L.\ L.rvr votc.,"o: r'from thc ffil_lo,vefils L .rf Kauai tr' 4 f. 1 cr-ct t :ure5 t. l.hc rdinalridgc I I {'o,;:;'t,r,. i_ I (tl.t rri; ll()w1 \)fms A l()\\ i1,l.l seo,,"cnt ,- -, I ().rlhw.rv:; t-romprrc: 160" n on l \tdc' n '.izc.irn.t Fig. 5. Tracing from GLORIA image showing contrasting features of the South Kauai debris avalanche and the \urfttcc i.r. Waianae slumo. ma\imunr on 1.5-kHz general po\tlililurc. slump which partly disintegrated to form the hummocky The steeptoe, large width-length ratio, and lack ofa rund r cltre- debris avalancheapron. hummocky surface indicates that it moved as a few large 3\\ \CCnr)n blocks rather than as a mass of broken fragments.However, failed near the center producing the rt this slidc Slide 4: Waianae Slump the steeptoe apparently smaller debris avalanche which moved l5 km farther out The Waianae slump heads in a 130-km-widezone extend- from the main slump mass. ing from the Kaena Ridge northwest of Oahu to a point west of Penguin Bank (Figures l, 2, and 5). This feature was Slida 5: Nuuanu Debris At'olanche originally calledthe West Oahu Giant Landslide IHussong et amphilhc- al., 19871.However, to avoid confusion with other Oahu The Nuuanu debris avalanche covers 23,000 km2 (the Ridge.thc mass wasting features, we call it the Waianae slump. be- largestlandslide mapped), on the northeast flank of Koolau ,hu torr ard cause of its location primarily on the flank of Waianae volcano, the shield comprising the easternhalf of the island .)rc than 9(1 volcano which makes up western Oahu. However, the of Oahu (Figures l,2, 6, and 7). The debris avalanche is I t ri herc it southeastern part of the slide apparently belongs to the about 230 km long as measured from its headwall at Nuuanu The upper presumablyyounger eroded and submergedvolcano forming Pali on Oahu to its toe half way up the southwest flank of the righamphi- Penguin Bank, and the northwestern part may lie on a third Hawaiian Arch. In crossing the Hawaiian Deep, the debris r hose long volcano forming the Kaena ridge. The great width of the avalanche (and the Wailau sister avalanche to the east) One such head of this slump (Figure 4), together with the fact that it produced a welt acrossthe deep bounding two depressions, re than -3(X) apparentlyis rooted on more than one volcano, suggeststhat the Kauai Deep on the west, and the Maui Deep on the east Figure81. multiple slide events occurred. The surfacefeatures seen in (Figure l) IWilde et al., 19801. c depositir GLORIA images, however, do not provide a basis for After crossing the axis of the Hawaiian Deep, the ava- 'fhe nages. subdividing the slump. lanche moved uphill a distance of about 140 km from a d. suggest- The slump extends about 90 km offshore and covers an presentdepth ofat least 4600m in the deep to its terminus at edimented. area of 6100 km2. Large transversescarps or ridges tens of about 4300 m on the arch (Figure 6). Hence the vertical o the east. kilometers long and spaced 5-20 km apart divide the slump upslope transport is greater than 300 m. This does not lump: it is mass into several large fault-controlled blocks (Figure 5). consider the thickness of the slide in the deep which will ure 4) and The slide generally lacks hummocky topography except in increasethe upslopedistance, nor does it considerthe extent r I km high one lobe at its central, distal part. Most of the slide toe is of postavalanchingridge subsidence,downwarp of the moat. as a block steep and meets the adjacent seafloor abruptly (Figure 3). and uplift of the arch. t7,472 Moonr Et AL.: SuBMARTNELANDSLTDEs. He.warr

0 averagemaximum thickness of sedimenton theseblocks (as \ Nuuanu 5 E ,\ determined from 3.5-kHz echo records) is 9 m. Y. Longitudinal bathymetric profiles 3 h that show the size and ''.... height of the large blocks in the central part of the Nuuanu o n -5 debris avalancheled Moore [1964]to postulatethat the bases of the large blocks probably extend somewhat farther below 0 the general surface than they rise above it. This model .1 Wailau 6 E suggestsa thicknessof about 2 km for the landslide.Insights J on debris avalanchethickness are also obtained by compar- t ison of longitudinal profiles down the axis of the avalanche o -5 with parallel profiles on the side (Figure 6). These profiles -5 suggestan averageminimum thickness ofthe central part of 0 50 100 lso 200 Distance,km the avalanchebelow the amphitheaterofabout 400 m and an averageof perhaps 200 m over the entire landslide area. Fig. 6. Longitudinal profiles from Nuuanu and Wailau debris A detailed seismic reflection and refraction line crosses avalanches.Dotted prolilesare on northwest margin of Nuuanu and the western part ofthe debris avalanche east margin of Wailau landslides.Projected trace of Hawaiian Deep about 100km north axis shown by triangles. See Figure 2 for location. Vertical exas- of the east cape of Oahu near the axis of the Hawaiian Deep geration is 10. (fine ESP-9of Watts cl a1.il9851 and Brocher und ten Brink ll987l). l'his 100-km-longeast-west line extends from thc eastern part of the Kaena debris avalanche deposit to thc. western part of the Nuuanu deposit. Analysis of the seismic It is important to note, however, that the deep and arch returns inclicatesthal the total moat fill is about 2.6 km thick. continue around the southeasternend of the Hawaiian Ridge but a higher-vclocitylayer (0.9 km thick) is sandwiched ringing the youngest volcanoesof the chain on the island ol' betweentwo lower-vclocity layers (a L-5-kmlayer below and Hawaii. Because the depth interval betwcen the deep axis a 0.2-km layer above).We regardthe higher-velocitylayer a' and the arch crest off southeast Hawaii is also several landslidematerial, which overliesand is overlainby prefail- hundred meters, a similar slope probably existed off north- ure and posttailuresedimcnt (probably moat-fillingturbidirc east Oahu, at the time of slope failure, and the Nuuanu material): il'so, the averagethickness of slide material herc avalanchemust have traveled up that slope. The momentum is 900 m. Of the fbregoing thickness limits, we favor thc necessaryfbr the debris to ride up such a slope requiresthat conservative estimate of 200 m over the entire area of thc it moved rapidly. A fast moving submarine lanclslideof this slopefailure (23,000 km: ), which yieldsa volumeestrmate ol size probably produced strong bottom-scouring currents as aboul -5000kmr fbr the debris avalanche. well as large surface waves. In the regionwhere the Nuuanu debrisavalanche and thc Two gravity cores taken about 130km north of the north neighboringWailau avalanchc on the easl meet, the major limit of the blocky tacies imaged with GLORIA (ar elongate blocks are more nearly normal to the long axis ol 24"04.9'N; 'N; 157"04.7'Wanlava yet thin, blocks requires that they slid to their present of the North Arch volcanic field and in turn are overlain br position. younger lava of the same volcanic field (Figures 2 and 71. The distal 50 km of the debris avalanche deposit is Therefore age constraints for the North Arch lavas will well-definedon the GLORIA images which show scattered better bracket the agesof these slope failures. blocks ranging up to about I km in size (Figure 7). The Severalsubmarine canyons incise the headwall area ofthe Moonr ET AL.: SuBMARINELANDSLTDEs. HAwArr 17,473

!N\ l.i\ 40 KM g2a t

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- ,'if.! Lrll!C '.ri LlfC. .lchrtr .,r'rhc . \u- r: li1\ ti .iln bV -). 1J . \\ ill Fig' 7 ' Mosaic of GLORIA sonographs showing the Nuuanu and Wailau debris avalanches. Light shades designate regions of greater acoustic backscatter. The apparent elongation of hummocks parallel to ship iracks is an artifact caused by yaw of the towed vehicle. trl'th€ 11,474 Moone Er el-.: SusvenrNe LlNosI-tops, Hlwnrr

Fig. 8. Bathymetry north of Molokai from A. Malahoff and C. D. Hollister (written communication, 1987);contour interval 100m. Margins of the Wailau debrisavalanche shown by heavy dashedlines and trend of dike swarms is shown (shown by double dashed lines [from S/earn.s and Ma<'donuld, 1947]. Note that major canyons above sea level by curved |ines) are commonly aligned with submarinecanyons. The postslideKalaupapa volcano (K) grew in the slide amphitheater after subsidence was complete and partly filled the upper part of adjacent. submerged canyons'

Nuuanu debris avalanche (Figure 2). These submarine can- water depth provides someconstraints on the ageof the slidc yons feed sedimentdown the slopewhere it has accumulated (Figure 8). This terrace extends completely acrossthe heacl- to produce a 5OO-m-thickdeposit behind and between the wall amphitheaterof the Wailau landslide, and caps a scarfi blocky seamountswhich are a part of the upper landslide about I km high lSheportl und Dill, 19661.The nearlr deposit. The fact that the northwest canyons are clearly horizontal nature ofthis bench indicatesthat it is a sca levcl associatedwith land canyons led Andreu's and Buinhridge feature that postdates the debris avalanche. Hcnce thc |9721 and Coulbourn et al. |9741 to conclude that the failure occurred whcn the volcano stood at least 1300 nr canyons were carved as subaerial canyons. and subse- higher than a1 present, or nearly 3 km above sea level. Thc quently drowned during subsidenceof the Hawaiian Ridge. terracewas either built by ( l) fbrmation ol'a lava delta during This subsidencehas been as much as 1800m becausethe the end of shield-buildingactivity of East Molokai volcano. canyon morphology is well-developed to that depth [An- or (2) growth ofa particularly vigorous carbonatereefduring 'I'he drews and Bainbridge,19721.Because the canyonsare cut in a period of stable shorelineconditions. rccf hypothesit the upper part of the slide scar, we infer that the ridge stood is supportedby the appearanceon the terrace of mounds in at least 1800m higher at the time of failure. echo records that are interpreled as reefs IMathev'stttt. The 40-km-long,northwest trending dike swarm of Koolau l 9701. volcano lWalker, 1987] at the head scarp of the debris The 1300-m-deepterracc is about 800 m dceper than thc avalanche is aligned normal to the downslope direction of tilted H terrace which extends liom east Molokai to easl movement of the avalanche. Although the dikes generally Maui lMoore, 19871and is about -500m deep east of thc tend to dip outward on the two sidesol the swarm, most dip 'lhis Wailau landslide headwall. This H terrace can be traced part of the swarm. dip was northeast at the southeastern south of east Maui, where it is about 2 km deep. A compar- perhaps controlled by gravitational failure and removal of ison of the depth of this terrace with thclseof dated reefr support from that side of the volcano. west of northern Hawaii, which are currently subsiding at about 2.5 mm/yr, suggeststhat the H terrace was formetl Slide 6: Wailau Debris Avalanche about 850 ka fMoore ancl Campbell, 198'71.lf the Molokai The Wailau debris avalanche,named for a major canyon subsidencerate was also 2.5 mm/yr during the period be- on the north side of Molokai Island, has removed the north tween formation of the 1300-m Wailau terrace and the H part of East Molokai Volcano and left a well-defined amphi- terrace, then the estimatedage of the Wailau terrace is 320 theater 40 km wide at its head (Figures l and 7). The debris thousand years older than the H terrace or about 1170ka. avalanche apparently overrode the south side of the older and the landslide somewhat older. The landslide could not Nuuanu debris avalanche. and the extent of its distal facies have been much older than this terrace, however, because and its area is unknown becauseof our inability to distin- postfailure volcano growth has been quite minimal. The guish debris from the two features with available data. A estimatedage of the Wailau debris avalancheis 1400t 200 known fact, however, is that the largest block (Tuscaloosa ka, which is in general agreement with K-Ar ages of lavas Seamount, Figure 1) in this crossover zone is not a part of that mark the end of the shield stage of the East Molokai the Wailau debris avalanche but belongs to the Nuuanu volcano about l-500kalClague and Dalrymple,19871. avalanche, as based on the composition of lava dredged from The landslide scar is largely unfilled except that the small it (M. S. Pringle, written communication, 1988). Kalaupapa volcano has grown within the upper part of it A regional terrace that abruptly steepens below 1300 m (Figure 8). This small volcano (which has been dated by Moonr er eL.: SusN4lntNELANDSLIDES. Hlwelt t1 ,475

K-Ar at 570-340ka [Clague et al., 1982])grew after com- southeastern segment of the slide is apparently more sedi- pletionof the 1300-msubsidence of the HawaiianRidge that ment covered than the northwestern segment and is regarded followedlandsliding. The lack of post-Kalaupapasubsidence as the older feature. is indicatedby the fact that a notable slope changeon the \->< volcano flank that presumablyformed at sea level is now Slide 8: Clark Debris Avalanche only slightly below presentsea level lMoore, 1987]. S - The 6100 km2 Clark debris avalanche can be traced from i. The presentposition of the greatnorthern sea cliff of East west of Lanai south for 150 km (Figures I and 2); its lower Molokaiis not entirelyof landslideorigin. Those parts of the part divides into two lobes which flowed on either side of cliffthat flank the seaseem to stand,on average,about 500 Clark seamount, a 1500-m-highvolcanic ridge (apparently of m south of the part which is behind (and protectedby) the Cretaceous age) situated beyond the axis of the Hawaiian Kalaupapavolcano. A similarrecession (and heightening) of Deep. The landslide heads in the steep southwest facing the sea cliff due to marine erosion must have taken place amphitheater west of the island of Lanai and is flanked on during the approximatelyone million years after the debris the north by Penguin Bank and on the south by the west 20 and before growth of the Kalaupapa avalancheoccurred plunging ridge south of Lanai. The head of the debris volcano. avalanche is apparently marked by the major northwest Several major submarinecanyons occur in the Wailau trending system of normal faults which transectthe island of debrisavalanche amphitheater and most of them head off- Lanai fstearns. 19401.These faults, which are dominantly shorefrom majorcanyons above sea level on EastMolakai downthrown toward the southwest,probably mark the upper volcano (Figure 8) and Dill, 1966',Mathev'son, [shepard limit of the region of mass failure. 19701.Some of thecanyons cross the Wailau1300-m terrace The debris avalanche separatesinto two branches 80 km and extendseveral hundred meters below it. The canyons from the source region. The eastern branch, which moved were chiefly carvedby subaerialstream erosion before the almost directly south, extends an additional 80 km to termi- volcanosubsided about 1300m to its presentdepth. Subse- nate on the lower slopes of Perret and Jaggar seamounts. quently, some submarineerosion extendedthe canyons The western branch extends60 km southwestand ends after below the terrace level, supportingthe notion that the I thc.it.:r flowing between Dutton and Clark seamounts. These two Wailau terrace is composedof softer sedimentand coral, thc hc.:.:' branches may actually be two independent debris ava- r\ \!,i:: ratherthan harderlava. .r lanches with the eastern branch originating from the north If the Wailauterrace indeed is composedof sedimentor 1C nC.ri wall of the amphitheateron the south flank of Penguin Bank then the questionis raised as to why sea level r .ct.r icl i coral, and the western branch originating to the east on the west remainedfor sucha long periodat this levelduring terrace Icnec t: - flank of Lanai volcano. '-' construction?A possibleanswer is thatthe ongoing isostatic ,l l.ltxt Most of the surface of the debris avalanche in water subsidenceofthe volcanocaused by theaddition ofvolcanic crcl. |:.- deeperthan 3.-5km is characterizedby a speckledpattern on material to the ridge was temporarily interruptedand re- 'll.r .l Ltt ttt - GLORIA images,indicating hummocky terrain with individ- versedby the rapidand massivetransfer ofmaterial from the | \ (rla.rl1,, ual blocks generally less than a few kilometers in size. crestto the flankof the ridgeby masswasting. Perhaps this ccl rltlrttl- Maximum sedimentthickness on theseblocks averages9 m, masstransfer caused the ridgeto stabilizefor a periodbefore \ p(rthC.i. suggestingthat the slide may be roughly the age of the resumingits subsidence.Because the debrisavalanche vol- nt\Und. I:' Nuuanu slide which appears to be mantled with about the umeis roughly1000 kmr (assuminga thicknessof 75 m over jii tt rr ',,, same amount of sediment. of 13,000km'), it representsl0 ka oi Hawaiian an area The steep scarp west of Lanai at the upper part of the hotspotvolcanic production at the currentrate o[ 0.I kmr/ :'thun tnr feature is surmountedat a depth of 1200m by gently sloping yf. .ti lrt c.r.i terrain that is the subsided subaerial surface of Lanai vol- gentle ,r.l ol' thc cano fMoore and Camphell, 1987,Figure 2l' On this r\c lreec!: Slide 7: Hana Slump surface is a steplike series of five coral reefs. This general surface apparently was constructed by subaerial, postava- \ a()nlplri' The Hana slumpis centerednorth of the eastcape of Maui lanche volcanic activity, after which reefs successivelygrew .rtcd rccl. on the northeastflank of Haleakalavolcano (Figures l and as the region subsidedbelow sealevel. The lowest and oldest f'.iding.'l 2).The 4900km2 slumpseparates the two deepestsegments reef, at a depth of 1000m, has been estimatedto be about 650 ,r. tbrntc.: of the HawaiianDeep, the Maui Deepon the westand the ka fMoore antl Campbe1l, 19871, which is therefore a .'\lolokrrr the east e/ c1., 1980].The Hawaiian Trough on lWilde younger limit on the age of the Clark slope failure. This lcnod hc- northeastdirected feature shows no hummocky surfaceon observation requires a revision in the assumption that the rnd thc fl images.Instead, it is characterizedby subdued GLORIA wave-deposited gravels on Lanai (dated at 105 ka) and rcc is ll(l transverseridges, probably reflecting deeply sedimented similar deposits on adjacent islands were deposited by a t I 170kl. block slide scarps,and a steeptoe area (Figure 3). At the giant wave caused by a landslide (here named the Clark ar)uld n()t headof the slump,two broad amphitheatersseparated by a debris avalanche) on the ridge flank southwest of Lanai r. hccau\c low ridgesuggest that two slopefailures may havemerged to 'l'hc fMoore and Moore, 19881.Age relations indicate, rather, rmal. producethis feature. * that the giant waves probably were produced by movement .r00 l(x) The western amphitheateris flanked on the north by a of the Alika debris avalanche west of the island of Hawaii r\ ()f lava\ northeasttrending ridge that may representan olderrift zone lLipman et 41., 19881. .t \ilolokat ridge. South of this ridge, the slump surfaceis definedby e87l. repetitivenorthwest trending, regularly spaced linear ridges Slide 9: Pololu Debris Avalanche t the small 2-8 km long and 1-4 km wide. The slumpsurface, below the part of it easternamphitheater, is markedby larger northwesttrend- Kohala volcano, which forms the northern peninsula of r dated b1 ing blocky ridges up to 25 km long and 8 km wide. This the island of Hawaii, has undergone a major slope failure 17.476 Moonn rr er.: SugvenlNE LANDSLIDEs.Hnwett

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Conlour l6lcrv.l IOO m.r.rr

Fig. 9. Bathymetric-topographicmap of the upper part of the Pololu debris avalanche(margins shown by dashed line). Also shown are the edge of Mauna Kea lava flows lapping onto Kohala volcano (dash-dottedline), the Kohala shelf break (open triangles),and the Mauna Kea shelf break (solid triangles).Bathymetry south of 20'22' contoured from National Oceun Survcvl l98l I and north of20'22' from Cunpbcll l l9U7l.and land contours from USGS I : 100,000 1980topographic map. Contour interval above and below sea level is 100m. that covers 3500km'ofits east flank (Figures1,2, and 9). beyond the mouth of the Kohala-East Haleakalatrough and 'lhe This featureis calledthe Pololu debris avalancheafter Pololu produced a broad lan in the Hawaiian Deep. landslidc Valley, a major subaerialcanyon in the northwest part of the can be traced a total of 130km from the shoreline to its toc headwall area of the feature. The landslide amphitheater by means of the subtlc hummocky character shown on extendsfrom above sea level down to a depth ofabout 900 m GLORIA images. and producesa marked reentrantofthe shoreline20 km long The age of thc landslidecan be estimatedfrom its relation and extending2 km inland (Figure 9). The submarineamphi- to the abrupt change in slope of Kohala volcano at about theater is lO-20 km wide and its central depth is 150-400m 1000 m depth (Figure 9). This slope change or terrace i: below the regional slope, causing the bathymetric contours believed to have formed at the point in the evolution of thc to be displaced shoreward about -5km. volcanowhen shieldbuilding ceased, that is, when copious The upper part of the amphitheater is incised by four lava flows no longer crossed and remade the shoreline. submarine canyons lMoore, 19871that can be traced on Subsidencerates and model agesof coral reef.s indicate that availablebathymetric charts to depths somewhat below 400 this slope break was formed (at sea level) about 170 ka m. These canyons are directly on line with prominenl fMoore und Cumpbe11. 19871. l'he debris avalancheappears subaerialcanyons on Kohala volcano. It is notable that the to be slightly older than the formation of this terrace because submarinecanyons occur only within the landslidereentrant the terrace is partly constructedacross the landslidesurface. and the large subaerialcanyons occur only upslopefrom the particularly on its eastern side (Figure 9). The terrace reentrant. Submarine canyons are not known to occur apparently formed after slope failure when subaerial lava elsewhereon this volcano. flows erupted from southeastern Kohala (in a region now Downslope from the amphitheateran irregularbulge in the covered by Mauna Kea lava flows) crossedthe subaerialpart regionalcontours definesthe main landslidewelt. The debris of the landslide and extended the shoreline. The limited avalancheturns east where it enters the trough between the coverage of the landslide topography by these lava flows steepflanks of Kohala and the eastridge of Haleakalaon the indicatesthat mass failure took place not long before 370 ka north. The west flank of the landslide has dammed and and that the volcano stood 1000m higher (or about 2.7 km isolated an unfilled depression, l3 km long, 6 km wide, and above sea level) at the time of failure. about 800 m deep, along the axis of this trough (Figure l). Kohala volcano is mantled by the postshieldHawi Volca- This major trough, within which the depressionoccurs, must nics which unconformably overlie the older shield stage be an important sediment pathway. but surprisingly, the Pololu Basalt. K-Ar agesdetermined on the Hawi Volcanics closed depressionhas not yet filled with sediment. are 261 + 4 ka to 6l + I ka, and on the Pololu Basalt.459 t The lower part of the debris avalanche extends 60 km 28 ka to 304 t 9l ka fMcDougall and Sv,anson, 1972]. The Moons r.t nr.: SugvlntNE LANDSLIDES.Hlwltt l'7.41'7

terrain is marked by benches and t55'r: Pololu debris avalanche apparently occurred during the late The offshore slump particularly steep shield stage during eruption of the Pololu Basalt; Pololu scarps fLipman et al., 19881and shows a and Valley itself, carved in the landslidescar. is partly filled by a regional slope near the base of the volcano flank fMark late Hawi lava flow. Hence the age estimated from the Moore. 19871.This broad, oversteepenedslump has under- produce morphology and relations ofthe 1000-mterrace is in accord gone younger local collapse in four places to over- kilo- with available radiometric ages of Kohala lavas. riding debris avalanchesthat have moved out tens of the The end of shield building is considerably younger on meters beyond the base of the slump to cross the axis of Mauna Kea volcano than on Kohala. A submergedterrace Hawaiian Deep. These include the Alika debris avalanche and 7-15 km wide occurs for a distance of 70 km along the (phasesI and 2) in the center of the slump and the west margin of northeast flank of Mauna Kea volcano. The depth of the east Ka Lae debris avalanchesnear the southern shelf break is 37-5* l5 m which corresponds with oxygen the slump. isotope stage 6 ending at 128 ka fMoore and Cttmpbell. 19871.This terrace (in addition to the older 1000-m-deep Slides1l ond l2: Alika DebrisAvulun<'hes has also developedacross the southernpart Kohala terrace) The Alika debris avalanchesare probably among Hawaii's of the Pololu landslideamphitheater tbr a distanceof about youngest lLipntan er 41., 19881.These two partly overlap- (Figure terrace overlap apparentlyresults from 5 km 9). This ping debris avalanches, designatedphases I and 2. cover growing Maun:r Kea shield spilling acrossthe lavasfrom the 2300 and 1700 kml, respectively (Figures I and 2). They east flank of Kohala. These Mauna subsided. slide-scarred moved nearly 100 km from their source near the prescnt also apparently filled and covered the submarine 'fhc Kea lavas shoreline to depths of 4800 m in the Hawaiian Deep. Waipio valley, the prominent easternmostKo- extension of cstimated thickness of the debris avalanchcs is -50-200m h:rlacanyon. basedon the sizc ol'the blocks and the heightofthe lcvees, systcm of normal faults about A major northwest trending yielding an estimatcd volume ibr both tirilures together of near the summit of Kohala mountainhas been l3 km long 200-800km'. trncl Mu

rsr'6o'

a v

tg' 20',

-\ ta\ \ N \

\l ll l\ U A lt \l \\

o246 l--l-- | t VECTOR SCALE

-\

Fig. 10. Bathymetric-topographic map of the Hilina slump area. Bathymetry from National Ocean Survey ll98l, 19821and land contours from USGS I : 100,000topographic map. Contour interval is 200 m. Heavy dashed line is margin of Hilina slump, and dotted lines bound smaller overriding Papa'u and Loihi landslides with movement direction shown by short arrows. Dash-dotted line is boundary between Kilauea volcano and Mauna Loa volcano to north. The epicenter ofthe magnitude 7.2 (1975)earthquake is shown by star, and dashed arrows are geodetically measured vectors showing horizontal movement associated with the earthquake fafter Lipman et al.,1985]. A-A' indicates location of section of Figure I l.

southmostpoint of the island of Hawaii) and moved down field at its terminus suggeststhat this may be one of the the slopeon either side of Dana seamountto the Hawaiian youngestdebris avalanches imaged by GLORIA. Deep (Figures I and 2). The western Ka Lae debris ava- lanche (850 km2) extends 85 km south between Day and Slide l5: Hilina Slump Dana seamountsand spreadsout in the HawaiianDeep as a broad field of faintly hummocky terrain. The east Ka Lae The Hilina slumpis an active landslidethat involvesmost debris avalanche(950 km2) moved 75 km down a narrow of the southflank of Kilaueavolcano (Figures l, 2, and l0). chute betweenDana seamounton the west and the steep The slumpcovers about 5200 km2, an areamore than 3 times west slopeof the southrift zoneridge of MaunaLoa volcano that of the volcanoabove sea level. Its locationon an active on the east. The excellent preservationof the hummocky volcanoatop the Hawaiianhotspot placesit at the locus of Moonr er e.L.: SusNI,{nINeLlNosLIors. HlwltI 17.479

A, 5

HILINA FAULT SYSTEM ) -=..=rE;;=; 0

)< -5 z.) r ,/t 1 ,4 tU Pre-Mauna Loa oceanic crust -10 t, tl -t3 n204060 DISTANCE.KM Fig. ll. Section through Hilina slump with no vertical exaggerationmodified frctmLiprnun el 41. [198-51.Location l of section shown in Figure 10. Dashed pattern forming base of volcano is subaeriallyerupted clastic rocks inclucling i phreaticexplosion ash, litloral cone ash. flow fbot breccias.minor pillowed flows. and pillow breccias,landslide debris. and turbidity flow deposits.Most of this materialwas probably erupted from Mauna Loa except for the uppermostfew ')..] kilometers. Lined pattern forming upper part of volcano is subaeriallyerupted tholeiitic basalt. predominantly thin (2-10 m thick) pahoehoeand aa lava fl6ws. but includingash layers. Lava from Mauna Loa predominatesover that from avalanchefavored by steepregional -l Kilauea in north part of section. Dotted line indicatespossible sole of future debris slooe.

sustainedmagmatic. seismic, and hydrothermal activity, all subsided up to 3.5 m and moved seaward scver:rl mcters; of which probably influenceslope failure. This creepingzone morcover a lO-km scgment of the Hilina lirult system was is about 100km wide; it extendswest approximatelyto the reactivated.and a small summit eruptionbroke out I'fillirtg south rifl zone ridge of Mauna Loa and eastas fbr as the east ct ul ., l9'761.Geodetic mcasurements shclw that subaerial cape of the island. The slump is bounded at its upslope vertical and horizontal ground deformation rclated to the margin on the northwest by the southwest rift zone ot' quake was as much as 3.-5m down and 8 m southeast, Kilauea vcllcanoand possibly the Kaoiki fault system (which respectively(Figure l0). perhapsgreater bclow sea levcl. I separatesKilauea volcano from Mauna Loa volcano) and on Similar episodeshave occurred previously,as in 1823and I the northeast by the cast rilt zonc of Kilauca. The Hilina l86tt. If the 197-5displacements are represcntativc, the (downthrown fault system is a major system of normal faults long-termdeformation of the south flank of Kilauca must be I of to the south) parallelto the south coast and 5-10 km south dominated by cpisoclic large-scale gravitational slumping \ the north boundary ot' the sliding zone that apparcntly lTilling at ul., 1976 Liptnan c/ ai., 19851with a seaward definesa major dislocationin the slump (Figuresl0 and ll)' thc [,ove anci \ '[his movement of the ordcr ol'0. I m/yr. Analysis of slope is moving around the vast, actively sliding Rayleighseismic waves fiom the 197-5carthquakc led Eis.r/r'r t buttressformed by Loihi volcano and shows an anomalously untl Kunumortil9871 to concludethat the bulk of the seismic steep slope where it meets the flatter seiifloorboth west and I radiation was produced by large-scaleslumping of a part o[- eastof LorhiIMurkuntl Moore,1987].The southeastflank of I the south flank of Kilauea. the slide impinges on Hohonu seamount. an apparent Cre- The main hypocentcr of the 197-5shock, and mosl after- I taceous volcano from which weathered basalt has becn a shocks,were about l0 km decp,situated in the vicinity olthe recovered and Fiske, 1969].Marking the middle of [Moore prevolcanicseafloor (Figure ll), showing thal the cntire the slump northeast of Loihi is a conspicuous midslope volcanic mass down to thc top of thc Cretaceouscrust was bench 2600-2800 m deep, 15 km wide. and 60 km long probably involvcd in block slumping l()ntsscn antl Entlo, al. (Figure l0). The general backtilted nature of this bench. as 1982',Lipmun et ul.. 1985 Holcornh, l9tl7l. Much of thc gtn shown by a closed depressionon its shoreward side. sug- ,$n lateral movement may have been accommodirted within gests that it representsthe top of a giant rotational slump fhe poorly consolidated submarine volcanic rubble, material block (Figure ll). Alternatively, features such as these 0r\ which earlier in its history had already experienceddowns- t trf "bulges" lMark and Moore. 1987] may be giant folds, a lope sliding. and also within the pelagic sediment accumu- notion suggestedby the aseismic aspect of the middle and lated on the Cretaceousseafloor before Kilauea (and Mauna lower part of the Hilina slump [Cro.r.ren ttnd Endo, 1982]. began growing. South of the steep slump toe are three small elongate Loa) Although the measured horizontal displacementsare di- one of the seamounts (3-8 km long) which apparently represent the toward the unbuttressed south tops of buried slide blocks. The steep toe of the main slump rectcd uniformly seaward pattern vertical and horizontal zone is about 35-km offshore, but faintly hummocky terrain flank of the volcano. the of summit caldera continuesanother 40 km into the axis of the Hawaiian Deep. movements. on a smaller scale, reflects the partly the move- This subdued, irregular backscatter on GLORIA images and rift zones and apparently is related to ment of magma. The slumping results not only from the r olves mort probably representsmaterial depositedby early debris ava- gravitational instability of the volcano flank but apparently .l. andl0t. lanches, now largely covered by finer volcanoclastic sedi- forceful injection of magma into the summit than3 times ment. also from (Fig- rift zones et ul.,1976:-Lipmun et ul.. on an active Coincident with the 1975magnitude 7.2 earthquake reservoirand [Sx'anson the locus of ure l0). much of the south coast for a distance of 60 km l98,sl. 17.480 Moonr er er.: SusN{nntNeL,lNosLIops. He,weII

Slide 16: Papa'u Sand-Rubble Flov, ratio exceeds one. The slumps maintain physical continuitl . that is, they apparently The Papa'u landslide lFornari et al., 19791is a 200 km2 never broke loose and moved far from their source. Several lobate debris tongue directly south of Kilauea caldera on top lack well-defined amphitheaters. probably because they were of the Hilina slump (Figures l, 2, and l0). It is l9 km long, 6 continually fed at their head br addition volcanic km wide, and up to I km thick with a volume of about 40 km 3 of material during the period of episodic movement. that has moved downslope from near the shoreline to a depth The slumps are commonly marked by transverse ridges and scarps generally of 1850m. Dredge hauls, remote camera photographs,and that are bounded by faults bur perhaps submersibleobservations indicate that it is largely composed also in some places represent bulges or folds. Thc toe of the slumps of unconsolidatedangular glassy basalt sand and blocks up commonly forms a distinct scarp where ir meets the preslide to about I m in size. Because of the small size of the seafloor. Taking constituent fragments and the regular lobate shape, the the Hilina slump as a model, these slope failure. apparentlydeveloped during the active feature has been termed a sand-rubble flow lFornari et al., stageof shield build- t9791. ing, when rift zones were well established,and owe their movement The landslide was apparently fed from a vasr oversreep- not only to the gravitational instability of thc growing and widening ened, nearshore bank of fragmental basaltic material that shield but also to the lateral hydraulic jacking resultingfrom injection of magma into the rift zone, was generated as subaerial lava flows from the summit 'Ihe slumps region of Kilauea poured into the sea, quenched, and shat- move intermittently at a rate controlled by thc accumulation tered. The Iandslideappears to have been emplacedduring a of lava flows on their upper reaches,as well a. by magma intrusion singleflow event as indicated by its simple lobate shapeand in the rifl zones which mark the bound- ary between the fact that multiple small debris flows would have filled in the slumps and the more stableupper part of thc volcano. the saddle landward of the high point of the flow lobe. Seismic shaking contributes to the development ot' these Estimates based on the present lava production of Kilauea slope failures and is also a direct result of their movement. volcano suggest that the landslide was emplaced several Earthquake thousand years ago IFornuri et ul.. 19791. focal depths for the 197-5magnitude 7.1 Kilauea earthquakeindicate that the baseof the Hilina slumn is close to the l0-km-deep, pre-Hawaiian seafloor surface Slidc l7: Loihi Lundslidc.s Becausethe top of the oceanic crust dips under the volcant, Loihi is an active submarine volcano, growing againstthe at irbout3" lHill und Zu<'t.u,19871,the soleof the slumpmo,i south flank of Kilauea [Moore et al., 19821.The volcano is likely slopesupward at this angle, if indeedit is controlled hr elongatenorth-south and has a generallyflat summit about I the pre-Hawaiian ocean floor. Upslope movement on thi. km deep indented by several pit craters (Figures t. 2, and surface can only occur if the overall top of the slump dip. l0). The site ofthe volcano at the extreme southeasternend seaward more steeply. Measurementsof the averagedou,n- of the Hawaiian Ridge in close proximity to the Hawaiian slope anglesof all the slope failures show that generally thc hotspot suggeststhat it will develop into the ncxr ma.ior slumps exceed 3'. whereas the debris avalanches harc shield.The Hilina slump on the south flank of Kilauea is smallerslope angles (Figure 4). moving south against Loihi and divides around it on its The debris avalanches generally are longer and not a. course down to abyssalclepths in the Hawaiian Deep. thick as the block slumps. The amphitheater at their head Detailed bathymetric mapping utilizing multibeam sonar marks the source area from which the lower avalanchc 'fhe systemshas revealedseveral amphitheaters on the flanks of material was derived. debris avalanches typically in- Loihi (the "armchair indentations" of Maluhr[J' [1987]; see clude a long middle and distal train of hummocky debris alsoFornari el a1.[1988j), which are interpretedas landslide Blocks up to tens of kilometers in size may be presenr scars. Below these amphitheatersa zone of irregular, hum- immediatelybelow the amphitheater,and blocks l km in size mocky topography apparently representinglandslide debris are common in the distal part of the deposit. The depositsarc is evidenl, but these surveys did not extend far enough of variable thickness along their course! but the greate\r downslopeto map and characterizethe lower courseof these average thickness in the blocky and hummocky sectors is landslides.The GLORIA imagesof Loihi are of limited value probably 0.4-2 km, and the overall averagethickness may be becauseof the small size of the failures ancl their location as little as -50m in the smaller avalanches.Little debris mar near steep slopes in relatively shallow water. These land- remain in the upper part of the amphitheater,and the distal slides cover more than three-quartersof the volcanic cone part is thin and perhapstransitional to turbidity flows. Somc' above 2-500m depth and the slide headwalls are extraordi- of the debris avalanchesmay develop by partial or completc. narily steep, up to 40" in the first vertical kilometer (Fieure failure of the volcano slope already oversteepened by a l0). slump and therefore provide a mechanism to reduce the overall gradient of the slump and carry material farther out beyond the base of the volcano. Unlike the slumps, SrnucrunE AND NATUREoF LANDSLIDL.s mosr debris avalanchesprobably move at high velocity in a single general The two types of large Hawaiian landslides, episode,as shown by the uniform mound size and spacingin slumps and debris avalanchesfVarnes,19781, range from the the distal hummocky facies and the fact that some ava- more coherent to the more disaggregatedforms respectively. lancheshave moved uphill several hundred meters. No doubt a whole spectrum of intermediateand compound landslidesexists between these end-members. LnNosr-torNc AND VoLCANo DEVELopMENT- The slumps are particularly steep and thick (up to l0 km) and are wide relative to their length (Figure 4). The overall Major landsliding occurs throughout the growth of Hawai- gradient generally is greater than 3", and the width/lensth ian volcanoes and apparently continues at a reduced rate Moonr, et nl.: SugventNE LANDSLIDES,Hewltt 17,481

'l continuity. long after dormancy. More than half the surface area of skeletal, shape with sharp ridges radiating from the center I moved far Loihi, the youngest volcano in the chain. is extensively (the "starfish" shapeofVogt and Smoot t19841).Submerged rphitheaters. modified by landsliding even though its summit is still I km 43 Ma volcanoes of this character fSmoot, 19851near the heir head b1 below sea level and the volcano is perhaps a hundredth the Hawaiian-Emperor bend (about 3500 km northwest from the l of episodic size of a mature shield. Hawaii) probably owe their shape to dense dike rock of '\ transverse Some ofthe largestlandslides occur near the end of shield rift zones exposed by downslope movement and removal of .r taults but building, when the volcanoes stand 2-4 km above sea level less resistant. weathered lava flow material between the riIl 'r folds. The and are erupting and extending their shorelinesmost vigor- zone dike swarms. Between the ridges deep amphitheaters :arp where rt ously. Mass wasting probably occurs, however. intermit- above and fanlike debris welts below apparently mark the tently throughout most of the shield-buildingstage with the site of landslideevents. type Iope failures slope failure deposits being incorporated into the edihce as it The widespreaddevelopment of landslidesof diverse .hield build- grows so that little surface evidence remains of their exist- along the submarine slopesof the Hawaiian Ridge suggests rd owe their ence. Fragmental landslide debris, associatedwith normal that analogous slides should be present on other oceanic bility of thc faults and slide slip planes. are believed to be important volcanoesconstructed in deep water. Comparablebathymet- la ral hydraulic elementsin the layered structureofthese volcanoesfrom top ric and sonar data exist only for a f'ew areas. Isle de Reunion. in the Indian Ocean, closely resemblesthe volca- re rift zones. to bottom. 'Ihe -olledby thc Some of the debris avalanches (Wailau and Pololu) are nic island of Hawaii. presenceof large-scalesubmarine the :.. as well iir characterized by prominent submarine benches I km or landslides on the east flank of Piton de la Fournaise' ul. k the bound- more deep which representa postfailureshoreline (Figures 7 active volcano on La Reunion, was inferred by Du.lJieldet of cr partofthc and 8). Such benches indicate that significantextension of tl982l, in part by analogy with the structural features ' elopment oi the shoreline by accumulation of volcanic and/or coralline Kilauea volcano in Hawaii. Recent Sea Beam bathymetric (SLAR .ult of thcir material occurred after movement. Additionally, they indi- mapping,high-resolution sonar imaging system;0.3- cate that after failure, major regionalsubsidence occurred. a m resolution),se:rfloor photography. and dredging along the rgnitude7.1 process that typically occurs only during. and shortly after submarine east flank of Piton dc la Fornaise has confirmcd -the material Hilinaslump the end of, shieldbuilding lMottre untl Cumpbell. l9tl7l. the widespread presence of submarine landslide Ioor sutfacc. apparent hiatus in subsidence,which allowed considerablc ll.cnut ct 41.. 19891.Hummocky terrain' similar to the km ' thc volcant, shorelineextension, may result from the rapid removal. by Hawaiian debris slide deposits, extends at least 40 which offshore to 2-500m depth. Photographedand dredged mate- "'slump most landsliding,of a substantialpart of the volcanic load Iava. :ontrolledh\ causesthe subsidencein the first place. rial consistsentirely of subaeriallyerupted, fragmented rrain nr'nt on lhit The subaerial structural f'eaturesof the volcanoes com- The full climensions of the large slumped submarine te yet c slumpdip. monly show an apparent relationship to the major slope on the eastflank of La Reunion have not bcen dclimited. shows cragedown- failuresof the submarineflanks of the volcanoes.Most of thc Additionally, the regional bathymetry fbr thc island (Piton generallythc major failures have moved in directions normal to the rift steepsubmarine slopes along older parts of the island gently .rncheshavc zones of the volcanoeson which they occur. The upper limit des Nciges) that grade down into a broad sloping is of most of the southward movement ol the Hilina slump' for apron on the lower slopes and suggestthat mass tailure and not a' example, is bounded by the east and southwestrifl zones of widespreadaround the cntire island. rt thcir head Kilauea volcano, and horizontal displacement vectors iire subperpendicularto the rift zones et rr1.. 198-5]. :r avalanchc fl'ipmarr RCU-IION OI.- L,tNIIST-IOES'I'O SUBMANINE C,INVONS tr pically in- Moreover, some volcanic activity is related to landslide Hawai- trckl debris. activity as shown by the 1877 submarine eruption in The known submarinecanyons offshore from the be present Kealakekua Bay that occurred near the north margin of the ian islands occur on thc upper part of the major debris . I km in size South Kona slump, the small Kalaupapa volcano growing avalanches.and typically, these canyons extend shoreward subaerial : depositsare within the amphitheaterof the Wailau debris avalanche,and to connect with major subaerial canyons. The the the greatest the 1975 Kilauea summit eruption apparently triggered by counterpartsof the submarine canyons include some of those of rl sectorsl\ movement of the Hilina slump. largest canyons of Hawaii lFigure 2) including \nessmay bc The Nuuanu. Wailau, and Pololu debris avalanchesand northwest Kauai. northeast Molokai (Figure 8), and north- c debris ma1 the Waianae, Hilina, and South Kona slumps (Figures I and east Hawaii (Figure 9). However, connection between sub- Lndthe distai 2) all head at, and have moved perpendicularto. the primary aerial and subsea canyons is everywhere obscured by a tlows. Somc' rift zones of their host volcanoes.Other slope failures. such shallow submarine shelf, a few kilometers wide adjacent to Lor complete as the Kaena debris avalancheand the Hana slump' head on the coast, in which no canyons occur. Seismic reflection -'pened by a the sidesof long, shallow submarineridges which apparently profiling has shown that buried canyons. filled by sediment' . reduce the represent submerged rift zones. This rift zonelgravity failure are present beneath the shelf and provide connections be- rl farther out relationship is part of the cause-effect paradox of the gravi- tween the subaerial and submarine canyons lCoulhourn et ,lumps,mosl tational instability of these volcanoes.Rift zones form along al..1974lr. tr in a single the tensional zones at the head of landslides. but also Submarinecanyons occur on the upper parts ofthe North nd spacingin landsliding is induced by the lateral shoving caused by Kauai. Nuuanu. Wailau, and Pololu debris avalanches' ,t some ava- magma injection into the rift zones. These canyons are developed mostly above the shoreline :!ers. Continued mass wasting long after volcanic activity has terraces. produced shortly after slope failure occurred. ceasedis suggestedby the modification of submarineslopes which are 800, 900. 1300,and 1000 m deep on these four on the older volcanoes of the Hawaiian-Emperor Ridge avalanches, respectively. Hence the submarine canlL)n\ \IENT toward the northwest. Detailed multibeam sonar bathymetry were subaerially carved to about their present depth alir.r th of Hawai- shows that the form of some volcanoes is modified by landsliding. Subaerial canyon cutting in the harcl bri'altt. reduced rate extensivemass wasting that has produced an angular,almost rock was promoted by the oversteepened' and rc.!.ntl\ 17,482 Moonn Er aI-.: SunM,cRtNeLeNoslroes, Hewett stripped, slopes of the upper amphitheatersof the debris F-1 avalanches, especially in the amphitheaters formed on the E windward (northeast) volcano slopes having high rainfall. Y-z Subsequent regional subsidence carried all but the upper parts of the canyons below sea level and permitted them to o-5 be alluviated in a coastal strip a few kilometers wide. ln from the sediment source generatedby deeper water, away 0 400 800 1200 1600 subaerialcanyon cutting and coastal erosion, the soft sedi- Distance.km ment filling the upper parts of the submarine canyons was Fig. 12. Bathymetric profile along axis of Hawaiian Deep pro- currents and turbidity flows, and some clearedby submarine ceeding clockwise from the north side of Kauai, around the island of submarineerosion extended the canyonsbelow the shoreline Hawaii. and northwest to the south side of Kauai (see Figure 2 for that existed initially after sliding. location). Humps produced by distal parts of landslidesare North Kauai (NK). Nuuanu (Nu), Wailau (Wl), Hana (Ha), Pololu (Pol. Alika (Al), Clark (Cl). Waianae (Wn). and South Kauai (SK). REI-ertoN on LeNnsltoEs t'o rHE HaweltnN DEEp

The Hawaiian Deep or moat is a bathymetric trough which flanks both sides of the Hawaiian Ridge about 100km from the ridge center and loops around its southeastend. The axis of the deep is representedby a seriesof closedbasins: the along the Hawaiian Ridge. Slumps and avalanches dccpest seafloorin the Hawaiian region, exceeding-5600 m. ^debris were found to cover nearly 100,000km' of the ridge and occurs in one such basin north of Maui. The deep is adjaccntseafloor fiom Kauai to Hawaii, an area more than -5 produced by the downwarp of the oceanic crust by the timcs the combined land area ol the islands. Some of the weight of thc volcanic ridge which pushesdown and outward individual debris avalanchesare morc than 200 km long and to produce a broad upbowing beyond the deep called thc -50(X)kmr in volume, ranking them among the largest Hawaiian Arch. The distance between the arch crests on about on E,arth.These slope failures begin carly in the history ol' both sides of the ridge is about -500km (Figure 2). individual volcanoeswhen they are small submarinesea- Virtually all of the debris avalanchcsextend outward mounts.culminale near thc end o{'subaerialshield building. acrossthe axiso1'the deep and hencehave apparently moved and continuc long aller dclrmancy. Hcnce mass-wasting uphill onto the flank of the arch (Figures 2 and 3). The and structurcsare no doubt an important clement in long-term efl'ectof major mass wasting on the flanks of the deposits thc internalmakeup of'the volcanoes. Hawaiian Ridge is to transport volcanic material outward Most of the landslidesflowed into, and locally across, the fiom eruptivc vents and thus widcn the ridgeand force the 'fhe axis ol'thc Hawaiian Dcep which flanks the Hawaiian R.idge. axis ol'the deep to migrateoutward. continucdsubsid- Therefore mass wasting is the primary mechanismfor filling cnce ol'the ridgecounters this efl'ect.narrows thc ridgeand the deep and widening the ridge. The eruptive behavior of' causesthe axis of the dccp to migrate inward. Hence the the volcanoes can be modulated by landsliding' and thc present configuraticlnof the ridge and deep are a balance subaerial geometry ol the slope failures is related to the between these two proccsscs. structural f'eaturesof the volcanoes including rifi zones and The expectedcycle of mass wasting during thc life history 'l'hc iault systems. landslidcs are of two general types. of' an individual volcano is lirr the largcst and most f'ar 'l'he slumps and debris avalanches. slumps are slow moving. traveled debris avalanchesto occur during the final stagesof wide (up to ll0 km). and thick (about l0 km) with transverse the shicld building cycle when volcanic productionpeaks, 'fhc blocky ridgesand stecp toes. debris avalanchesare f'ast and when the volcanoes stand highest above sea level. moving.long (up to 230km) comparedto width, and thinncr Before and alier that time smaller landslides that travcl (0.0-5-2km): they commonly havc well-defined amphithe- shorter distancesare expccted to occur. Hence we inf'erthat aters at their head and hummocky terrain in the lower part. an onlap-offiap stratigraphic sequence of landslide and re- Water disturbancescaused by the rapid movement of the lated deposits should occur on the inner flank of the Hawai- avalanchesprobably produced the high waves which ian Arch. The landslidedeposits that lap furthesl up the arch debris have lefl deposits on Lanai and adjacent islands. Most flank should be closest in age to the end of shield building of canyons were carved subaerially in the upper the host volcano. submarine amphitheatersof debris avalanches. Subaerial canyon cut- Where the landslidescross and partly fill the axis of the ting in the hard basalticrock was promoted by the oversteep- deep, they produce low ridges that compartmentalize the and recently stripped. nature of these amphitheaters. deep into a chain of separatebasins (Figures I and l2). The ened, differencesin depth betweenthe tops of theselandslide welts and the bottoms of the basins range fiom 0.4 to 1.5 km. primary on which this work is These depth differencesprovide a conservativeestimate of At'kruttlcdgntent.s.The resource basedis the GLORIA surveyof the U.S. exclusiveeconomic zone the thicknessof the landslidewelts where they have ponded from Kauai to Hawaii. We are indebtedto the crewsand technical in the deep (Figure l2). staffsof the M/V Furnellufor their outstandingwork during these surveycruises in 1986and 1988and have benefited from discussions with our colleaguesof the U. K. lnstituteof OceanographicSci- CoNcLustoNs ences.R. Belderson.J. Campbell,M. Harris,R. G. Rothwell,R. C. Searle.M. L. Somers.and J. B. Wilson. R. M. Carter, R. P. (GLO- A primary result of long range side looking sonar Denlinger.M. L. Holmes.and A. N. Shor helpedwith the data RIA) surveys of the Hawaiian exclusive economic zone was collectionand providedimportant geologic insights' We apprectate to document the enormous scale of submarine slope failure thoughtfulreviews by H. J. Lee, R. I. Tilling,and D. J. Fornari. MoonE ET AL.: SUBMARINE LANDSLIDES, HAWAII 17.483

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