Chapter 3 – paleotsunami research—current debate and controversies

Chapter 3 – paleotsunami research—current debate and controversies Chapter 3 – paleotsunami research—current debate and controversies Chapter 3 – paleotsunami research—current debate and controversies Chapter 3 – paleotsunami research—current debate and controversies Chapter 3 – paleotsunami research—current debate and controversies Chapter 3 – paleotsunami research—current debate and controversies

of debate that still exist, in particular the identification of paleotsunami sediments ingeoarchives, the discussion of boulder movements induced either by strong storms ortsunami waves, the enigma of impact tsunamis during the Quaternary, and modelingversus field observations in paleotsunami science.

3.1 INTRODUCTION

One of the earliest research papers that attributed sedimentary evidence topaleotsunami events is the publication ofAtwater (1987)and the detection ofthe so-called “Orphan Tsunami” of 1700 AD in Washington State, UnitedStates (with the source along the coast of Japan), and the article byDawson

et al (1988) on the submarine Holocene Storegga slides, west of Norway.Since then several reviews of tsunami research have been published (e.g.,

Dawson and Shi, 2000; Synolakis and Okal, 2005; Dominey-Howes et al.,2006; Tappin, 2007; Dawson and Stewart, 2007; Shiki et al., 2008; Synolakis

et al., 2008; Scheffers et al., 2009; Satake et al., 2011) The aim of thiscontribution is to highlight some aspects of palaeotsunami research that arecontentious in the current debate Catalogs on historical tsunamis have beenCoastal and Marine Hazards, Risks, and Disasters http://dx.doi.org/10.1016/B978-0-12-396483-0.00003-0

published for many sections of the world’s coastlines, for example,Altinokand Ersoy (2000) for Turkey; Baptista and Miranda (2009) for Portugal;

Papadopoulos and Chalkis (1984), Soloviev (1990), Tinti and Maramai (1996),Soloviev et al (2000), Papadopoulos and Fokaefs (2005), Ambraseys andSynolakis (2010) for the Mediterranean Sea; Lander et al (2002) and

O’Loughlin and Lamb (2003)for the Caribbean;Goff (2008), as well asGoff

et al (2010c) for New Zealand;Hamzah et al (2000) for Indonesia; or theNational Geophysical Data Center (NGDC) in Boulder, Colorado (2013),which aims to provide a global dataset for certain coastal areas and spansnearly 4,000 years with >2,000 tsunami events in different probabilitycategories

The NGDC dataset not only lists tsunamis based on historical reports butalso those from “mythical” sources such as the collapse of Santorini volcano inthe Aegean Sea (w1628 BC) Hitherto, this event attracted public attention,several publications, and also speculations, though substantial field evidence islacking compared to other events, with the exception of deep sea tsunamites(Augias turbidites), which have been associated with the last Santorini caldera-forming event (Cita and Aloisi, 2000; Hieke and Werner, 2000; McKoy andHeiken, 2000; Dominey-Howes, 2004; Gutscher, 2005; Sironi and Rimoldi,2005; Bruins et al., 2008; Goodman-Tchernow et al., 2009)

More problematic is the reliability of these ancient reports The questionthat must be asked is how much of their content is simple fantasy or exag-geration, whether for political or religious reasons? The NGDC dataset clas-sifies the reports that describe events before the year AD 1500 into differentcategories of reliability; nevertheless, an uncertainty remains In some in-stances, these historical reports could be verified by field research and nu-merical dating, but so far, even events that have been classified as reliable havebeen elusive with regard to physical evidence as sedimentary signatures in thelandscape

On the other hand, paleotsunami research has unearthed and dated manytsunami events of often strong impacts during historical times, which are notlisted in any of the catalogs Whether this conundrum can be attributed to therather young scientific discipline of tsunami research and the gap will close inthe years to come, or the catalogs and conclusions of field event studies arefalse for a large proportion of cases, has to be elaborated by future andinterdisciplinary research

Well-studied examples of ancient tsunami events include that of CaesareaMaritima, Israel, in AD 115 (Reinhardt et al., 2006), and multiple tsunamiimpacts in the Lechaeon region north of Corinthos in Greece (Hadler et al.,

2011) Often cited is the coseismic sudden uplift of western Crete in 365 ADwith several written sources describing associated strong tsunamis in theeastern Mediterranean (Pirazzoli, 1986; Kelletat, 1991; Stiros, 2001; Stirosand Drakos, 2006; Scheffers, 2006; Shaw et al., 2008) In a Europeanperspective, the Lisbon earthquake and tsunami of 1755 AD continue to

receive attention because of the great number of fatalities, that is,>35,000victims (Baptista et al., 1999; Martinez-Solares, 2001; Scheffers and Kelletat,2005; Gracia et al., 2006; Lario et al., 2010; and many others) In contrast andquite surprisingly, the Messina earthquake and strong tsunami of 1908 AD(with>60,000 fatalities) is much less investigated.

Strong tsunami events outside of Europe have only attracted scientists inexceptional cases, such as the explosion of Krakatoa volcano in the SundaStrait between the Indonesian Island of Sumatra and Java in 1883, or the

1960 Chile event where one of the strongest earthquakes since instrumentalrecords began caused a destructive tsunami event (Eaton et al., 1961; Pfafkerand Savage, 1970) A comparable seismic energy was released by theAndamaneSumatra earthquake of December 26, 2004, the first megatsunami

to be documented continuously with photographs, videos, and immediatepostevent field research The magnitude and the unpreparedness of theaffected countries, with approximately 230,000 fatalities along thousands ofkilometers of coastline in the northern Indian Ocean, triggered tsunami andpaleotsunami research worldwide This tsunami disaster forced political andscientific organizations worldwide to deal with tsunami as natural hazardsand associated socioeconomic risks much more intensively than in the past.Ten years have passed since that event, and hundreds of tsunami studies havebeen published, older tsunami events have been reinvestigated, new physicalmodels for tsunami wave propagation have been developed, and warning sys-tems have been discussed and established Still, the AndamaneSumatra earth-quake event will have an impact on tsunami and paleotsunami research formany years to come (compare, e.g., Bishop et al., 2005; Lay et al., 2005;Lavigne et al., 2006; Tsuji et al., 2006; Richmond et al., 2006; Bahlburg andWeiss, 2007; Kelletat et al., 2007; Choowong et al., 2007; Goto et al., 2007;Sawai et al., 2009) The Tohoku Tsunami of March 2011 along the northeastcoast of Japan had a similar earthquake energy released (9.3 on the Richterscale) to that in AndamaneSumatra, but there were significantly fewer fatalities.Seismic activity along plate boundaries (mostly subduction zones) ac-counts for >85 percent of the generation of tsunami events, includingoceanwide megatsunamis In several cases, it is difficult to discriminate aseismic origin from a volcanic induced tsunami event as active volcanoes areconcentrated along the “Ring of Fire” and collision structures that are parallel

to subduction zones Volcanoes in the sea or near the coast may collapse ascalderas (e.g., Santorini and Krakatoa), or breakdown because of steep slopesand a very rapid construction of the volcanic edifice The latter has been re-ported from Mt Fogo on the Cape Verde Islands off West Africa (Figure 3.1)with a failure volume of>100 km3 that occurred over 10,000 years ago, orcollapses of Mt Etna on Sicily, Italy (Pareschi et al., 2006)

Advances in mapping sea floor topography have provided ample evidencefor slides, slumps, or mass failures of (steep) slopes mobilized either by seismicshocks, oversteepening of the slope profile, or occasionally by gas hydrate

eruptions The largest of these submarine slides have been found (and partlydated) around the volcanic group of the Canary Islands, Spain (Carracedo et al.,1999; Masson et al., 2008) and the Hawaiian Islands in the central PacificOcean (e.g., Moore et al., 1989) They have been active during the LastInterglacial period of high sea level (w125,000 years ago) back to the earlyQuaternary, >1.2 Million years ago One prominent Holocene slide that leftdeposits along many coastal sites around the northeastern Atlantic Ocean is theStoregga Slide event (in fact, probably three different slides) with the oldestaround 7,200/8,200 BP and the youngest slide occurring about 1,500 years BP(Bondevik et al., 1997 a, b;Haflidason et al., 2004; Bryn et al., 2005; see also

Figures 3.2 and 3.3) Examples from smaller submarine slides and tsunamis aredescribed inCharalampakis et al (2007)andTinti et al (2007)

Studies by the IFM-Geomar at Kiel University, Germany (cf Kopp andWeinrebe, 2009) showed that seamounts on the subducting sea floor area mayaffect the overriding plate as instruments triggering faults and seismic activitythat may initiate submarine slides with possible tsunami generation (Figure 3.4

as well asBrune et al., 2010)

Based on the knowledge of past submarine slides around volcanic islands,the question arose whether similar disastrous events may occur in the future.Computer simulations have been conducted to model a collapse of the westernslope of La Palma island in the Canaries, Spain (Figure 3.5(a)e(c)), in particularthe Cumbre Vieja region in the south, which developed an NeS-trending openfault in 1949 This site has the potential for many hundreds of cubic kilometers

of land being set in motion, thereby triggering a megatsunami with high velocityover the deep water of the open Atlantic Ocean The resulting tsunami wouldaffect the east coast of the United States with wave heights surpassing 30 m and

FIGURE 3.1 Scar of the collapsed Fogo volcano (8.5  11.3 km wide) on the Cape Verde Archipelago off western Africa The slide reaches nearly 15 km out into the sea A new volcanic cone has grown within the slide area Image credits: ©Google earth 2014.

FIGURE 3.2 Three slides in the Storegga area west of southern Norway They ran out for

>300 km on a very flat sea floor The two smaller slide sections have a volume of about 1,700 km 3

, the largest of 3,880 km3(compare also Bondevik et al., 1997; Bondevik et al., 2003) The youngest slide is the smallest event, while the largest submarine slide was the second event.

FIGURE 3.3 Upper section of the Storegga slide off western Norway Credit: British Geological Society http://www.bgs.ac.uk/research/marine/marineGeohazards.html.

also regions from the British Isles in the north to South America in the south(Ward and Day, 2001) Submarine slides in general are a constant risk togenerate far-field strong tsunamis even outside tectonically active regions.3.2 CURRENT RESEARCH SUBJECTS UNDER DEBATE

A review of the tsunami and paleotsunami literature reveals several topics thathave attracted interest of researchers from different scientific disciplines withthe following topics discussed most contentiously:

1 The possibility of extraterrestrial impacts and the forming of chevrons

2 Identifying overwash and lagoon sediments of tsunamigenic origin and thecontrol of older cores (commonly from geoarcheological studies) in respect

we know that thousands of meteoroids and asteroids are moving in theasteroid belt between Mars and Jupiter, or may approach as comets from theOort’s Cloud or the Kuiper Belt We are aware of nearly 200 impact craters

on planet Earth, some of them billions of years old, though only about onedozen are land based from Holocene times Therefore, it is highly likely thatmore than twice this number have collided into the oceans, and if they werelarge enough (100 m in diameter), they would cause significant tsunamisaffecting near and far coastlines This potential has been calculated andmodeled in tsunami research (Kristan-Tollmann and Tollmann, 1994; Hillsand Mader, 1997; Powars, 2000; Abbott et al., 2006; Tester et al., 2007;Bryant et al., 2007; Bunch et al., 2008; Bryant, 2008; Goto, 2008; Bryant

et al., 2010; Goff et al., 2010) Melted quartz and other shocked minerals,iridium peaks in fine sediments, or foraminifera attached to molten minerals,have been observed in coastal deposits likely associated with these (possible)events, and as a landform, chevron deposits have been described andcautiously attributed to cosmogenic tsunami eventsdalthough this hypoth-esis is controversial (Scheffers et al., 2008; Bourgeois and Weiss, 2009: butcompareFigures 3.6e3.10)

3.2.2 Fine Washover Sediments Stored in Geobioarchives

The identification of tsunami deposits has long been a subject of scientificdebate A still unresolved problem is the type of fine tsunami sediments that

FIGURE 3.6 One to five km long sandy “chevrons” south of Shark Bay, Western Australia, starting at a near vertical cliff top 85e143 m asl Tip of the small northern form is at

2655018.8900S and 11346058.3900E At the base of the cliff and because of its plunging character, no sand deposit is available (Credit: Google earth.) Dr Phillip Playford, geologist in Western Australia, just found megablocks up to 700 t nearby on Dirk Hartog island at 15 m above sea level and 250 m inland (Source: Michelle Wheeler in “The West Australian”, June 29, 2013.), and more sites with large boulders high on cliff tops can be found in that area.

can be accurately distinguished from fine sediments left by storm waves Inthe early phase of paleotsunami research, the catalogs of signatures fortsunami deposits have been rather extensive, describing a large number ofspecifications for typical tsunami sedimentation of fine material But, as aconsequence of the rising number of contemporaneous inspections of theimpacts of recent tsunamis, in particular that of the 2004 event, it becameclear that tsunamis may leave sedimentation characteristics very similar oridentical to those of storms during overwash (Figure 3.11) This may be the

FIGURE 3.8 Holocene chevron (coral rubble), a breach in the atoll rim of Rangiroa, Tuamotus, French Polynesia Image credits: ©Google earth 2014.

FIGURE 3.7 Nearly 3 km long, young Pleistocene chevron (sand and coral debris, partly cemented) of the Exumas, Bahamas Image credits: ©Google earth 2014.

reason why during decades of corings in coastal sediments, pre-1990 ADtsunami layers have not been identified, simply because they did not differsubstantially from other wave deposits During 1994e2014, only threesedimentological features remained as diagnostic, and these are mentioned inmost modern paleotsunami papers (besides the deposition of a generally

FIGURE 3.10 Washover deposits at the westernmost part of the Rhone delta (scene is 700 m wide) Image credits: ©Google earth 2014.

FIGURE 3.9 Oblique aerial photograph of two chevrons composed of coarse coral rubble on the east coast of Bonaire The features reach about 300-m inland at þ5 m asl and dated to approxi- mately 5,000 years BP Credit: A Scheffers.

coarser grain size unit): (1) a sharp basal discordance/unconformity; (2) amudcap; and (3) fining up sequences or a graded bedding that might not befinely developed (e.g.,Engel et al., 2010) However, we found two of these

“distinct” pieces of evidence of tsunami within storm washover deposits inWestern Australia (Figure 3.11) Japanese colleagues, who have in thesouthern part of their country a high number of recent and historicallyconfirmed tsunamis and cyclone landfalls, came to the conclusion that there

is not a single signature that can be exclusively connected to a tsunami onone side, or a strong storm on the other side (see several contributions in

Shiki et al (2008)) Today, in combination with coarse onshore deposits and

a crosscheck of numerical dating, paleotsunami deposits have quite oftenbeen identified (and sometimes connected to known historical events) incores from lagoonal geobioarchives (Andrade, 1992; Nichol et al., 2003;Kelletat and Scheffers, 2004; Switzer et al., 2005; Vo¨tt et al., 2006; Hori

et al., 2007; Switzer and Jones, 2008; Goff et al., 2009; Sawai et al., 2009;Vo¨tt et al., 2009; Fujino et al., 2010; Engel et al., 2010;Bahlburg and Spiske,

2011;Brill et al., 2011; Vo¨tt et al., 2011a,b; Scheffers et al., 2013, and manyothers)

As strong tsunamis tend to inundate wide flat landscapes with a ratherstrong flow, 2e6 m/s or even faster as detected from recent video tapes(e.g., in Thailand, 2004 or Japan, 2011), it is astonishing that ripple marks,

as a typical surface pattern of flow, have been reported as developing almostexclusively from surfaces on Bonaire island (Figures 3.12 and 3.13) Theseripple marks exhibit individual forms many meters wide and long andseveral decimeters high in multimodal sediments (sand to medium size

FIGURE 3.11 Washover deposit of shell and coral debris on a coastal dune with a silty cap,” from tropical cyclone Laurence in 2009 (Wallal Downs, NW coast of Australia) Photos: A Scheffers.

“mud-FIGURE 3.13 Backwash in tsunami ripple marks, east coast of Bonaire, southern Washikemba section The tsunami reached at least 500 m inland from the cliff The backwash lobe is >350 m

“long” to seaward and about 500 m wide at around þ7 m asl From Scheffers (2005).

FIGURE 3.12 Ripple marks in onflow tsunami deposits, east coast of Bonaire, southern section

of Washikemba Track crossing is >200 m from the coastline (at right) The NeS extension of the ripple mark field is >1 km Farther north in the Seru Grandi section, radiocarbon dating of coral fragments in a similar ripple field give possible dates for these features between about 1600 and

3200 BP (Scheffers, 2005).

coral debris), reaching a few hundred meters inland and nearly þ10 m asl

on the surfaces of a cliff and terrace Even backwash lobes are clearlydeveloped (Scheffers, 2005)

3.2.3 Boulder Movement: Observations, Calculations,

Modeling and the Contrast to Storm-Wave Impacts

The discussion and debate on the origins and implications of coarse coastalsedimentary deposits, in particular large boulders, are also unresolved Thelogical conclusion is that with the large flow depth of a strong tsunami, itshigh velocity passing the shoreline, its substantial inland inundation, and itslong duration compared to storm-wave impacts, much larger bouldersshould be transported farther inland than by the strongest storm waves.However, this is not generally accepted (Weiss, 2012) For example, thiscan be seen in the paper ofBourrouilh-Le-Jan and Talandier (1985) on theTuamotu boulders that have maximum weights of around 2000 metric tons.Also, there is the Tongan boulder that weighs, at a minimum, 1600 t(Frohlich et al., 2009), and that was found 9.4 m asl and at a distance of

130 m landward from the cliff top The authors keep the conclusion open onthe transport modus, tsunami, or cyclone The argument against tsunamis isgenerally that no tsunamis have taken place in the respective area, but this

is a weak argument that should merely be interpreted as no tsunami natures have been found so far! In fact, during the megatsunamis of the lastfew decades, which are well investigated by task forces, the transport ofcoarse debris has rarely been described Recent coastal scientific researchhas begun to address this void (compare, e.g.Nott, 2000, 2003a,b; Bryantand Nott, 2001; Bryant, 2008; Bryant et al., 1997; Whelan and Kelletat,

sig-2002,2003;Mhammdi et al., 2010;de Martini et al., 2010; Noormets et al.,

2002, 2004; Scheffers, 2002, 2004, 2005, 2008; Scheffers and Kelletat,2003a,b, 2005, 2006a,b; Scheffers and Scheffers, 2007; Scheffers et al.,

2005,2008, 2009a,b, 2010a,b; Scicchitano et al., 2007;Williams and Hall,2004; Hall et al., 2006, 2010;Morton et al., 2008; Hansom and Hall, 2009;

Etienne and Paris, 2010)

Fortunately, parallel to the descriptions resulting from field observations,models are being developed and tested on the transport energy of tsunami flowand extreme storm waves The results of these models vary significantly Atleast as important as theoretical tests and models are the real-time observations

of the wave power associated with storm systems Therefore, some of therecently published results concerning storm-wave transport are reviewed Ofparticular interest should be observations and investigations at coastal sitesthat exhibit modern (or historical) strong tsunamis as well as strong cyclonelandfalls This is the case, for example, along the coastlines of southern Japan,where the best evidence for the comparison of the energy of both processes can

be expected to be found (e.g.,Goto and Imamura, 2007;Imamura et al., 2008;

Goto et al., 2007, 2009, 2010)

First, we will refer to examples where strong storm waves have beenobserved and the transport of boulders onsite has been checked in closetemporal proximity:

l Lorang (2000) observed a maximum boulder of 1.1 t moved by waves of

8 m at the coastline

l Bartel and Kelletat (2003)visited the coastlines of Mallorca Island, Spain,after the strongest hurricane ever observed in the western Mediterranean inDecember 2001 They found the maximum fresh boulder transport caused

by waves of 9 m immediately along the coast to be 2.5 t atþ3.5 m asl with

a transport distance (sliding) of 27 m (Figure 3.14) Another example of a

FIGURE 3.14 A 27-m long, slide track of a 2.5-t boulder at 3.5 m asl during a “Medicane” (hurricane cat 2) in December 2001 on Mallorca island, Mediterranean Spain Photo: D Kelletat.

boulder slide on a rough surface has been documented from the east coast

of Curacao (Figure 3.15;Scheffers, 2002)

l Saintilan and Rogers (2005)found that an 11.8-m deep water wave at thecoast of NSW (Australia) had transported boulders weighing 9.7 t and 6 t adistance of 40 m onshore at an elevation ofþ2 m asl

l Scheffers and Scheffers (2006) observed the impact of hurricane Ivan

in 2004 along the cliff coasts of Bonaire (Netherlands Antilles, bean), measuring hundreds of strong waves at with heights of at least

Carib-12 m, with an onshore bore flow The largest fresh boulder broken fromthe cliff top weighed 6 t, and was found at þ5 m asl after being trans-ported inland for 30 m Much larger boulders further inland were moved

by the hurricane bore for a few meters, but were never transported uphill

l Etienne and Paris (2010) investigated the boulder movement at thesouthern spit of the rocky Reykjanes peninsula south of Reykjavik, in aseverely energetic storm-wave environment They measured 47 transportedboulders, and 10 had weights >4 t They found the volumes of the twolargest moved boulders to be 22 and 26 m3, with maximum altitudes ofdeposition around 11e14 m and 17e18 m asl The most extreme examplesare as follows:

l A boulder of 3 m3and 7.2 t atþ6.5 m transported across 105 m

l A boulder of 26 m3and 70.3 t atþ6 m that had been moved 65 m

l A boulder of 6 m3and 16.2 t atþ8 m that moved 105 m

l Suanez et al (2009)studied the boulder dislocation by a “one in 30 years”storm (March 10, 2008) on Banneg Island of Brittany, France Flow wasdetected at elevations up toþ19 m, which is 5e10 m higher than the clifftops Boulders were dislocated at elevations that wereþ7.5 to þ14.5 m, at

a distance 14 m from the þ9 m cliff top The largest boulder in this sition weighed 32 t, although the median boulder measurements were only

po-FIGURE 3.15 Sliding of a large old tsunami boulder for several meters in cliff top position (þ4.5 m asl) on the east coast of Curacao by a storm before AD 2001 Oblique aerial photograph

2001 by A Scheffers.

0.19 m3and 0.6 t The bore flow transported boulders of 0.3 to 1.4 t tances up to 50e90 m inland.

dis-l Goto et al (2009) observed boulder movements by strong typhoons onislands of southern Japan As a consequence of a storm with open waterwave heights of 11.9 m that occurred in association with spring tide, theyfound the average size of translocated boulders to be 3.7 t, but the largestwas 127 t, occurring 29 m from the reef edge without a significant verticaldislocation Boulders <47 t have been found up to 200 m from the reefedge The boulders moved by typhoons are definitely much smaller, closer

to the reef edge and in lower position than those from historical tsunamis.The measured change of bore flow velocity across the reef was from aninitial 6.5 to 3.1 ms1across a distance of 260 m

l Khan et al (2010) observed and measured boulder movements fromfour recent hurricanes of category 4e5, which directly passed Jamaica.The maximum values for freshly emplaced boulders were found to beweights of 1.77e26.4 t at 0- to þ2-m elevation, which had been moveddistances up to 25 m The maximum values were 4.48 t and found at aþ0- to 25-m distance, and 26.4 t at a þ0- to 6-m distance

Existing boulders have been moved as larger fragments and for largerdistances, for example, 0.08 to 79.74 t,þ0 to þ14 m up to 76 m distant fromthe shoreline, with maximum values observed:

79.74 t atþ12- and 55-m distance moved 2 m

50.4 t atþ0- and 30-m distance moved 8.5 m

5.39 t atþ8- and 22-m distance was overturned at its place

0.08 t atþ14- and 76-m distance was overturned at its place

The discussion on boulder movement onshore is difficult for severalreasons:

l The first is the debate on boulder volume Several authors just multiply thelengths of the three axes a, b, and c, which may, in any instance, yieldvolumes that are up to 200 percent greater than those obtained by multi-plying mean axis length (e.g., Scheffers, 2002), or by determining theboulder volume by laser scanning or other modern methods (Engel andMay, 2012) A test by Kelletat (2012, unpublished) of 16 boulders (mostlycuboid to platy forms) from Mallorca found that estimating bouldervolume by simply multiplying the dimensions of aebec axes will givevolumes that were between 80 percent and 215 percent too large, with amean deviation ofþ154 percent!

l The second is the debate on bulk density, which gives (when multiplied

by the volume) the mass that may be moved Comparing measurements

of bulk densities from reef rock and coral species therein (all from MIS

5e-terrace, i.e., Younger Pleistocene from the east coast of Bonaire), thefollowing deviations and accordances have been found:

16 samples, the boulders were found to have bulk densities that ranged from2.363 to 2.412 g/cm3with a mean value of 2.388 g/cm3 This is close to theBonaire values reported byEngel and May (2012), but about 15 percent lessthan those measured bySpiske et al (2008), also from Bonaire

Frohlich et al (2009)describe very large boulders in the west of Tongatapunear the village of Fahefa, at an elevation of þ10 m asl and 130 m inland.Others can be found up toþ20 m asl and 400 m inland The long axis of thelargest boulder measured 15 m with a height of 9 m and volume around

1200 m3 The material is broken from reef rock that builds the base along anactive cliff As the age of the reef rock is from the last interglacial, the dislo-cation has clearly taken place during younger Holocene times This is evident

by the stripping of soil cover due to flow erosion around the boulder, as well as(missing) karstification differences between the wider environment of the rockplatform and the boulder-resting place More boulders of a similar size havebeen found on the Tuamotus, French Polynesia (cf Bourrouilh-Le Jan andTalandier, 1985; Talandier and Bourrouilh-Le Jan, 1987) In many coastalenvironments, the sheer size of boulders, their isolation from cliff top sources orbarriers and location many meters above sea level, their apparent age, and thelack of other, smaller boulders from frequent tropical cyclones in their vicinitywould point to their origin from rare and extreme powerful events, that is,tsunamis (Figures 3.16e3.18)

As the comparison of field facts and models, as well as laboratory wavetests have documented, a lot of unexplained features and processes exist incoastal boulder dislocation, and most questions remain open for continueddebate The main reason is that although storm wave models produce resultsthat match observations rather well, the dimensions of runup, inundation,and flow depth and velocity of tsunamis are not easily translated into