1 The importance of geologists and geology in tsunami 2 3 science and tsunami hazard 4 5 DAVID R. TAPPIN 6 7 British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK, 8 [email protected] 9 10 11 Abstract: Up until the late 1980s geology contributed very little to the study of tsunamis because 12 most were generated by earthquakes which were mainly the domain of seismologists. In 1987–88 13 however, sediments deposited as tsunamis flooded land were discovered. Subsequently they began to be widely used to identify prehistorical tsunami events, providing a longer-term record than pre- 14 viously available from historical accounts. The sediments offered an opportunity to better define 15 tsunami frequency that could underpin improved risk assessment. When over 2200 people died 16 from a catastrophic tsunami in Papua New Guinea (PNG) in 1998, and a submarine landslide 17 was controversially proven to be the mechanism, marine geologists provided the leadership that 18 led to the identification of this previously unrecognized danger. The catastrophic tsunami in the 19 Indian Ocean in 2004 confirmed the critical importance of sedimentological research in under- 20 standing tsunamis. In 2011, the Japan earthquake and tsunami further confirmed the importance 21 of both sediments in tsunami hazard mitigation and the dangers from seabed sediment failures 22 in tsunami generation. Here we recount the history of geological involvement in tsunami science and its importance in advancing understanding of the extent, magnitude and nature of the hazard 23 from tsunamis. 24 25 26 27 28 Until the late 1980s tsunami science was mostly The involvement of geologists in tsunami 29 the province of seismologists, numerical modellers, research began in the early 1980s, with the first 30 geophysicists and historians; tsunamis received papers on deep-sea deposits in the Aegean Sea. 31 little attention from geologists (e.g. Bailey & Weir Here, unusual seabed sediments, termed homoge- 32 1933; Coleman 1968, 1978). Earthquakes were nites, were proposed as deposited from a tsunami 33 considered the main, if not the only, mechanism generated by the Late Bronze Age eruption of San- 34 that could generate elevated tsunami waves that torini (Kastens & Cita 1981; Cita et al. 1984). In the 35 were very destructive at the coast. Other tsunami- Hawaiian Islands, boulders and coarse-grained 36 generation mechanisms, such as submarine land- gravels preserved at high elevations (hundreds of 37 slides, were not considered a major hazard despite metres) were considered to result from tsunamis 38 evidence to the contrary such as from the Grand generated by large-scale volcanic collapse (Moore 39 Banks event of 1929 (Bardet et al. 2003). Numerical & Moore 1984). The early results from the Mediter- 40 tsunami modelling of submarine landslides was in ranean and Pacific were controversial due to the 41 large part theoretical (Jiang & LeBlond 1992, uncertainty over whether there were tsunamis gen- 42 1994). The slow landslide failure velocities were erated at these locations. In addition, this approach 43 perceived as inhibiting tsunami generation, in of using sediments to identify tsunamis from their 44 contrast to earthquake-generated tsunamis where sedimentary record had not been attempted before. 45 rupture velocities of kilometres per second were The tsunami from the Late Bronze Age (LBA) erup- 46 regarded as instantaneous in the context of the tion of Santorini had been a major controversy for 47 relatively slow (metres per second) velocity of tsu- decades (Marinatos 1939). Sand had been found 48 nami wave generation (Geist 2000; Ward 2001). in Minoan buildings on the coast and used as evi- 49 Historians provided evidence on older tsunamis, dence for tsunami inundation; its origin was how- 50 hopefully for use in estimating tsunami fre- ever disputed as it was located at sea level and the 51 quency–magnitude relationships for future tsunami sand could have been deposited from storms or 52 risk, although the limitations of historical data were was present for religious purposes. Subsequent 53 recognized (Ambraseys & Jackson 1990). When a validation has been impossible because the sand 54 paper (rarely) considered the geology of tsunamis, deposits were destroyed during subsequent excava- 55 it was on the sediments deposited from inundation tions. The Aegean Sea homogenites provided the 56 and authored by seismologists (e.g. Wright & first geological evidence in support of the tsunami 57 Mella 1963). hypothesis but, even today, the origin of the 58 From:Scourse, E. M., Chapman, N. A., Tappin,D.R.&Wallis, S. R. (eds) Tsunamis: Geology, Hazards and Risks. Geological Society, London, Special Publications, 456, https:// doi.org/10.1144/SP456.11 # British Geological Survey@ NERC. Published by The Geological Society of London. All rights reserved. For permissions: http:// www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics D. R. TAPPIN

59 homogenites remains controversial (see Pareschi determine tsunami characteristics (Jaffe et al. EDQ1 60 et al. 2006; Shanmugam 2006; Weiss 2008; Polonia 2007). More recent, devastating, events in the Indian 61 et al. 2013). The suggestion that the Hawaiian Ocean (2004) and Japan (2011) have expanded 62 deposits were from tsunamis was groundbreaking this geological involvement in tsunami sediment 63 because of their use in identifying tsunamis from characterization (e.g. Paris et al. 2007), in inverse 64 volcanic collapse. Previously, the deposits were modelling of tsunami-generation mechanisms 65 interpreted as laid down during sea-level highstands (Spiske et al. 2010; Sugawara et al. 2014) and in 66 (Stearns 1978). Their origin is also still disputed research on submarine landslides in tsunami gener- 67 (Stearns 1978; Rubin et al. 2000; McMurtry et al. ation (Tappin et al. 2007, 2014). Storegga, PNG 68 2004a). and most recently Japan have led to an increased 69 In 1987 and 1988, two groundbreaking papers realization of the tsunami hazard from submarine 70 published on tsunami sediments demonstrated landslides. 71 their potential in evaluating tsunami hazard. The Here it is recounted how over the past c. 30 72 first (Atwater 1987) was on prehistoric sediments years geologists became increasingly involved in 73 in Cascadia, which identified a sequence of earth- tsunami science and how they have contributed to 74 quakes and their associated tsunamis that extended an improved understanding of tsunami mechanisms 75 back in time to over 10 ka before present (BP). and their hazard. Although there were earlier, iso- 76 The second paper (Dawson et al. 1988) on sedimen- lated precursors to the main ‘catalyst’ events iden- 77 tary deposits in Scotland identified a major prehis- tified above, it was during the 1980s that ‘geology’ 78 toric tsunami from a massive submarine landslide became significant in advancing tsunami science; 79 (Storegga) located off Norway (Bugge 1983; this advancement was initially from the application 80 Bugge et al. 1988). These sediments motivated the of tsunami sediments, followed by an improved 81 first attempt at numerical modelling of a tsunami understanding of tsunami frequency, hazard and 82 from a submarine landslide mechanism (Harbitz risk, and more recently, with the Japan tsunami, 83 1992). In 1998, a devastating tsunami struck the from the use of sediments as a basis for inverse 84 north coast of Papua New Guinea (PNG) killing numerical models of tsunami-generation mecha- 85 over 2200 people (Kawata et al. 1999). The associ- nisms (e.g. Sugawara et al. 2014). The motivations 86 ated Mw 7 magnitude earthquake was too small to for this paper were the two meetings held in Japan 87 generate all of the elevated local run-ups of 15 m. and the United Kingdom in 2014 and 2015 on the 88 Amid confusion and controversy (e.g. Geist 2000), theme of ‘Tsunami Hazards and Risks: Using the 89 marine surveys organized in response to the disaster Geological Record’. The focus of the Japan meet- 90 acquired hydroacoustic and sample data offshore ing was the use of tsunami sediments in the mitiga- 91 of the impacted area. These surveys identified an tion of tsunami hazard. The aim of the UK meeting 92 offshore slump, which preliminary numerical simu- was to bring together geologists and hazard and 93 lations demonstrated to be the local tsunami mech- risk modellers. In this paper the use of the geolog- 94 anism (Tappin et al. 1999). The PNG event was ical record in contributing to tsunami hazard is 95 seminal in identifying the major hazard from sub- extended by the addition of how the identification 96 marine landslides in tsunami generation. It was the of submarine landslides have led to their recogni- 97 first tsunami to be studied from responsive marine tion as a major hazard in tsunami generation, 98 surveys led by geologists. Submarine landslides a hazard previously overlooked (Bardet et al. 99 were well researched previously, but rarely in the 2003; Løvholt et al. 2015). To underpin realistic 100 context of tsunami generation and not by collabora- modelling of tsunamis generated from submarine 101 tion between geologists and numerical modellers landslides requires their architectures to be deter- 102 (e.g. Grand Banks in 1929). mined from hydroacoustic data, including multi- 103 The 1987–88 research on tsunami sediments in beam echosounders (MBES) and seismics. 104 the USA and Europe, together with the landslide- Mapping of seabed failures has led to the transition 105 generated PNG tsunami, resulted in the major from theoretical numerical models, which sug- 106 involvement of geologists in tsunami science and gested that landslides were ineffective in tsunami 107 the recognition of the contribution which geology generation (LeBlond & Jones 1995), to realistic, 108 could make to an improved understanding of tsuna- event-based models that prove otherwise (Harbitz 109 mis and their hazard. Demonstrating the mechanism 1992; Tappin et al. 1999). The use of the term 110 of the PNG tsunami was seminal in confirming the ‘geology’ is interpretative in the sense that sub- 111 tsunami hazard from submarine landslides. It was seabed structure can be determined from remote 112 based on marine surveys carried out by geologists, data such as multibeam echosoundings and seis- 113 with the geological interpretations underpinning mics. It is also recognized that the characterization 114 the numerical tsunami models. Much later, the of tsunami sediments and their discrimination from 115 PNG tsunami was the first attempt to use inverse other depositional mechanisms is still undergoing 116 and forward modelling of tsunami sediments to review (see Shanmugam 2012), so here the focus IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

117 is on well-studied examples from well-established (Dawson 1994; Paris et al. 2007); however, in the 118 mechanisms. ocean they are poorly researched. Interpretations 119 are speculative because there are no reliable mea- 120 surements of this process and few recent examples 121 Tsunami sediments (Fig. 1; Dawson & Stewart 2007; Ikehara et al. 122 2014). The best evidence for backwash flow is 123 Sediments deposited from tsunamis are mainly rec- from video footage of sediment plumes moving off- 124 ognized on land. Those described from the seabed shore from the Indian Ocean and Japan events (see 125 are rare and their identification and application in Tappin et al. 2012). The backwash sediment flushed 126 tsunami hazard assessment are more controversial seawards has rarely been studied, but from the few 127 (see Shanmugam 2006; Dawson & Stewart 2007), case histories published on shelf deposits its long- 128 but they are covered here briefly for completeness term preservation potential is probably low because 129 (Fig. 1). Seabed tsunami sediments are found in of reworking by longshore currents (Tappin et al. 130 deeper, oceanic water depths, include the homoge- 2012) and storms (Noda et al. 2007; Sakuna et al. 131 nites in the eastern Mediterranean noted above. 2012; Feldens et al. 2012). Reworking compromises 132 There are also shallow-water shelf deposits result- discrimination between tsunamis and storm depos- 133 ing from tsunami backwash (or backflow). Turbi- its. On the outer shelf of Japan, the preservation of 134 dites in the deep ocean have been well studied backwash deposits from the 2011 tsunami is consid- 135 (e.g. Heezen & Ewing 1954; Kuenen 1957; Piper ered likely only over short timescales (Ikehara et al. 136 et al. 1988). Triggered by earthquakes, and in com- 2014). Nearer shore, the sediment flushed seawards 137 bination with onshore sediments, turbidites can be was soon eroded by longshore drift and redeposited 138 proxies for large-scale prehistoric earthquake on the adjacent coast, where it repaired major 139 events. Cascadia, on the west coast of the USA, is coastal breaches (Tappin et al. 2012). The use of 140 the best-studied area (Goldfinger et al. 2012). shallow-water tsunami deposits on the continental 141 Here, turbidites along the oceanic margin were shelf in hazard assessment at present is therefore 142 first researched in the 1970s (Griggs & Kulm considered too poorly understood to be considered 143 1970), with the first attempt to use these to date further. 144 earthquake cycles in 1990 (Adams 1990) and sube- Our imperative here is therefore on coastal 145 quent research developed by Goldfinger et al. deposits, particularly those from recent, historical 146 (2012). As the distal parts of submarine landslides, and Quaternary events. Onshore sedimentary depos- 147 turbidites can be used in dating these (e.g. Normark its from the older (pre-Quaternary) geological 148 et al. 2004). High-resolution age dating of subma- record have been attributed to tsunamis (e.g. Le 149 rine landslides is important in establishing relation- Roux & Vargas 2005). Where the tsunami mecha- 150 ships to climate change, which is a major control on nism can be established, for example Chicxulub 151 sediment failure (Maslin et al. 2004) although still (e.g. Goto et al. 2008b), deposit origins can be val- 152 poorly understood (Urlaub et al. 2013). Backwash idated; where the mechanism is more uncertain, as 153 flow follows the maximum inundation of tsunami for many of the deposits on the west coast of 154 waves, after which the water recedes seawards. South America, there may however be considerable 155 Backwash deposits on land are well described uncertainty (e.g. Bailey & Weir 1933; Spiske et al. 156 157 158 159 160 161 162 163 164Colour 165online/ 166mono 167hardcopy 168 169 170 171 172 173 Fig. 1. Schematic illustration of principal pathways of tsunami sediment transport and deposition (Reproduced from 174 MikeNorton; Wikipedia). D. R. TAPPIN

175 2014). The focus here is therefore on those mainly Hawaii (Shepard et al. 1950). The first observational 176 recent, on-land deposits which can improve hazard evidence for tsunamis transporting sediment was a 177 assessment through: (1) better understanding of tsu- series of photographs also from Hawaii from the 178 nami frequency and impact; (2) the validation of 1957 earthquake in the Aleutians (referred to in 179 numerical models; and (3) as a basis for inverse Bourgeois 2009). Observations were reported of 180 modelling of tsunami inundation and tsunami- sediment deposition from the Chile tsunami of 181 generation mechanisms, although the application 1960 (Wright & Mella 1963) and the Alaska earth- 182 of these methodologies is still in its infancy (Hun- quake of 1964 (Reimnitz & Marshall 1965), but 183 tington et al. 2007). These deposits include both no detailed interpretations made. The 1960 Chile 184 fine-grained sediments and coarse-grained gravel/ event left a 1–2 cm veneer of sand over the coastal 185 boulder deposits. lowlands (Wright & Mella 1963). There are numer- 186 ous descriptions from Japan (e.g. Kon’no 1961), but 187 Early history of research (pre-1980) these publications are mostly in Japanese so less 188 accessible (Bourgeois 2009). Descriptions of the 189 Written observations of tsunami sediment deposi- Suva earthquake and tsunami of 1953 were perhaps 190 tion date back to 1868 from the Arica earthquake some of the first of the modern era to suggest an 191 off Chile, where the US Postal Steamer Wateree associated submarine landslide-generated tsunami, 192 (Fig. 2) was carried inland 430 m by a wave 18 m which deposited coral boulders on the adjacent 193 high at the coast and buried under a mass of sand reef (Houtz 1962). Probably the first modern ‘geo- 194 and water (recorded in Myles 1985). The first scien- logically’ focused paper that described tsunami sed- 195 tific publication to suggest that tsunamis might be imentation was by Coleman (1968). 196 responsible for sediment deposition is Bailey & 197 Weir (1933) on sediments of Jurassic age located Seminal events of the 1980–90s 198 on the west coast of Scotland. No further studies 199 of tsunami inundation and sedimentation from his- The first geological evidence for sediments depos- 200 torical or recent large earthquakes were published ited from prehistoric tsunamis, which identified 201 until the 1946 Aleutian tsunami, which struck their potential for use in mitigation, was from 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 Fig. 2. USS Wateree (1863) beached at Arica, Chile, 430 yards above the usual high water mark, after she was 231 deposited there by a tidal wave on 13 August 1868. Note the tsunami sand in the foreground (U.S. Navy 232 photograph). IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

233 North America (Atwater 1987). The sediments were from a submarine landslide event, the first time 234 up to 10 ka old and preserved in sand sheets inter- that numerical modelling of a recorded submarine 235 bedded with marsh muds from the outer coast of landside tsunami had been attempted. 236 Washington State. They were interpreted as depos- The evidence for the origin of the sands in Cas- 237 ited from earthquake-generated tsunamis. Recogni- cadia and Scotland was circumstantial because their 238 tion of the tsunami origin of the sediments led to the deposition from tsunamis had not been observed. 239 identification of earthquakes and tsunamis of much For the first time the research was also by sedimen- 240 earlier age than that provided by historical records tologists, unlike observations made at earlier histor- 241 (up to 200 years old). A year later Dawson et al. ical events noted above which were by non-experts. 242 (1988) described an unusual sand deposit (Fig. 3) Confirmatory evidence for the Cascadia research on 243 within uplifted coastal sediment sequences in Scot- the tsunami origin of the sands came first from a his- 244 land, which they attributed to a tsunami generated torical event in Japan. In the east coast of Honshu 245 from the Storegga submarine landslide off the Nor- Island, sands preserved in lake sediments were 246 wegian coast (Jansen et al. 1987; Bondevik et al. also identified as laid down by a tsunami, generated 247 2005a). Deposits from Storegga had been described from the 1983 Sea of Japan Earthquake (Minoura & 248 from the Shetland Islands by Birnie (1981), but had Nakaya 1991). Older, underlying sands were dated 249 not been identified as originating from a tsunami. as far back as 2700 years BP, confirming the associ- 250 Along the Washington coast, the only explanation ation between tsunami inundation and earthquake 251 for the sediments was by rapid coastal subsidence subsidence as observed previously in Cascadia. Fur- 252 from local earthquakes that generated tsunamis. In ther confirmation of the Cascadia and Scottish inter- 253 Scotland, radiocarbon dating of the sediments and pretations of tsunami sand deposition was lacking 254 Storegga landslide proved a close coeval correspon- because of an absence of post-1987 tsunami events. 255 dence. The identification of the deposits in Scotland A devastating tsunami struck Flores Island in 256 motivated Harbitz (1992) to simulate a tsunami December 1992 killing 2190 people, over half of 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280Colour 281online/ 282mono 283hardcopy 284 285 286 287 288 289 Fig. 3. Tsunami sediments (pale grey by the handle of the Nejiri-gama) from the east coast of Scotland generated 290 by the Storegga tsunami of 8.2 ka BP. Nejiri-gama 23 cm long (Photograph, D.R. Tappin). D. R. TAPPIN

291 whom died in the tsunami. The Flores tsunami was 2004, based on the new and developing quantitative 292 one of the first investigated by specifically orga- approaches developed first for PNG Huntington 293 nized, responsive, field surveys (Yeh et al. 1993). et al. (2007) identified two further key challenges: 294 It was the first of the modern age to include geolo- (1) closing the knowledge gap in linking modern 295 gists in the field team (Shi et al. 1995). The 1991 events to their deposits with an improved under- 296 Japanese research by Minoura and Nakaya was standing of tsunami sediment transport; and (2) 297 based solely on core samples, but at Flores field adapting this relationship to better interpret the geo- 298 surveys reported on both the geomorphology of tsu- logic record. 299 nami impact and the sediments deposited (Shi et al. 300 1995; Minoura et al. 1997). For the first time, there Early advances (1990s–2004) 301 was a direct correlation between an observed tsu- 302 nami and the sediments deposited. Although the From the early 1990s there was a steady increase 303 Flores surveys were aimed at understanding the in research on tsunami deposits as reflected in the 304 sedimentary processes associated with tsunami number of peer-reviewed papers published (see 305 inundation, they were also planned as the first Bourgeois 2009, fig. 3.2). The research was from 306 attempt to use a recent event to improve the identi- responsive field surveys, which became the norm 307 fication and interpretation of older, palaeotsunami after the Nicaragua tsunami of 1992 (Satake et al. 308 deposits (Shi et al. 1995). 1993), but also because of continued work on semi- 309 nal events such as Storegga and in the Hawaiian 310 Scientific objectives Islands. The focus was mainly on the convergent 311 margins of the NW Pacific and Japan, and the 312 Two primary objectives of sedimentological re- north Atlantic passive margin. Research in other 313 search emerged from the early tsunami studies. areas where the hazard is significant, such as New 314 The first was to establish unequivocal sedimentary Zealand, was constrained because of the challenges 315 characteristics of tsunami deposits that would in proving the tsunami sediment provenance (Goff 316 allow their discrimination from other, high-energy et al. 2001). The experience gained from the early 317 depositional mechanisms such as storms (e.g. Shi studies on Cascadia and Storegga and more recent 318 et al. 1995; Dominey-Howes et al. 2006). The sec- events (e.g. Flores in 1992) was used to research 319 ond was to use these sedimentary characteristics to older historical events including Lisbon in 1755 320 identify older, prehistoric sediments. Identification (Dawson et al. 1995), Grand Banks in 1929 (Tuttle 321 of ancient, prehistoric tsunami sediments as a record et al. 2004; Moore et al. 2007) and events in New 322 of older events, together with their reliable age dat- Zealand (Goff et al. 2001). In Cascadia, further 323 ing, would allow the quantification of tsunami recur- research identified six major earthquakes over the 324 rence intervals, improving tsunami hazard and past 7 ka, three of which at least were associated 325 risk assessment. Initially, discrimination of fine- with tsunamis. During the past 3.5 ka, there were 326 grained tsunami sediments was based on simple seven extensive tsunamis (Atwater & Hemphill- 327 criteria such as the extent of deposits, landwards Haley 1997; Peters et al. 2007). At some locations, 328 grain-size fining and deposit thinning (fine-grained the record extended back to 14 ka BP, with three 329 tsunami sediments were normally graded and events older than 3.5 ka (Peters et al. 2007). 330 storm deposits laminated), and rip-up clasts were Research on the Storegga tsunami confirmed previ- 331 significant (Morton et al. 2007). Discrimination of ously established relationships between the land- 332 coarser-grained, boulder deposits was from imbrica- slide and the sediments in Scotland. This was 333 tion and boulder orientation. There were also the based on additional evidence from sediments pre- 334 first attempts at mathematical modelling of boulder served in elevated lakes in Norway (Bondevik 335 transport (Moore & Moore 1988; Nott 1997, 2003; et al. 1997a, 2005a) and in the Faeroe Islands 336 Weiss 2012). (Grauert et al. 2001). At the end of the period 337 Based on the analysis of sediments from more 1990–2004, the study of tsunami sediments had 338 recent events, such as PNG and the Indian Ocean, expanded from a few publications based on circum- 339 objectives that are more ambitious were identified. stantial relationships between cause and effect (as 340 Detailed grain size analysis of tsunami deposits with Cascadia and Storegga) to the reporting of 341 from PNG for the first time were used to model recent events validated by observations of tsunami 342 onshore flow depth and speed, from which tsunami inundation. The tsunami record at some locations 343 size could be quantified (Gelfenbaum & Jaffe now extended back beyond historical reporting. Pre- 344 2003). The results were to provide the key for long- liminary identification of simple sediment charac- 345 term hazard assessments based on tsunami source teristics led to optimism that, with time, absolute 346 mechanisms (e.g. earthquake fault slip or submarine criteria would be identified for discrimination of tsu- 347 landslides) inverted from calculated tsunami wave nami deposits from other high-energy mechanisms 348 characteristics. After the Indian Ocean tsunami of of deposition. IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

349 Considering the limited published research, Gelfenbuam 2007; Spiske et al. 2010), are depen- 350 there were a large number of review papers (Bour- dent on the successful discrimination of inflow 351 geois 2009). New terms, such as tsunamites, were from backflow deposits (Bahlburg & Spiske 2012). 352 introduced to describe the sediments, although Most studies are of tsunami deposits from earth- 353 these were later subject to some controversy over quakes; there are still only a few descriptions of 354 their definition (Shanmugam 2006). There were sev- deposits from submarine landslides (Dawson et al. 355 eral special journal issues produced (Einsele 1996; 1988; Bondevik et al. 2003; Gelfenbaum & Jaffe 356 Shiki et al. 2000), and an edited book was in prepa- 2003). Submarine landslide deposit research is still 357 ration (later published as Shiki et al. 2008). Then the focused on Storegga and their sediments deposited 358 most catastrophic tsunami ever generated struck in on the coastlines of the north Atlantic (e.g. Bonde- 359 the Indian Ocean. vik et al. 2005a). 360 Intensive studies of the catastrophic Indian 361 Catastrophic events, advances and Ocean, 2004 and Japan, 2011 events show that pre- 362 controversy: post-2004 viously proposed ‘simple’ discriminants described 363 above are not necessarily unique to tsunami sedi- 364 The Indian Ocean tsunami led to major advances in ments; they may also be found in storm deposits. 365 tsunami sediment science because it generated a The assumption that tsunami sediments are only 366 major surge in research. Observations of the tsunami deposited from suspension settling (giving fining- 367 flooding the land confirmed the sedimentary evi- upward beds) has given way to the recognition 368 dence for the tsunami source of the coastal sedi- that they also result from bedload (traction) trans- 369 ments deposited. The Indian Ocean tsunami was port (giving laminated sediments), which compro- 370 the largest recorded event where there was a positive mises previous discrimination (Paris et al. 2007). 371 association between inundation and deposition. The Commonly deposited sediments during both the 372 responsive tsunami surveys carried out since 1992 Indian Ocean and Japan events were laminated 373 (on Nicaragua) provided the basis for the manage- (from traction currents; Paris et al. 2007; Szczucin´- 374 ment of the Indian Ocean field surveys which, car- ski et al. 2012). In addition, marine microfossils 375 ried out over the whole region impacted, provided (diatoms and foraminifera), previously considered 376 a comprehensive database of the tsunami impact. characteristic of tsunami sediments as in the Indian 377 An even greater surprise than the Indian Ocean tsu- Ocean (Sawai et al. 2009), were rarely found in sed- 378 nami was the Japanese event of 2011. Japan had the iments from the Japan 2011 tsunami (Szczucin´ski 379 most sophisticated mitigation and response strate- et al. 2012). Without observational evidence of tsu- 380 gies of any country in the world in contrast to the nami inundation, the identification in isolation of 381 Indian Ocean where there was no warning system, ‘absolute’ sedimentological criteria that allow iden- 382 yet 19 000 people died. The research on the Indian tification of a palaeotsunami sediment does not yet 383 Ocean tsunami resulted in improved understanding seem possible. 384 of qualitative depositional mechanisms, whereas the Despite the recognition that simple discrimina- 385 advances from Japan were quantitative. Although tory criteria are not as ‘absolute’ as previously pro- 386 the advances from these events (and other smaller posed (e.g. Morton et al. 2007), there have been 387 events) were significant; the original objectives major advances in characterizing fine-grained sedi- 388 identified above – tsunami deposit discrimination ments and the controls on deposition. Tsunami 389 and its use in identifying older deposits – were not deposits vary because controls on sedimentation 390 quite as successful as envisaged. Sedimentary char- are richly variable. These controls include: (1) 391 acteristics have been identified, but absolute charac- coastal and nearshore morphology; (2) the elevation 392 terization of tsunami deposits has not been achieved of tsunami waves at the coast; (3) run-up (maximum 393 (Shanmugam 2012). Coarser-grained (boulder) inland elevation reached by the inundation); (4) the 394 deposits have been especially difficult to character- nature and amount of the sediment available for 395 ize (Morton et al. 2006). erosion; and (5) the preservation potential of the 396 Sedimentary characteristics of fine-grained, tsu- sedimentary imprint laid down during inundation. 397 nami sediments are now being used more effectively As a result, tsunami deposits are complex. There is 398 to estimate flow velocities from which wave magni- positive news, however. Whereas there may not be 399 tudes are derived. Preliminary studies (e.g. Jaffe absolute distinguishing sedimentological criteria 400 et al. 2011; Sugawara et al. 2012; Tang & Weiss that can be extrapolated between different locations, 401 2015) show that inverse modelling of these charac- at any single location it is possible to discriminate 402 teristics can be used to identify tsunami-generation between tsunami and storm deposits where both 403 mechanisms (MacInnes et al. 2010; Sugawara are present (e.g. Nanayama et al. 2000; Goff et al. 404 et al. 2012). However, palaeotsunami magnitudes 2004; Tuttle et al. 2004; Switzer et al. 2005). 405 and inflow characteristics derived from inverse Research on the sediment history of tsunamis at 406 numerical models, such as TsuSedMod (Jaffe & individual locations can be used in improved D. R. TAPPIN

407 understanding of event recurrence and frequency, as sedimentary fabric of fine-grained unconsolidated 408 in Cascadia and Japan (Atwater & Hemphill-Haley tsunami deposits at resolutions down to particle 409 1997; Minoura et al. 2001; Ishimura & Miyauchi scale. The results are validated by comparing to 410 2015). The surveys focused on the catastrophic data obtained using other techniques such as laser 411 events in the Indian Ocean and Japan yielded diffraction, AMS and X-ray microfluorescence. 412 major advances in understanding of the (mainly) 413 fine-grained sediments deposited, but also boul- Numerical modelling 414 ders. With the Indian Ocean tsunami, understanding 415 was improved on characterizing the sediments and The study of deposits from the PNG, the Indian 416 investigating their preservation and alteration over Ocean and Japan tsunamis has resulted in improved 417 time. With Japan, the advances were in the geo- insights into the processes and forces acting during 418 chemical characterization of the sediments and tsunami inundation based on the hydrodynamic 419 their use in numerical models (see ‘Tsunami sedi- characteristics of the tsunami that control sediment 420 ments in numerical modelling’ below). In both deposition (Jaffe & Gelfenbaum 2002; Spiske 421 instances, the focus was also on characterizing the et al. 2008; Witter et al. 2012). Numerical inverse 422 deposits to provide valid diagnostic criteria to help modelling, however, is still at an early stage of 423 identify palaeotsunami deposits at specific loca- development and a more general physical under- 424 tions, from which the tsunami hazard at the location standing of hydrodynamic processes and their inter- 425 is better understood (Jankaew et al. 2008; Sugawara play with sediment is required to advance this 426 et al. 2012). approach (Cheng & Weiss 2013). For instance, stud- 427 ies of the deposits may be used to assess the tsunami 428 Analysis: new technological developments flow velocity and the depth of tsunami inundations. 429 As noted above, however, inflow and backflow 430 Sedimentological analysis of deposits has advanced deposits have to be reliably identified for the numer- 431 considerably, but new analytical techniques provide ical models to be valid. Notwithstanding, tsunami 432 additional methodologies to support classical sediments have been used to identify their earth- 433 approaches. The use of geochemical profiling (‘tool- quake source mechanisms. On the west coast of 434 kits’) of recent deposits has advanced considerably Kamchatka, alongshore distribution of tsunami sed- 435 (Chague´-Goff et al. 2011, 2012). For example, geo- iments was used to discriminate between two earth- 436 chemical analysis of soil profiles landwards of the quakes that took place in 1969 and 1971 (Martin 437 limit of sand deposition now allow the maximum et al. 2008). A study of the Kamchatka earthquake 438 extent of tsunami inundation to be identified tsunami of 1952, based on the variable distribution 439 (e.g. Goto et al. 2011; Chague´-Goff et al. 2012). of tsunami deposits, resulted in new models of earth- 440 There are constraints with older deposits, how- quake slip (MacInnes et al. 2010). 441 ever, where the use of geochemistry may be limited The major scientific response to the Indian 442 because of poor sediment preservation and post- Ocean tsunami resulted in impacted coastlines 443 depositional alteration (taphonomy) and reworking being researched for tsunami sediments (e.g. Bor- 444 by rainfall, wind action, bioturbation and human rero 2005; Kench et al. 2006; Paris et al. 2007; 445 activity (Szczucin´ski 2011, 2013). Other developing Goto et al. 2008a; Morton et al. 2008). The devas- 446 branches of study on tsunami sediments include the tating Japan tsunami of 2011 resulted in the acquisi- 447 use of heavy minerals in their characterization, tion of an extensive and comprehensive dataset of 448 anisotropy of magnetic susceptibility (AMS) and the sediments deposited. It provided another major 449 X-ray tomography. Analysis of heavy minerals is opportunity to improve understanding of tsunami 450 now being used to discriminate tsunami deposits sedimentation and its use in mitigation (e.g. Goto 451 from other high-energy depositional mechanisms et al. 2011; Szczucin´ski et al. 2012). The response 452 such as storms (e.g. Costa et al. 2017). AMS pro- to the coastal inundation was the most intensive of 453 vides the sedimentary fabric of the tsunami deposits any previous event, with both national and interna- 454 from which flow directions are identified. There are tional teams involved in field studies (e.g. Goto 455 still too few events studied, but this technique has et al. 2011; Mori et al. 2011). From the Japanese 456 been used successfully on sandy deposits in northern field surveys, there were a number of new insights 457 Sumatra from the Indian Ocean tsunami (Wassmer into tsunami sediment deposition: (1) tsunami inun- 458 et al. 2010) and pyroclastic volcanic deposits dation much greater than the depositional limit of 459 from the Krakatau eruption of 1883 (Paris et al. sand; (2) geochemical analysis of muddy sediments 460 2014). X-ray tomography is the most recent devel- that identify tsunami inundation beyond the limit of 461 opment, but has only been used on sandy deposits sand deposition; (3) a minor component of marine 462 from the Lisbon tsunami of 1775 (Falvard & Paris microfossils in tsunami sediment; (4) coarse gravel 463 2017). It allows the characterization of grain-size deposits thicker than sands; and (5) varied beach 464 distribution, structures, component analysis and erosion, limited in some areas but intense in others IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

465 (Goto et al. 2011). These new observations formed but surprisingly did not cite the earlier 1933 paper. 466 the basis for improved understanding of older This is unfortunate, because the opportunity to 467 deposits preserved in the region of the 2011 inunda- reconsider the previous interpretation of a tsunami 468 tion. One of the most devastating older historic origin in the context of improved sedimentological 469 events was the Jogan tsunami of 869. Comparison understanding was missed. Interpretation of other 470 between the sediments from the two events (Min- boulder deposits has also been controversial, for 471 oura et al. 2001) showed the inland inundation of example discrimination between recent storm and 472 Jogan to be far greater than previously recognized. boulder deposits on the Aruba, Bonaire and Curac¸ao 473 In addition, from inverse modelling of the deposits (ABC) islands in the southern Caribbean (see Mor- 474 the 869 earthquake magnitude was re-evaluated ton et al. 2006). 475 and found to be much greater than previously esti- Some of the most controversial coarse-grained 476 mated (Sugawara et al. 2013). The result was a tsunami deposits are in the Hawaiian Islands. 477 major advance in understanding earthquake fre- Here, boulders and marine gravels up to 200 m 478 quencies. The Japan, 2011 event therefore offered above present sea level (Fig. 4) are attributed to tsu- 479 an important opportunity to improve: (1) how namis from volcanic flank collapse (Moore 1964; 480 inland inundation can be identified; (2) the method- Moore & Moore 1984, 1988; Moore et al. 1989). 481 ology of using inversion of sedimentary data to The deposits were first interpreted as marine high- 482 establish the 2011 earthquake rupture magnitude stands (Stearns & Macdonald 1946; Stearns 1978), 483 and extent; and (3) existing models of older, com- then as tsunami deposits (in the Moore papers) and 484 parable, events in the same region (Sawai et al. then again as highstands (Rubin et al. 2001). The 485 2012; Sugawara et al. 2013). The scientific advances best-preserved deposits are on Lanai Island, and 486 were both qualitative (e.g. Goto et al. 2011) and recent interpretations attribute their origin to the 487 quantitative (Sugawara & Goto 2012; Sugawara last two interglacial highstands at 120 and 240 ka 488 et al. 2014). BP (Grigg & Jones 1997; Felton et al. 2000; Rubin 489 et al. 2000). A problem in interpreting the origin of 490 Boulder/gravel deposits the deposits is the similarity between the ages of the 491 highstands and the triggering of the landslides. The 492 Tsunamis deposit individual boulder trains and submarine landslides from volcanic collapse are 493 boulder groups (Goto et al. 2007; Ramalho et al. probably triggered at the end of glaciations, at 494 2015) as well as boulders entrained in finer-grained which time sea levels were rising rapidly (McMur- 495 sediment (Paris et al. 2004; Yamada et al. 2014). try et al. 1999; Quidelleur et al. 2008). The origin 496 The first attempts to understand the hydrodynamic of the deposits are therefore quite complex. The 497 mechanisms of deposition of both types of deposit importance in establishing the origin of the Hawai- 498 were by Nott (1997); there are still uncertainties ian gravels is two-fold: (1) to prove (or not) the tsu- 499 however because boulders are moved differently nami hazard from the volcanic collapses mapped 500 by storms and tsunamis, resulting in different offshore the islands (Fig. 5); and (2) to validate tsu- 501 deposit characteristics (Weiss 2012). Discrimina- nami run-up elevations from the numerical models 502 tion of tsunami boulder deposits from storm depos- of volcanic collapse (McMurtry et al. 2004b, fig. 503 its is problematic (Felton 2002; Hall et al. 2006; 3). The controversy over the origin of the deposits 504 Morton et al. 2006), but recent work suggests that (from highstands or tsunamis) compromises their 505 tsunamis produce disorganized boulder deposits use in identifying the tsunami-generating potential 506 whereas, storms organize boulders along lines and from volcanic collapse. 507 clusters (Goto et al. 2010b; Weiss 2012). Individual The basis of the controversy over the interpreta- 508 boulders cast ashore have been used to identify tsu- tion of the Hawaiian deposits is the uncertainty in 509 nami impact (Goto et al. 2010b) as well as in vali- the elevations of the islands when the sediments 510 dating tsunami-generation mechanisms and their were deposited. The flank collapses (and sediments) 511 magnitude (Moore et al. 1995; McMurtry et al. are thousands to hundreds of thousands years old. 512 2004a). As with fine-grained sediments, discrimina- The elevations of the Hawaiian Islands have 513 tion is possible where both storm and tsunami changed since these deposits took place; some 514 deposits are preserved at the same location (Goto have been uplifted whereas others have subsided. 515 et al. 2010b). Only the Big Island has a well-established vertical 516 Boulder deposits of tsunami origin first de- tectonic history, which shows subsidence for hun- 517 scribed as far back as 1933 from eastern Scotland dreds of thousands of years (Ludwig et al. 1991). 518 are coarse-grained conglomerates attributed to The elevation of the sediments at their time of dep- 519 earthquake-triggered landslides, which were re- osition is calculated from their age, present eleva- 520 worked by an ensuing tsunami (Bailey & Weir tion and the known subsidence rate of the island. 521 1933). Subsequent research by Pickering (1984) In the north of the Big Island at Kohala, corallifer- 522 proposed only a fault-scarp origin for the deposits, ous gravels are now at sea level. These deposits D. R. TAPPIN

523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541Colour 542online/ 543mono hardcopy544 545 546 547 548 549 550 Fig. 4. Tsunami sediments from Lanai Island, Hawaii deposited from large-scale volcanic flank collapse. Note the 551 two coarsening-upwards cycles. Geological hammer 40 cm (Photograph, D.R. Tappin). 552 553 554 are dated at 120 ka BP, the same age as the Alika 2 depositional mechanisms such as storms, especially 555 landslide just offshore (Fig. 5). Alika 2 is the most where both storms and tsunamis affect the same 556 likely source of the tsunami which laid the deposits coast (Hearty 1997; Noormets et al. 2002; Mastro- 557 down (McMurtry et al. 2004a). Since deposition nuzzi & Sanso 2004; Mylroie 2008; Switzer & Bur- 558 120 ka ago, Hawaii has been subsiding at a rate of ston 2010; Weiss & Diplas 2015). The elevation of 559 3.4 mm a21. The elevation of the sediments at the deposits is an important discriminant (Ramalho 560 time of deposition was therefore 400 m above et al. 2015) but local tectonic history needs to be 561 present sea level. Numerical tsunami modelling of established to avoid conflict, such as in Hawaii 562 the failure of the Alika phase-2 giant submarine (Rubin et al. 2000; McMurtry et al. 2004a). Apart 563 landslide results in tsunami run-ups of hundreds of from boulder elevations, there are geomorphologi- 564 metres on Hawaii and Lanai (McMurtry et al. cal and sedimentological methodologies proposed 565 2004b, fig. 3). The most recent evidence from the that might discriminate between the different mech- 566 Big Island therefore confirms that the deposits are anisms (e.g. Morton et al. 2006; Spiske et al. 2008; 567 indeed from a tsunami hundreds of metres in eleva- Goto et al. 2010a). Unfortunately, there are still too 568 tion. The fringing coastal (? tsunami) deposits on few case studies based on a systematic sedimento- 569 both Hawaii and Lanai are compositionally very logical approach to deposit analysis to formulate 570 similar. It therefore seems likely that the explana- robust criteria for distinguishing between coarse- 571 tion for the gravels at both locations (The Big Island clast storm and tsunami deposits (Morton et al. 572 and Lanai), at least for the youngest deposits dated 2006). 573 at 120 ka BP, is a tsunami generated during postgla- 574 cial volcanic collapse. 575 Individual boulders without associated finer- Submarine landslide tsunami 576 grained sediment are common along many shore- 577 lines, and have the potential to improve tsunami The potential for submarine landslides to generate 578 hazard assessment. As with fine-grained tsunami tsunamis has been known for over 100 years 579 deposits, however, there is considerable controversy (Milne 1898; Montessus de Ballore 1907; Guten- 580 over their discrimination from the other potential berg 1939), yet most research has been on IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605Colour 606online/ 607mono 608hardcopy 609 610 611 612 613 614 Fig. 5. Hawaiian seabed morphology and locations and extents of the Giant Submarine Landslides as mapped by 615 EDQ2 Moore et al. (1989) (From Tappin, 2010). 616 617 618 EDQ3 mechanisms of failure (Piper et al. 1987; Talling Aleutians in 1946 (e.g. Heidarzadeh et al. 2008), 619 et al. 2007; Masson et al. 2009; Talling 2014) and but no submarine landslide has been found or, 620 very little published on their potential to generate indeed, even looked for. The tsunamis generated 621 tsunamis (Masson et al. 2006). Significant proven by the earthquake of 17 August 1999 at Izmit, Tur- 622 historical submarine landslide-generated tsunamis key are also probably associated with submarine 623 before 1998 include those of the Grand Banks in landslides (Altinok et al. 2001). 624 1929 (Heezen & Ewing 1952), Nice in 1979 The best-known historical submarine landslide 625 (Assier-Rzadkiewicz et al. 2000; Dan et al. 2007) tsunami before PNG was in 1929 on the Grand 626 and Skagway in 1994 (Kulikov et al. 1996; Rabino- Banks, Canada but surprisingly, considering the 627 vich et al. 1999). Both Nice and Skagway landslides importance of the event, most research has been 628 were probably triggered by human impact, although on the landslide (e.g. Piper & Asku 1987) and earth- 629 the tsunami mechanisms are controversial. Both quake (e.g. Hasagawa & Kanimori 1987) and not the EDQ4 630 these events came sharply into focus after the tsunami. Only one paper is published on numerical 631 PNG tsunami (Synolakis & Bernard 2006). Other tsunami modelling of the submarine landslide 632 controversial events include the tsunami generated (Fine et al. 2005), but this is based on a theoreti- 633 by the earthquake of 1946 in the Aleutians, where cal mechanism. The most significant prehistorical 634 the 40 m local run-ups were most probably from a tsunami from a submarine landslide is Storegga 635 local submarine landslide triggered by the earth- which, because of the discovery of the Ormen 636 quake (Fryer et al. 2004; Okal & Herbert 2007; Lange Gasfield, is also the best studied. Research 637 von Huene et al. 2014). The 1945 tsunami in the on tsunamis generated from volcanic collapse has 638 Indian Ocean off Pakistan is a similar event to the identified another submarine landslide mechanism, D. R. TAPPIN

639 analogous to clastic sedimentary events. However, 2015) and possibly underestimated (Simon Day, 640 volcanic collapse tsunamis, even those in the pers. comm., 2016). 641 Hawaiian Islands, have not generated as much 642 interest in their hazard to the same degree by The impact of Papua New Guinea, 1998 643 which PNG raised the profile for non-volcanic sub- 644 marine landslides. The exceptions are the Canary It was not until 1998 in PNG, when over 2200 people 645 Islands landslides perhaps, which have had a consis- died in a tsunami now recognized as generated from 646 tently high media profile because of their potential an offshore seabed sediment failure (Kawata et al. 647 to generate high-elevation, ‘megatsunamis’ in the 1999; Tappin et al. 1999), that the hazard from sub- 648 far field, which could be a significant hazard to the marine landslide tsunamis was fully recognized 649 east coast of the USA (Ward & Day 2001; Gisler (Bardet et al. 2003; Løvholt et al. 2015). The PNG 650 et al. 2006; Løvholt et al. 2008; Hunt et al. 2013; event was the first investigated by a responsive pro- 651 Tehranirad et al. 2015). A point to note is that the gramme of marine surveys (Fig. 6), and scepticism 652 Hawaiian collapses are submarine, whereas those greeted the initial results (e.g. Geist 2000). The land- 653 in the Canary Islands are partly sub-aerial. Because slide architecture was constructed from the marine 654 of their retrogressive failure mechanisms however, dataset, and for the first time used as the basis for 655 which initiate on the seabed, the Canary Island col- realistic numerical models of tsunami generation 656 lapses are included here. The Canary Island col- (Tappin et al. 1999, 2001, 2008). Although the haz- 657 lapses are analogous to those of the Hawaiian ard from submarine landslides is now more generally 658 volcanoes but smaller in volume which, together recognized, the marine surveys required to map the 659 with their multistage, bottom-up collapse mecha- submarine landslide hazard are expensive so only a 660 nisms proposed for the largest events such as those few countries have so far carried these out to the 661 on Tenerife (Hunt et al. 2011), suggest that elevated degree necessary to fully identify the hazard (e.g. 662 tsunami wave heights could be more localized and Grilli et al. 2009; ten Brink 2009; Clarke et al. 2014). 663 concentrated near to source. Recent numerical tsu- 664 nami modelling indeed suggests that, in the far How submarine landslides generate tsunamis 665 field, frictional effects of propogation over the 666 wide US coast shelf significantly reduces their Before PNG, simulations of tsunami generation were 667 onshore impact, although the volumes used in tsu- mainly confined to earthquake sources (Satake et al. 668 nami generation may be small (Tehranirad et al. 1996). Numerical earthquake tsunami-generation 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685Colour 686online/ 687colour hardcopy688 689 690 691 692 693 694 Fig. 6. Papua New Guinea regional bathymetry viewed from the north, showing the convergent margin in the 695 foreground and the Papua land mass to the rear. Depths in metres. Black square the location of Figure 7 (From 696 Tappin, 2010). IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

697 models are based on an assumption that the initial Submarine landslide numerical models 698 water surface deformation is instantaneous and 699 equal to that at the seabed. For the rise time of Storegga (8.5 ka BP). The first realistic attempt at 700 most earthquakes, the long-wave phase velocity in numerically modelling a submarine landslide 701 the ocean is slow enough so that displacement can based on a slide architecture from seabed morphol- 702 be considered instantaneous. There are slight modi- ogy was Storegga (Harbitz 1992). Validation of the 703 fications to the tsunami wave field for earthquakes tsunami generated was from run-up recorded in 704 of slow rupture duration (tsunami earthquakes). coastal sediments deposited on the east coast of 705 Seabed deformation is calculated from earthquake Scotland (Fig. 3; Dawson et al. 1988; Long et al. 706 fault parameters using theoretical deformation 1989) and uplifted lake sediments in the west of 707 models such as Okada (1985). Underwater land- Norway (Svendsen & Mangerud 1990). The land- 708 slides were considered to be ineffective at generating slide modelled was translational, moving at veloci- 709 significant tsunamis because of their longer source- ties of 20–35 m s21 which were taken from 710 generation times, smaller areas of seabed disturb- measurements of the Grand Banks tsunami of 711 ance (compared to earthquakes) and the directivity 1929 (Heezen & Ewing 1952). Three major slide 712 of the tsunami produced (Hammack 1973; LeBlond events were modelled: the first and third as partially 713 & Jones 1995; Geist 2000). Before the PNG event, liquefied debris flows; and the second retrogressive 714 one of the major challenges was in understanding failing from the bottom upwards. Individual slide 715 how the relatively slow-moving submarine land- volumes were between 1700 and 3880 km3. Aver- 716 slides generate tsunamis. age slide thicknesses used were between 88 and 717 A further complication in understanding tsuna- 114 m. The recorded run-up heights of 4 m on the 718 mis generated by submarine landslides is the range east coast of Scotland were best reproduced by slide 719 of failure mechanisms which vary according to velocities of 35 m s21. The 1992 paper was a bench- 720 morphology, sediment type and/or kinematics mark in tsunami numerical models as it was based 721 (see Hampton et al. 1996; Turner & Schuster 1996; on both a realistic landslide model and run-up data 722 Keating & McGuire 2000; O’Grady et al. 2000). measured from sediments deposited on land. The 723 Theoretical numerical modelling of submarine land- numerical model was a major advance at the time, 724 slides was visualized as a Bingham-type fluid flow, because only a few earthquake tsunamis had been 725 analogous to a translational mechanism, where simulated and the controls on tsunami generation 726 large blocks disintegrated on travelling downslope by landslide architecture were not recognized. Sub- 727 to form turbidites (Hampton 1972; Geist 2000). marine landslide tsunamis were only identified 728 Modelling of solid block landslides at the time of along two ocean margins: Storegga (Norway) and 729 the PNG tsunami was in its infancy (Watts 1998). Grand Banks (Canada). In 1992, there were no 730 Submarine landslide failure is dependent mainly on numerical models of the Grand Banks tsunami. 731 sediment composition, which controls landslide Subsequent numerical models of Storegga post- 732 morphology and kinematics. Numerical tsunami- dated the PNG event (Bondevik et al. 2005a; Hill 733 generation models were initially based on depth- et al. 2014) and made significant improvements on 734 averaged wave equations that represented immisci- the 1992 results of Harbitz as they were based on 735 ble liquids or water as a Bingham plastic (e.g. a more comprehensive dataset of geophysics and 736 Jiang & LeBlond 1992, 1994). While depth-averag- coring of the landslide (Bryn et al. 2005). The moti- 737 ing accurately applies to tsunami generation from vation to investigate Storegga was not the 8.2 ka BP 738 earthquakes, it is questionable when applied to land- tsunami, but to ensure that exploitation of the under- 739 slide tsunamis because it does not allow for vertical lying Ormen Lange gas field would not create 740 fluid accelerations, important during submarine another hazardous submarine landslide similar to 741 landslide motion and tsunami generation (Grilli the 8.2 ka BP event (Bryn et al. 2005). Validation 742 et al. 2002). In 1998, landslide constitutive equations of later numerical models was based on a more 743 used in modelling were largely untested by labora- extensive dataset of tsunami run-up data from Nor- 744 tory experiments or case studies (Tappin et al. way (Bondevik et al. 1997a, b), Faroe Islands 745 2008). Submarine landslide models were idealized, (Grauert et al. 2001), Shetland Islands (Bondevik 746 and not based on geological data. There was no et al. 2003, 2005b) and mainland Scotland (Smith 747 established method of merging geological data et al. 2004, 2007). With the later numerical models 748 with numerical landslide models. In total, there based on improved landslide architecture, maxi- 749 was little appreciation of the complexity of model- mum tsunami run-ups on the Shetlands increased 750 ling tsunamis generated by the different submarine to 20 m which agreed with new studies on the eleva- 751 landslide mechanisms. All this was to change tions of tsunami sediments on the islands (Bondevik 752 because of two major events: one prehistoric and et al. 2003). Sea levels were much lower when the 753 thousands of years old, Storegga; and the other Storegga landslide took place (8.2 ka BP) than at 754 recent and devastating, Papua New Guinea in 1998. present. The sea-level curve for the Shetlands is D. R. TAPPIN

755 still uncertain, so these run-up elevations are most that devastated the coast around Sissano Lagoon. 756 probably still underestimated. It was not a ‘tsunami’ earthquake based on Newman 757 The later numerical models of the Storegga tsu- & Okal’s (1998a, b) discriminant of E/M0 (E/M0 758 nami also used a more comprehensive geotechnical is the ratio between high-frequency energy E and 759 and morphological dataset from the landslide (Fors- low-frequency seismic moment M0), because it 760 berg 2002; Haflidason et al. 2005). These confirm did not have the required ‘slow’ source characteris- 761 that the Storegga failure mechanism was retrogres- tics. Several other aspects of the event also sug- 762 sive, with large block failure initiated at the base of gested the earthquake was not responsible for the 763 the slide at a water depth of c. 1000 m. The time lag tsunami. The aftershock distribution indicated a 764 between the individual block failures as the slide shallow-dipping thrust (McCue 1998; Hurukawa 765 retreated landwards is a major control on tsunami et al. 2003) rather than a steeply dipping event 766 generation (Bondevik et al. 2005a; Løvholt et al. necessary to generate the recorded tsunami. The 767 2005). Tsunami run-up elevations from sediments peaked run-up distribution along the coast sug- 768 preserved at the different locations constrains the gested a local focused source rather than the broad 769 timing of block slide development. The shape and source usually associated with an earthquake tsu- 770 volume of the modelled slide used by Harbitz nami. The 20 minute time lag between the felt earth- 771 (1992) were adjusted until they fitted the new and quake and the tsunami striking the coast did not 772 much more detailed slide reconstruction. Modelling agree with an epicentre located just offshore. The 773 of Storegga was not just to simulate the tsunami, tsunami in the far field was very small and undoubt- 774 but also to establish the slide mechanics and trigger- edly generated from the earthquake (Kikuchi et al. 775 ing (Kvalstad et al. 2005). In the revised slide model 1999), but it seemed unlikely that the local tsunami 776 the maximum thickness (400 m) of the slide is near was from this mechanism. 777 the upper headwall, gradually becoming thinner All evidence therefore initially converged on a 778 towards the slide front in the offshore direction. submarine landslide mechanism located close off- 779 The slide volume generating the tsunami is esti- shore. This conclusion was highly controversial 780 mated to be 2400 km3. The Storegga slide is proba- (e.g. Geist 2000), so marine hydroacoustic and 781 bly the best studied of any tsunami from a sampling surveys were required for confirmation. 782 translational-type failure mechanism. For the first time after a major tsunami, respon- 783 sive marine surveys funded by Japan and the USA 784 Papua New Guinea tsunami (1998). The PNG tsu- acquired a comprehensive hydroacoustic dataset 785 nami struck during the initial development of the with some sediment sampling off northern PNG 786 Ormen Lange gas field in 1998. PNG was an impor- (Tappin et al. 1999, 2001; Sweet & Silver 2003). 787 tant precursor to the Indian Ocean tsunami of 2004 The hydroacoustic data comprised over 19 000 km2 788 because the loss of life was greater than from any of multibeam bathymetry (Fig. 6), 4.2 kHz high- 789 previous recent event. It resulted in a major revision resolution, sub-bottom seismic lines (SBSL), and 790 in understanding of tsunami-generation mecha- both single (SCS) and multichannel seismic (MCS) 791 nisms from submarine landslides; in this context it data. In the region of the landslide, four 7 m long 792 is second only in importance, although the genera- sediment piston cores were recovered together 793 tion mechanisms are different. Identifying the with numerous shallow (30 cm) push cores of sedi- 794 mechanism of the PNG tsunami required marine ment, rock samples and marine organisms. Still and 795 geophysical and geological data and, as with tsu- video photography of the seabed were acquired 796 nami sediments, it utilized the expertise of geolo- from a tethered remotely operated vehicle (ROV) 797 gists (Tappin et al. 1999, 2001, 2008). PNG also and a manned submersible (MS). 798 provided an important context to the identifica- Before the surveys, a translational landslide was 799 tion of tsunamis from tsunami earthquakes (Kana- proposed as the most likely tsunami mechanism 800 mori 1972), where the associated tsunami is much (e.g. Geist 2000) and the first published numerical 801 larger than expected from the earthquake magni- models were based on this mechanism (e.g. Hein- 802 tude. It showed that although an earthquake might rich et al. 2000). These numerical models success- 803 be small it might not be slow, and the tsunami fully reproduced the recorded on-land tsunami 804 mechanism could alternatively be from a submarine elevation data, with the translational landslide 805 landslide. The discrimination between tsunamis treated as a fluid-like flow of a cohesionless granular 806 generated by ‘tsunami’ earthquakes and submarine material (Heinrich et al. 2000). From the marine sur- 807 landslides is still uncertain, as initially suggested veys, however, this translational mechanism was 808 by Kanamori (1972) and further supported by events shown to be unrealistic because seabed morphology 809 such as Java in 2006 (e.g. Kanamori 1972; Fritz and seismic data revealed a rotational slump (Fig. 7; 810 et al. 2007). Tappin et al. 1999). Theoretically, slumps by vol- 811 The earthquake magnitude of the PNG event was ume generate the largest tsunami (Tappin et al. 812 small in comparison to the 10–15 m high tsunami 2008). At the time of the PNG tsunami, theoretical IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838Colour 839online/ 840mono 841hardcopy 842 843 844 845 846 847 Fig. 7. 3D cutaway image of the PNG slump C that generated the 1998 tsunami, including a seismic section, 848 viewed from the NE. Vertical exaggeration ×3. Location shown in Figure 6 (from Tappin et al. 2008). 849 850 851 numerical models of tsunamis were based on: (1) landslide mechanism at PNG (see Tappin 2010a) 852 sliding blocks (e.g. Watts 1997; Grilli & Watts multibeam echosounder (MBES) technology was 853 1999); (2) finite volume discretization–volume of used to map the detailed seabed morphology, the 854 fluids (VOF) of the Navier–Stokes equations (Hein- first time this had been attempted. The MBES 855 rich 1992); or (3) deformable landslides, with the data, combined with sub-seabed seismic, led to the 856 generated waves governed by the finite volume dis- construction of the submarine landslide architecture 857 cretization (Jiang & LeBlond 1992, 1994). (Fig. 7) that underpinned the numerical wavemaker 858 To model the tsunami from the rotational slump, models. In addition, whereas earthquake rupture 859 new numerical models were developed based on models are mainly defined by strike, dip and rake, 860 rotational failure with travel distance being limited the failure mechanisms of submarine landslides 861 (800 m) (Watts et al. 2003). The results showed are many and varied as they are dependent on the 862 that non-linear and dispersive tsunami propagation nature of the sediment. As well as the differences 863 models were necessary to model submarine land- in initial tsunami wave generation between earth- 864 slide tsunamis, with the shape and motion of a real- quakes and submarine landslides, there are also sig- 865 istic submarine landslide wavemaker defined from nificant differences in how their tsunamis propagate. 866 marine survey data. This is unlike earthquake tsuna- Coseismic displacement from vertical seafloor 867 mis where numerical wavemaker models are based deformation usually generates tsunamis with longer 868 on ground deformation from fault slip, derived wavelengths and periods than those generated 869 from inversion of seismological, geodetic or tsu- by landslides, because of their larger source area 870 nami observations (Okada 1985). To identify the (Hammack 1973; Watts 1998, 2000). Coseismic D. R. TAPPIN

871 displacement generates tsunami amplitudes that wave elevations at the coast. This was a significant 872 correlate with earthquake magnitude (Hammack improvement over earlier simulations using tsunami 873 1973; Geist 1998) except for tsunami earthquakes; source and (non-dispersive) shallow-water, wave 874 submarine landslides produce tsunamis with ampli- tsunami propagation models (see discussion in 875 tudes limited only by the vertical extent of centre of Tappin et al. 2008). 876 mass motion or the water depth (Murty 1979; Watts 877 1998). 878 The first numerical simulation of the PNG slump Raised awareness after PNG 1998 event 879 was devised at sea during the first survey in 1999; it 880 was rudimentary, with many assumptions not vali- The great loss of life from the PNG tsunami and 881 dated. Initial tsunami-generation estimates were the demonstration of the non-earthquake mecha- 882 computed by hand, based on published (or soon to nism from numerical models were a catalyst to an 883 be published) literature (Watts 1998, 2000; Grilli intense period of research on how submarine land- 884 & Watts 1999). The slump architecture was provi- slides generate tsunamis (Synolakis & Bernard 885 sional because it was based only on the bathymetric 2006). Recognition of the submarine landslide haz- 886 data acquired during the survey (Tappin et al. ard in tsunami generation resulted in increased 887 1999). The tsunami source used was a solid block awareness of these events but also that there were 888 two-dimensional (2D) underwater landslide (Grilli no comprehensive models that covered all aspects 889 & Watts 1999). It did not use depth-averaging; in- of landslide-induced tsunamis from source mecha- 890 stead, it solved fully non-linear potential flow nism, through propagation to coastal inundation. 891 (FNPF) equations which allowed for vertical water Before the identification of the submarine landslide 892 acceleration. However, the tsunami propagation mechanism of the PNG tsunami, understanding of 893 simulation used linear shallow-water wave equa- the mechanics of landslide tsunamis was lacking. 894 tions. The maximum wave height of 6 m generated After the PNG event, there was a re-evaluation in 895 by the model was located offshore of the sand spit at the USA of other possible submarine landslide 896 the 10 m water depth contour. Although approxi- events such as Palos Verdes, California (Fig. 9) 897 mating the relative distribution of run-up along where the under-prediction of the height of the lead- 898 the coast, the maximum offshore water height was ing wave led to the dismissal of the local tsunami 899 not of the same magnitude as run-up measured on hazard. PNG also led to the resolution of the dispute 900 land by the responsive surveys. Despite these short- concerning the landslide trigger of the 1994 Skag- 901 comings, the results were a major advance; not only way, Alaska tsunami (Synolakis & Bernard 2006). 902 was it the first time that a landslide-generated tsu- Marine geophysical data now reveal that submarine 903 nami was modelled from a real event, but the mod- landslides are common along most continental mar- 904 elling was based on MBES data. Comparison with gins (Fig. 9), especially those of California, Oregon 905 the alternative earthquake-generation mechanism, and the east Coast of the USA. As a result, the level 906 which gave a maximum wave height at the shore of hazard posed by relatively moderate earthquakes 907 of 2 m based on a shallow-dipping rupture mecha- and submarine landslides was re-examined (e.g. 908 nism, demonstrated that it was the slump rather Borrero et al. 2001, 2004). There was an increased 909 than the earthquake that generated most of the recognition, by both the scientific research and tsu- 910 local tsunami. nami forecasting communities, that earthquakes 911 Subsequently, numerical models improved with affecting oceanic margins frequently trigger sub- 912 the definition of slump architecture (Fig. 8) (Watts marine landslides (e.g. Geist et al. 2009; Grilli 913 et al. 1999; Tappin et al. 2001). Most recent numer- et al. 2015). Although many of these might not be 914 ical modelling (Tappin et al. 2008) is from a slump tsunamigenic in the far field, they had the potential 915 modified from the marine survey data that included to generate significant local tsunamis, even if the 916 seismics (Fig. 7). Numerical modelling used an ini- earthquake magnitude was small and not of a suffi- 917 tial condition (wavemaker) using ‘Tsunami open cient magnitude to generate a significant coseismic 918 and progressive initial conditions system’ (TOP- event. 919 ICS) software that provides the vertical landslide Once the landslide mechanism became evident, 920 displacements as outputs, as well as a characteristic one of the first responses to the PNG tsunami was 921 tsunami wavelength l0 and a characteristic tsunami the workshop on landslide tsunamis sponsored by 922 period T0. To account for the dispersive nature of the National Science Foundation at the University 923 landslide tsunamis, the Boussinesq propagation of Southern California on 10–11 March 2000. The 924 models GEOWAVE (Watts et al. 2003) and the resulting special publication (Bardet et al. 2003) 925 later development FUNWAVE (Tappin et al. recognized that landslide tsunamis require multi- 926 2008) were used. The initial numerical models pro- disciplinary studies that build upon experience in 927 vided tsunami wave elevations offshore, not on-land engineering seismology, geotechnical engineering, 928 run-ups, and the later modelling provided tsunami marine geology, modelling of sediment deposition IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975Colour 976online/ 977mono 978hardcopy 979 980 981 982 983 984 Fig. 8. Eight snapshots of Papua New Guinea, 1998 tsunami propagation and inundation from a slump source. Light 985 blue are elevation waves and dark blue are depression waves. Numbers in the top right of each image are the tsunami 986 propagation times after the main earthquake shock trigger. The slump location is in yellow (from Tappin et al. 2008). D. R. TAPPIN

987 988 989 990 991 992 993 994 995 ted along convergent 996 997 es: landslides along 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 Global distribution of mapped submarine landslides. Green lines: landslides on continental shelves and fan systems. Yellow lines: landslides loca 1043 Fig. 9. 1044 margins. Red lines: locations ofstrike-slip landslide-sourced margins. tsunamis, Pale or yellow where lines: there landslides may along be fjord a margins. Significant landslide contribution. landslides Pale named. blue lines: volcanoes. Orange lin / colour Colour online hardcopy IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

1045 and run-off, and hydrodynamics. Another special simulations from earthquake mechanisms could 1046 publication (Tappin 2007) focused on tsunami sed- not reproduce the elevated (40 m) wave elevations 1047 iments. In approximately 35% of all tsunami events recorded along the coast between latitudes 398 30′ 1048 studied, nearshore waves from landslide tsunamis and 408 15′ N (e.g. Fujii et al. 2011). Secondly, 1049 could exceed those resulting solely from coseismic even when inverting tsunami waveforms and using 1050 ground motions (Watts 2004). In the USA, subma- dispersive-wave Green’s functions, the simulations 1051 rine landslide research programmes resulted in a could not satisfactorily reproduce the timing and 1052 special issue of the journal Marine Geology focused high-frequency content of tsunami waveforms 1053 on the tsunami hazard along the east coast of the US recorded at the nearshore GPS buoys located in 1054 (Chaytor et al. 2009; Geist & Parsons 2009; Grilli this area, nor the timing and dispersive-wave train 1055 et al. 2009; Lee 2009; ten Brink 2009; ten Brink at the Deep-Ocean Assessment and Reporting of 1056 et al. 2009; Twichell et al. 2009). Tsunamis (DART) buoy #21418 located 600 km 1057 Later tsunami landslide research made some sig- off the coast (Gusman et al. 2012; Iinuma et al. 1058 nificant counterintuitive discoveries. One of the 2012; Løvholt et al. 2012; Romano et al. 2012; 1059 most fascinating was that along the passive margins Grilli et al. 2013; Satake et al. 2013; Yamazaki 1060 of the North Atlantic (Fig. 9) the majority of poten- et al. 2013). These deficiencies were identified 1061 tially tsunamigenic submarine landslides were by comparison of the tsunami mechanisms from 1062 found to occur on seabed slope angles of less than ten earthquake source models, obtained by invert- 1063 58, with some of the largest slides failing on slopes ing seismic and geodetic data and tsunami wave- 1064 of less than 18 (Hu¨hnerbach et al. 2004). This evi- forms (see MacInnes et al. 2013, fig. 4). This 1065 dence on slope failure resulted in considerable con- comparison found that none of the mechanisms 1066 jecture on how submarine landslides fail (Canals satisfactorily reproduced the elevations of the 1067 et al. 2004; Smith et al. 2013; Talling et al. 2014). recorded run-ups on the Sanriku coast north of lati- 1068 Subsequently, there has been a large amount of tude 398 00′ N. 1069 physical and numerical modelling work devoted to Marine hydroacoustic data of the type previously 1070 studying tsunamis generated by submarine land- used to identify seabed failure elsewhere (such as 1071 slides (e.g. Heinrich 1992; Grilli & Watts 1999, PNG) were available in the region of the Japan 1072 2001, 2005; Watts 2000; Tinti et al. 2001; Ward earthquake; the area off Honshu Island had been 1073 2001; Grilli et al. 2002; Lynett & Liu 2002; Enet mapped by MBES both before and after the tsunami. 1074 et al. 2003; Watts et al. 2003, 2005; Locat et al. From these data, and a limited number of seismic 1075 2004; Enet & Grilli 2005, 2007; Fine et al. 2005; lines, the presence of submarine landslides offshore 1076 Haugen et al. 2005; Liu et al. 2005; Abadie et al. of the elevated Sanriku run-ups was confirmed 1077 2012; Ma et al. 2013, 2015; Løvholt et al. 2015; (Fig. 10). In addition, in the region of the earthquake 1078 Smith et al. 2016). there were offshore bottom sensors which recorded 1079 the frequency content of the tsunami waveforms. 1080 This was the first time that these marine seabed 1081 Improved understanding after data were available to record a tsunami generated 1082 Japan 2011 event by a large-magnitude earthquake. Analysis of these 1083 data provided the potential to discriminate between 1084 Up until March 2011, PNG remained the only different tsunami mechanisms, because the wave 1085 proven catastrophic tsunami generated by a subma- frequency content of tsunamis from earthquakes 1086 rine landslide. There were suspicions of a submarine and submarine landslides are quite different. Since 1087 landslide contribution to other events including the wave frequency content of tsunamis from earth- 1088 Java, 2006 (Fritz et al. 2007) and the highly focused quakes is much lower than from submarine land- 1089 tsunami at Riangkroko, Flores Islands in 1992, slides, it could be used to identify the submarine 1090 where coastal (sub-aerial) landslides were the prob- landslide location and to validate the numerical 1091 able cause (Yeh et al. 1993). Without marine sur- tsunami models. 1092 veys at these locations, their proposed submarine Although a submarine landslide was the most 1093 landslide tsunami-generation mechanisms remain likely second tsunami mechanism, an alternative 1094 unresolved. It was not until the Japan tsunami of was out-of-sequence (splay) faulting. There was 1095 11 March 2011 that exceptionally high (40 m) and no evidence for the splay faulting in the published 1096 focused run-ups along the Sanriku coast on northern seismic profiling data for the source region (Tsuru 1097 Honshu Island, north of the main earthquake rup- et al. 2002); however, there was considerable evi- 1098 ture, suggested another submarine landslide event dence supporting a submarine landslide. Large 1099 in addition to the magnitude Mw 9 earthquake (Tap- slumps had been found on the margin of the accre- 1100 pin et al. 2014). Two important aspects of this event tionary prism off the Sanriku coast (Cadet et al. 1101 suggested a second tsunami mechanism in addition 1987; von Huene et al. 1994; Tsuru et al. 2002). 1102 to the earthquake. Firstly, the numerical tsunami Data from two ocean bottom pressure gauge stations D. R. TAPPIN

1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117Colour 1118online/ 1119colour hardcopy1120 1121 1122 1123 1124 1125 1126 Fig. 10. Japan, 2011. Submarine landslides (black circles/ellipses) in the region off northeastern Honshu Island 1127 from bathymetry, viewed from the east. Red square: the location of the landslide triggered by the March 2011 1128 earthquake. Highest elevation observed tsunami run-up/inundation (around 39.58 N) along the Sanriku coast is also et al 1129 marked. Approximate location of this figure shown in inset (From Tappin . 2014). 1130 1131 1132 TM1 and TM2 off Kamaishi (latitude 398 12′ N) Undefined hazard 1133 indicated that at least part of the tsunami source in 1134 this region is a narrow area in the deeper part of Extensive mapping of continental shelves reveals 1135 the Japan Trench (Maeda et al. 2011). Seabed move- the common presence of submarine landslides, 1136 ment had been identified in the area of earthquake although large regions remain unmapped (Fig. 9). 1137 rupture from before and after bathymetry (Kawa- At present however, only four significant submarine 1138 mura et al. 2012), but this was south of the region landslide tsunamis have been mapped, modelled and 1139 of elevated onshore run-ups. Backward ray tracing validated: Storegga, Grand Banks, PNG and Japan 1140 using the higher-frequency, leading elevation (two of which have led to a significant loss of 1141 wave from the tsunami recorded by the seabed life). For volcanoes, only the flank collapse of 1142 buoys located offshore of the region of high run-ups Alika 2 on the Big Island of Hawaii is well studied; 1143 identified the most likely location of a submarine the tsunamis from the Canary Island volcanoes 1144 landslide that could have generated these. Within remain controversial. 1145 this area, north of that identified by Kawamura Regarding passive margins, the best-studied 1146 et al. (2012), a number of submarine landslides region is the North Atlantic (Fig. 9). Here, the gene- 1147 were identified from bathymetry acquired from ral controls on landslide failure are related to 1148 before and after the earthquake (Tappin et al. climate-controlled influences on sedimentation dur- 1149 2014). There were few seismic data in this region ing glacial and interglacial periods (e.g. Lee 2009; 1150 and none at the location identified by the ray tracing Talling et al. 2014), with earthquakes being the 1151 as the potential additionary tsunami source, so sub- most likely triggering mechanism. The specific rela- 1152 seabed structure was not available to confirm this tionships between climate and landslide failure are 1153 interpretation. A slope stability analysis confirmed still far from clear, however (Urlaub et al. 2013; 1154 that earthquake shaking could have triggered the Talling et al. 2014). Storegga is well studied in the 1155 landslide. Numerical modelling of a dual earth- NE Atlantic, and the associated tsunami was ele- 1156 quake and submarine landslide tsunami mechanism vated along the surrounding coasts and geographi- 1157 (Fig. 11) demonstrated that in combination they cally extensive. There is still no evidence for 1158 reproduced the waves recorded along the Honshu tsunamis from the other large-volume landslides 1159 coast, especially in the north of the inundated area off Norway (such as Trænadjupet; Fig. 9) located 1160 in the Sanriku region (Tappin et al. 2014). north of Storegga. Further south in the Shetlands IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199Colour 1200online/ 1201mono 1202hardcopy 1203 1204 1205 1206 1207 1208 Fig. 11. Japan tsunami, 2011. Numerical simulation of the 2011 tsunami using a dual earthquake and submarine 1209 landslide source mechanism, showing instantaneous surface elevations at time (a)5,(b) 10, (c) 15, (d) 20, (e)25 1210 and (f) 30 min. Labelled black dots mark the locations of GPS buoys and of DART buoy #21418. Note the highly 1211 dispersive nature of waves generated by the SMF source to the north, as compared to the longer-wavelength, 1212 long-crested, non-dispersive earthquake-generated tsunami waves to the south (From Tappin et al. 2014). 1213 1214 1215 (Fig. 9) there are tsunami sediments much younger submarine landslides. The most likely location for 1216 than Storegga, dated at 1.5 and 5 ka BP (Bondevik the landslides is off Norway, for example Trænadju- 1217 et al. 2005b), but there is no obvious tsunami mech- pet where two events are recognized, but the ages of 1218 anism for these events except the possibility of these were dated at 3–5 ka BP and 19–22 ka BP D. R. TAPPIN

1219EDQ5 (Laberg & Voren 2000), that is, different from the suggests that the hazard here may be limited, how- 1220 Shetland events. The mechanisms for the Shetland ever (Webster et al. 2016). 1221 sediments therefore seem to be more local, but are Along convergent margins, the hazard from 1222 yet to be found. submarine landslide tsunamis may be much greater 1223 To address the submarine landslide hazard in the than along passive margins. PNG is the best-known 1224 North Atlantic a major UK initiative was funded by and most comprehensively researched submarine 1225 the Natural Environment Research Council, with the landslide tsunami along a convergent margin. 1226 focus on high-latitude ocean warming and its poten- Japan follows as a close second, although further 1227 tial consequences on seabed slope stability. A confirmation is required to locate the exact position 1228 marine programme acquiring hydroacoustic and of the submarine landslide. Other convergent mar- 1229 sediment core data on the landslides off of Norway gin tsunamis where there is a possible landslide 1230 reappraised slope failure mechanisms and their component include those of Messina (1908), Mak- 1231 ages, addressing some of the deficiencies identified ran (1945), Aleutians (1946), Alaska (1964), Puerto 1232 by Urlaub et al. (2013) in the relationships between Rico (1918), Flores Islands (1992) and Java (2006) 1233 slope failures and their climate controls. Based on (Fig. 9). One of the most devastating historical, 1234 new data and numerical models, a further result of convergent-margin tsunamis was Messina, 1908 1235 the project is a new numerical tsunami model of (Fig. 9). A total of 50 000 people died in the earth- 1236 Storegga by Hill et al. (2014) which is the first to quake from collapsed buildings in Messina and 1237 address the palaeobathymetric effects of lowered Calabria with a large, but uncertain, number 1238 sea level at time of failure. drowned in the ensuing tsunami. Over 600 people 1239 In the USA, scientific programmes to determine died in the Java tsunami and 1000 at Flores, where 1240 the tsunami hazard from submarine landslides the highly focused tsunami flooded up to 25 m 1241 focus on the passive margin of the east coast and above sea level. There is a hazard programme off 1242 Gulf of Mexico where there are large population NW USA similar to that on the east coast, estab- 1243 centres and nuclear power plants (see special issue lished for dual earthquake and submarine landslide- 1244 in Marine Geology, eight articles of which present generated tsunami along convergent margins. This 1245 new research; ten Brink 2009; ten Brink et al. programme recently re-evaluated the Aleutian tsu- 1246 2009). Numerous submarine landslides have been nami of 1946 because of the controversy over the 1247 mapped off the east coast of the USA (Fig. 9), but submarine landslide mechanism of the local 40 m 1248 few have been well studied (Lee 2009). One of high tsunami (Fryer et al. 2004; Lo´pez & Okal 1249 the best-researched and largest slide is Currituck, 2006; Locat et al. 2009). Re-evaluation based on 1250 dated at 25–50 ka although it could be much youn- MBES and seismic data (von Huene et al. 2014) 1251 ger (J. Chaytor pers. comm., 2017), which has been now identifies the landslide location previously pro- 1252 numerically modelled for tsunami generation (Grilli posed by Fryer et al. (2004), but this has yet to be 1253 et al. 2015). Based on a slide volume of 134 km3 and numerically modelled. For most of the convergent- 1254 a rigid block failure, failure of Currituck would gen- margin tsunamis identified above, the submarine 1255 erate a local tsunami up to 5–6 m high, highlight- landslide contribution remains uncertain because 1256 ing the hazard along this coast. It is likely that not all have marine survey data on which to evaluate 1257 many of the passive margin landslides mapped on the seabed for landslides that can underpin numeri- 1258 Figure 9 generated tsunamis, but there is no histori- cal tsunami models. Hydroacoustic data have been 1259 cal or geological evidence preserved that records used to identify submarine landslides for the Aleu- 1260 their impact. Even without this evidence, the pres- tians, Alaska, Messina and Puerto Rico events 1261 ence and distribution of such a large number of sub- (Fig. 9) and re-evaluate their tsunami hazard. 1262 marine landslides, some of significant volume, 1263 identifies a potential risk from their associated tsu- 1264 namis (although statistical analysis suggests the Discussion 1265 risk is small; Grilli et al. 2009). Further research is 1266 required because the coast is densely populated The numerous tsunamis experienced since 1992 1267 and nuclear power plants are present (ten Brink suggest that we are living in a period where these 1268 et al. 2014). The tsunami hazard is mainly from sub- events may be more frequent than previously. Rec- 1269 marine landslides rather than earthquakes (ten Brink ognition of this (apparent) high frequency and con- 1270 2009; ten Brink et al. 2014). comitant hazard has resulted in a greater awareness 1271 The only other country where there has been a of the tsunami impact, which requires better un- 1272 concerted marine programme on a passive margin derstanding so the hazard and risk can be fully 1273 is Australia where, off the east coast, MBES map- addressed on a sound scientific basis. An improved 1274 ping reveals a large number of landslide scars off- understanding of the hazard and risk will necessarily 1275 shore of significant concentrations of population require a continued geological input. A major 1276 (Fig. 9) (Clarke et al. 2014). Numerical modelling requirement in hazard mitigation is for longer-term IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

1277 records so that the frequency of tsunami events can contributions both to understanding tsunami gener- 1278 be better understood. This longer-term record can ation and validation of numerical models. PNG 1279 only be gained from the sediments laid down as tsu- was the first recent event where there was a focused 1280 namis flood the coast. Geologists, as the arbiters of post-event marine survey organized and managed 1281 this record, can contribute based on major advances by geologists. For Storegga, the numerical models 1282 resulting from the recent catastrophic events of were based on marine data and validated by sedi- 1283 PNG (1998), the Indian Ocean (2004) and Japan ments deposited by the tsunami on adjacent coast- 1284 (2011). Tsunami sediments provide a longer-term lines. For PNG, although the tsunami sediments 1285 record for tsunami impact. Together with improved were analysed and in fact used as a basis for the 1286 understanding of inundation limits from new meth- first inverse modelling, the numerical models were 1287 odologies (such as geochemistry), the impacts of validated from tsunami run-up elevations acquired 1288 older historical and prehistorical tsunamis and during post-event surveys. 1289 their generation mechanisms can be revised as in Although only four (five with Alika 2) significant 1290 Japan with the Jogan (869) event. Based on recent tsunamis are positively identified as generated by 1291 events, where tsunami hydrodynamics are recorded submarine landslides, seabed mapping of continen- 1292 and the origin of the sediments established, tsunami tal shelves reveals their ubiquity (Fig. 9). Too few 1293 sediment can again be used through novel inverse landslides are yet dated, and even fewer studied to 1294 modelling methodologies of tsunami flow speeds a degree necessary to understand the controls on 1295 and depths to better understand older events at the their generation. These controls on sediment failure 1296 same locations. This methodology also has potential and landslide frequency are therefore poorly under- 1297 in regions where there are no observed recent stood, so the hazard from submarine landslides 1298 events, but where there is a historical or prehistori- remains undefined. Recent studies suggest that sub- 1299 cal record. Inverse modelling of tsunami sediment marine landslide failure is random (e.g. Urlaub et al. 1300 inundation provides new insight into generation 2013), but there are still too few events studied in 1301 mechanisms and magnitudes. sufficient detail or accurately dated to be certain of 1302 Although earthquakes are undoubtedly the most this. Urlaub et al. (2013) analysed 68 events, 50% 1303 frequent mechanism of tsunami generation, two of which they were unable to date to sufficient reso- 1304 events (PNG and Storegga) indicate that submarine lution to relate to potential landslide controlling fac- 1305 landslides are a secondary but important hazard. tors. Because the controls on failure are complex 1306 These latter events were recognized and researched (Tappin 2009, 2010b; Talling 2014) more events 1307 over a similar period of time in the late 1990s to require researching before reliable conclusions 1308 early 2000s; one (Storegga) was prehistoric so had can be drawn on the most important failure mecha- 1309 little human impact, and the other (PNG) was very nisms. Recent overviews (ten Brink et al. 2016) sug- 1310 recent, killing over 2200 people. The recognition gest that as more events are studied, improved 1311 of the Storegga tsunami was from a coeval relation- understanding of landslide failure and triggering 1312 ship between the tsunami sediments discovered in mechanisms, particularly in specific tectonic envi- 1313 Scotland and a submarine landslide off the Norwe- ronments, will result. Although earthquakes are 1314 gian coast, with the identification of the sediments still the most likely landslide triggers, these may 1315 motivating the first numerical modelling. The dis- not only be ‘tectonic’ in the context of convergent 1316 covery of the Ormen Lange Gas field in 1997 led margin environments. There are also strong climate 1317 to a major investigation into slope stability and tsu- controls on earthquake rupture, for example in high 1318 nami generation. When the PNG tsunami struck in latitudes due to glacioisostatic processes, such as 1319 1998 and the landslide mechanism was identified, established from research on Storegga (e.g. Bungum 1320 the Storegga gas field developers recognized its et al. 2005). In addition, new research suggests there 1321 importance. Storegga and PNG remain the best- may be triggering relationships between earth- 1322 studied and validated examples of landslide- quakes and continental shelf loading from sea-level 1323 generated tsunamis. The reasons for their study are rise not previously recognized (Brothers et al. 2013; 1324 entirely different. At Ormen Lange, the imperative Smith et al. 2013). Regarding tsunami generation, 1325 was to prove that exploitation of the gas field most submarine landslides have the potential to 1326 would not trigger another tsunami. With PNG, the generate hazardous events if of sufficient volume. 1327 scale of the loss of life and initial uncertainty in The preservation potential of tsunami sediment is 1328 the generation mechanism dictated that the cause low; so, even though evidence for tsunamis associ- 1329 of the tsunami had to be understood. Unlike Store- ated with the landslides may be absent, this may 1330 gga, where oil money was available to fund a not necessarily discount tsunami generation. 1331 comprehensive investigation of the landslide, with The Japanese tsunami of March 2011 led to the 1332 PNG there was no obvious donor to fund the marine confirmation that tsunami sediments were critical 1333 surveys which were organized on humanitarian in identifying inundation limits. It revealed that sub- 1334 grounds. In both instances, geologists made major marine landslides are an additional, yet unforeseen, D. R. TAPPIN

1335 major hazard even where a large-magnitude earth- similar to those for earthquakes and submarine 1336 quake generates a devastating tsunami. It also dem- landslides. 1337 onstrated the importance of new technologies, such 1338 as seabed pressure sensors and improved geological 1339 (geochemical) methodologies, in the identification Conclusions 1340 and discrimination between tsunami mechanisms 1341 and their coastal impact. Over the past 30 years, there have been major 1342 Challenges remain in improving our under- advances in understanding the mechanisms of tsu- 1343 standing of tsunamis and their hazard, to which nami generation and tsunami impact. Improvements 1344 geologists can contribute. The disintegration of in technology, documentation of events and ability 1345 translational landslides, and how this controls tsu- to model them have largely been because of devas- 1346 nami generation, has yet to be fully addressed by tating events. For the past 20 of these 30 years many 1347 numerical models, which need to be more complex of the advances have been by contributions from 1348 to be realistic. Geologists can contribute in develop- geology and by geologists. Most now-accepted 1349 ing more realistic landslide models from marine ideas, such as that tsunamis lay down sediment 1350 data and in their validation from sediment data. A when they flood the land and that submarine land- 1351 major challenge is in determining the hazard from slides generate hazardous tsunamis, were at first 1352 tsunamis generated from volcanic eruption, which controversial and considered unlikely. Now, post- 1353 is hardly researched. There have been some theoret- event surveys acquire geological data as a matter 1354 ical studies (e.g. Pareschi et al. 2006; Novikova of course. Earthquakes are undoubtedly the primary 1355 et al. 2011), but these have not been validated. Even tsunami mechanism, but improved understanding of 1356 validated studies leave uncertainties over mecha- earthquake mechanisms, magnitudes and frequen- 1357 nisms (e.g. Ulvrova et al. 2016), however. There cies in tsunami generation result from research on 1358 are numerical models of small-scale events, such their associated tsunami deposits. Studies of prehis- 1359 as from Montserrat (Pelinovsky 2004), but their rel- toric tsunami sediments have resulted in timescales 1360 evance to large-scale eruptions is uncertain. It is now extended back in time for thousands of years, 1361 almost certain that, as with recent events such as beyond historical records. These extended records 1362 the Indian Ocean, Japan and PNG where the semi- have improved the understanding of the frequencies 1363 nal research has resulted from a catastrophe, the of these events that together allow improved mitiga- 1364 challenge of eruption tsunami mechanisms will be tion and response strategies. Comparison of sedi- 1365 met only in response to a future major event with ments from recent tsunamis with those preserved 1366 significant loss of life and/or major economic in geological records at the same locations offers 1367 impact. The most recent (and only) catastrophic the opportunity to better understand past events. 1368 eruption tsunami in 1883 was when Krakatau in Recent major advances in inverse modelling of 1369 the Java Strait exploded, devastating surrounding earthquake tsunami magnitudes from sediments 1370 coastlines and killing 36 000 people. To understand indicate their potential in this field. 1371 the eruption tsunami mechanism demands validated Understanding tsunami generation from land- 1372 numerical models and, fortunately, Krakatau was slides, especially submarine events, is underpinned 1373 subject to the first post-event tsunami survey ever by geological research in mapping the locations 1374 carried out (Verbeek 1885). Although there was and architectures to identify failure mechanisms 1375 an immediate response to the tsunami impact, and and then using these mechanisms for realistic 1376 the volcanic eruption has been well studied (e.g. numerical simulations. Whereas the hazard from 1377 Self & Rampino 1981), there is still uncertainty submarine landslide tsunamis is now recognized, 1378 over the final cataclysmic explosion during which the extent and risk from these events is still uncer- 1379 the devastating tsunami was generated. Alternative tain. Submarine landslides are present along the 1380 possible tsunami mechanisms include pyroclastic margins of most continents; many of these margins 1381 flows, caldera collapse or both as a dual mechanism remain largely unmapped however, so their hazard 1382 (Francis 1985). There is only one comprehensive is not known. 1383 numerical modelling study, and this supports the Eruption-generated tsunamis remain poorly 1384 entry of pyroclastic flows into the sea (Maeno & researched, and their mechanisms are not well 1385 Imamura 2011). The results are however question- understood. Their global extent requires more 1386 able because of the validation used. Globally, focused programmes of research to address their 1387 there are 42 volcanoes similar to Krakatau which hazard and risk. Based on the most recent history 1388 could erupt with similar consequences. The uncer- of scientific advance for earthquake and landslide 1389 tainty over the tsunami mechanisms of eruption tsunami, it may well be that advances in understand- 1390 therefore remains an important issue that needs ing eruption events await the next catastrophic 1391 addressing if appropriate mitigation and response event. Nevertheless, continued research on tsunami 1392 strategies for eruption tsunamis are to be developed, sediments, submarine landslides and volcanic IMPORTANCE OF GEOLOGY IN TSUMANI SCIENCE

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