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Progress
in Physical Geography 31(6)
(2007) pp. 575–590
Tsunami geoscience
Alastair Dawson1* and Iain Stewart2
1Aberdeen Institute for Coastal Science and Management, University of Aberdeen, Aberdeen AB24 3UF, UK 2Department of Earth, Ocean and Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, UK
Abstract: Research in tsunami geoscience has accelerated markedly ever since the tragedy of the Indian Ocean tsunami of Boxing Day 2004. Yet, for many decades and centuries, scholars have been describing a multiplicity of tsunami events. Thus the Royal Society devoted a whole volume to the effects of the Great Lisbon earthquake and tsunami of November AD 1755 while in the early nineteenth century Charles Darwin was describing the great tsunami at Valdivia, Chile, in his account of the Voyage of the Beagle. Today, research in tsunami geoscience is still finding its feet. Thus, whereas there has been a wealth of publications on the reconstruction of Late Quaternary and Holocene tsunamis, the literature describing evidence for tsunamis in the geological record are rare. In this paper, we describe how our understanding of tsunamis has changed over time and we try also to identify areas of tsunami geoscience worthy of future study.
Key words: bolides, coastal flood risk, offshore earthquakes, submarine slides, tsunami desposits.
I Introduction
The tragedy of the Indian Ocean tsunami of Boxing Day 2004 made the world wake up to the dangers posed by tsunamis. From that day until the end of March 2005, some of the most intense earthquake activity of recent decades led to fault activity along a 1600 km length of plate with the first quake producing a tsunami that led to loss of life of more than 280 000 people. Evidence from history and prehistory, however, makes it clear that tsunamis are not only produced by offshore earthquakes; some have also been caused by underwater sediment slides, others by onshore landslides moving off the
flanks of oceanic islands into the ocean, still others by subsea volcanic explosions, while on rare occasions some have been caused by bolide (asteroid and cometary) impacts. With various tsunami threats now apparent along virtually every stretch of coastline in the world, there has been a recent upsurge in interest in tsunami studies. The clamour for assessments of tsunami risk for specific coastal zones has led to a dramatic acceleration in research into tsunami genesis and impacts, and led to an identifiable discipline: tsunami geoscience.
In the following sections, we summarize the progress recently made in various strands
*Author for correspondence. Email: [email protected]
© 2007 SAGE Publications DOI: 10.1177/0309133307087083
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of tsunami geoscience research with the overarching objective of providing the wider community, still trying to come to terms with how to deal with events of the scale of the Indian Ocean tsunami of 2004, with a clear idea of what has recently been achieved and what remains to be accomplished.
II What are tsunamis?
The word ‘tsunami’ is derived from the Japanese meaning ‘harbour wave’. Tsunamis are often described as tidal waves, but this view is nonsensical since they have nothing to do with tides. Tsunamis are generated by offshore earthquakes, submarine slides, by subaerial landslides (frequently on volcanic islands) that enter the ocean and, more rarely, by bolide (asteroid) impacts. In most cases where the sea-floor disturbance causes a lowering of the ocean surface, the initial water movement is drawn into this area from all sides, leading to a lowering of the sea sur-face at the coast. If the sea-floor displacement is upwards, water spreads outwards in all directions and throughout the entire ocean water column. In both circumstances, large kinematic waves are propagated outwards across the ocean. The waves travel across the ocean at very high velocities, often in ex-cess of 450 km hr–1, and possess very long wavelengths and periods. As water depths become shallower, the kinematic waves decrease in velocity. Thus changing water depths lead to complex tsunami behaviour (Murty, 1977). For example, a hypothetical tsunami travelling westwards across the Atlantic Ocean would decelerate at it ap-proached and passed over the mid-Atlantic ridge but would again accelerate as it moved across deeper ocean areas west of the ridge before decelerating again as the waves reached the continental shelf off the eastern USA.
At the coast, the tsunami flood level (run-up) is partly a function of the dimensions of the propagated waves but is greatly influ-enced by the topography and bathymetry of the coastal zone and as such the waves
can reach considerable elevations causing widespread destruction and loss of life. There are usually several waves in a tsunami wave train that may strike a coastline over a period of several hours. Due to wave resonance, it is quite possible for one of the smaller incoming tsunami waves to become amplified due to its interaction with the preceding outgoing wave, ultimately to become the largest tsunami wave that strikes a coastline.
It has only been in recent years that tsunamis have been extensively cited in the scientific literature, although there is a litera-ture for the nineteenth century including de-scriptions of the catastrophic tsunami caused by the 1883 eruption of the Indonesian volcano Krakatoa (Symonds et al., 1888) and the descriptions of Charles Darwin for the Valdivia region of Chile for February 1835 and graphically summarized in Voyage of the Beagle (Browne and Neve, 1989). The images of the effects of the Indian Ocean tsunami resonate with his descriptions of the town of Talcuhano where, ‘owing to the great wave, little more than one layer of bricks, tiles and timber, with here and there part of a wall left standing, could be distinguished … the Mayor told me of some cows which were standing on the steep sides of the island, were rolled into the sea’. He also referred to the descriptions of Captain Fitzroy who observed, ‘one unbroken swell of the water, but on each side, meeting with resistance, it curled over, and tore up cottages and trees as it swept onwards with overwhelming force’.
A body of tsunami literature also exists for the years prior to acceptance of plate tectonic theory when tsunamis were widely discussed (together with submarine slumps) as key processes in the formation of sub-marine canyons (Coleman, 1968). Tsunamis were given additional publicity through the descriptions by Holmes (1944) in Principles of Physical Geology and his photograph of a man standing in front of the 1946 tsunami in Hawaii. In the decades that followed, tsunami science attracted further interest due to the French nuclear bomb tests in the
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Pacific. Underground nuclear detonations were considered highly likely to produce tsunamis and thus the tsunami risk to Poly-nesian coasts had to be addressed. Later, defence strategists in the United States considered tsunamis as a serious risk to its eastern seaboard owing to the possibility of hostile actions that involved the triggering of Atlantic megatsunamis as a result of the bombing of mountain slopes on the Canary Islands and landslide-generated tsunami. Tsunamis have thus had a variable press with the majority of commentators adding to the confusion by referring to tsunamis as ‘tidal waves’.
Popular accounts of ancient tsunamis abound in the literature. Perhaps the most famous relates to the flight of Moses from Egypt and the parting of the ‘Red Sea’. Many have alluded to links between these events and a tsunami generated by the eruption and collapse of the Santorini (formerly Thera) volcano in the eastern Mediterranean. One opinion is that the ‘Red Sea’ translates in Hebrew into the ‘reed sea’ and that such descriptions correspond well with the marsh-lands along the southern part of the Gaza Strip. Thus the parting of the waters is con-sidered equivalent to the drawdown of the sea prior to the arrival of Santorini tsunami waves and the drowning of the Egyptian soldiers in pursuit of Moses (Myles, 1985).
Long record of human occupation of the coastal zone of Mediterranean shores means that accounts of tsunamis are not rare in Europe either. Probably the most destructive event during historical times took place on 1 November 1755. An earthquake took place offshore c. 200 km WSW of Cape St Vincent, on the Gorringe Bank on the sea floor west of Portugal and attained a magnitude estimated at 8.5 Ms. The epicentre of the earthquake was in an area along the Azores–Gibraltar plate boundary that forms the western part of the lithosphere boundary between the Eurasian and African plates (Moreira, 1985).
The considerable destruction that took place in Lisbon, in addition to widespread
fires, was mostly attributable to three tsunami waves, estimated to be 5–13 m high, that took the lives of 60 000 people in Portugal alone. There are also numerous reports of severe tsunami flooding and fatal-ities along the Algarve coast, southern Spain, Morocco and Algeria. The tsunami waves travelled westwards across the Atlantic. An officer on a man-of-war ship The Warwick off Antigua described it thus: ‘from many of the islands we are informed of a very surprising motion of the sea the 1st of November, much at the same time amongst them all. The sea, twice in some islands, and thrice in others, rose from 8 to 12 feet perpendicular, and suddenly retired as much below its usual height. In some places where the plantations were near the sea, on low grounds, much damage was done, by drowning and washing away houses, cattle etc.’ (Scots Magazine, 1756: 41). Even in SW England, contemporary observations by Borlase (1755; 1758) describe the arrival of the tsunami in Mounts Bay, Cornwall: ‘the first and second refluxes were not so violent as the 3rd and 4th (tsunami waves) at which time the sea was as rapid as that of a mill-stream descending to an undershot wheel and the rebounds of the sea continued in their full-fury for fully 2 hours ... alternatively rising and falling, each retreat and advance nearly of the space of 10 minutes until 5 and a half hours after it began.’
The relatively high frequency of Pacific tsunamis is due to the occurrence of severe earthquakes under or close to the seabed as a result of the subduction of oceanic crust adjacent to continental margins. Such geo-logical processes are a characteristic fea-ture of Japan where there is a long history of devastating earthquakes and tsunamis. Between 1596 and 1938 the Japanese islands were struck by no fewer than 15 major tsu-namis. For example, the Ansei-Nankai tsunami took place in western Japan in 1854 and led to the loss of life of around 3000 people. This tsunami is well-known in Japanese history in conjunction with the
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‘Fire of the Rice Sheaves’ since, during the evening of the tsunami, Mr Gohei, a village chief, noticed unusual vertical movements of the sea along the coast. He suspected that a tsunami was about to arrive and, in order to save the lives of the villagers, set fire to the rice sheaves in the fields surrounding his house on a hill overlooking the sea. The villagers noticed the fire and all ran up the hill. By the time they had arrived they turned round to witness the tsunami destroying their homes and crops.
III The need for tsunami chronologies
Historical accounts of former tsunamis have particular value because they can provide information on the timing and magnitude of events over the duration for which his-torical records are available. They also enable reconstructions of individual tsunamis by providing data on wave arrival, flood run-up and vertical changes in ocean level. Yet the nature and length of historical records for different areas are highly variable. Thus, whereas the chronology of historical tsu-namis in Italy begins with the AD 79 eruption of Vesuvius, the record of historical tsunamis for the west coast of the United States only begin in the mid-eighteenth century with the arrival of the first settlers.
Yet, for all coastal regions, those author-ities charged with coastal protection need to know how many (if any) tsunamis have taken place through prehistory, as well as the nature of each event. For any seaboard, a complete tsunami history is necessary if measures are to be put in place to protect lives and property. However, the task of identifying past tsunamis in prehistory is an extremely difficult one.
IV Tsunami sediment records
In recent years, numerous sedimentary invest-igations have been used to identify former tsunamis that took place in prehistory. This has proved possible owing to the recognition that most tsunamis deposit sediment in the coastal zone (Dawson and Shi, 2000). This
generally accepted statement, however, is accompanied by the caveat that sedimentary records exist only for coastal areas where there is a pre-existing sediment supply. A cliffed coastline, for example, may leave no trace of any past tsunami event. Yet many former tsunamis appear to have left sedi-mentary fingerprints.
So what do tsunami deposits look like and how can we distinguish these from sediments deposited during other extreme marine flooding events, such as hurricanes or storm surges? (Figure 1). This question lies at the core of tsunami research yet a satisfactory answer is fraught with difficulty. The most logical approach is to turn to sedimentary studies of coastlines where tsunamis are known to have taken place in the relatively recent past. Eye-witness accounts and post-tsunami surveys of sediment changes accom-panying modern events, such as the 2004 Indian Ocean tsunami, provide one line of evidence, but another comes from examining coastlines known to have been inundated by a large tsunami during recent historical times (eg, the Chilean tsunami of May 1960) and, consequently, likely to have preserved distinctive coastal sedimentary records.
V Tsunami coastal processes
Field observations of tsunami flooding usually describe the rapid lateral translation of water across the coastal zone (Figure 1). Frequently, this motion is associated with run-up and is amplified by local wave reson-ance. As the tsunami waves strike the coast, they are unlike waves associated with storm surges, because not only are they associated with considerably greater wave lengths and wave periods, but they are also essentially constructive (ie, depositional) as they move inland (Kortekaas and Dawson, 2007). Provided that there is an adequate supply of sediment in near-shore zone, the rapidly moving water carries everything from silt to boulders. As they surge onshore, individual tsunami waves reach a point of zero water velocity prior to backwash flow
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Figure 1 Cross profile showing key tsunami processes and sediment transport pathways (after Einsele et al., 1996)
where they reverse current direction. At this point, large volumes of sediment may be deposited out of the water column onto the ground surface. It is an important character-istic, and one that makes tsunamis distinctive from storm surges.
Recent studies of coastal sediments de-posited by palaeotsunamis have shown that tsunami sediment deposition is frequently associated with the deposition of sediment sheets that rise in altitude inland as tapering sediment wedges (Dawson, 1994; Nanayama et al., 2003; Cisternas et al., 2005). The high-est sediment accumulation always occurs below the upper limit of tsunami run-up. Similarly, in the Algarve, Portugal, tsunami deposits produced during the great Lisbon earthquake of AD 1755 locally occur as con-tinuous sheets of sediment inland from the coast, but farther inland are replaced by dis-continuous and eventually sporadic sediment lenses, until a point is reached when there is no sedimentary trace of the tsunami, despite historical observations that tsunami flooding took place to considerably higher altitudes (Hindson et al., 1996). Thus, the upper limit of sediment deposition is always less than the upper limit of run-up, therefore making it impossible to infer run-up levels from palaeotsunami deposits (Dawson, 1994).
The interpretation of any tsunami de-posit is complicated by episodes of sediment erosion during the passage of individual
waves. Successive waves may result in the erosion of pre-existing tsunami deposits and, on occasions, their complete removal. The pattern and processes of tsunami sedi-mentation is additionally complicated by the occurrence of episodes of backwash flow that move from landward to seaward and may cause additional fluvial erosion and sediment redeposition. Velocities of back-wash flow are, in turn, greatly influenced by the nature of the local coastal topography, and this factor may often play an important part in determining the nature and scale of backwash sediment transport and reworking. In addition, this process may introduce ter-restrial sediments including plant macro-fossils into the sediment assemblage, thus complicating palaeoenvironmental inter-pretation (Bondevik et al., 1997).
Another distinctive feature of tsunamis is their ability to erode, transport and deposit boulders of considerable size (Hearty, 1997; Kelletat and Schellmann, 2002; Scheffers and Kelletat, 2006; Kennedy et al., 2007; McMurtry et al., 2007) (Figure 2). Empirical evidence of coastal boulder deposition dur-ing the tsunami was provided by Yeh et al. (1993) in their study of the Flores (Indonesia) tsunami of December 1992, when blocks of eroded coral reef were recorded as having been transported by the tsunami and de-posited far inland. Bourrouilh-Le Jan and Talandier (1985) described areas of coral
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Figure 2 Coastal cliffs near Tipazae, Algeria, showing stacked boulder accumulations located c. 10 m above present sea level. The image illustrates the problem of whether such deposits are the product of past tsunami activity or former storm activity (photo courtesy of Christophe Morhange)
reef in the Tuamoto archipelago, SE Pacific, where large numbers of giant boulders (up to 750 m3) appear to have been carried across the atoll rim and dumped in the lagoon. However, although palaeotsunamis have been considered in this case as a possible depositional agent, similar-sized boulders may also be deposited as a result of very high-energy storm surges. The case is well illustrated by Noormets et al. (2002) who identified on a shore platform in Hawaii a 96-tonne boulder deposited during the 1946 Aleutian Islands tsunami but later moved on two occasions by storm wave action.
Despite the possibility that accumulations of so-called megaclasts are the result of extreme wave activity during storms, littoral boulder fields have been observed elsewhere in the world and typically attributed to tsunamis, for example in Japan (Nakata and Kawana, 1993; Scheffers and Kelletat, 2006). In southern Portugal, Scheffers and Kelletat (2006) identified boulders deposited by the Lisbon tsunami of AD 1755. Similar observations have been made on the New South Wales coastline of Australia by Young and Bryant (1992).
Although the sedimentary signatures of tsunamis are relatively well established, relatively few investigations have been undertaken on the microfossils contained within tsunami deposits. Those studies that have explored this aspect (eg, Hemphill-Haley 1995a; 1995b; 1996; Cisternas et al.,
2005; S. Dawson, 2007) show that brackish-marine diatoms within tsunamiites can be used not only to identify provenance but also to delimit the greatest inland extent of inundation, owing to the fact that certain assemblages occur beyond the landward limit of tsunami-deposited sands and silts. Likewise the occurrence of marine plankton in coastal freshwater lakes located beyond the reach of storms can be used to identify former incursions of tsunami waters. The character of the diatom assemblages within sediments can also imply a tsunami origin due to high percentages of broken species of Paralia sulcata (Dawson et al., 1996). Like diatoms, foraminifera also provide a useful means of identifying former tsunami deposits, especially where species typical of deep-water environments are contained within coastal deposits (Dominey-Howes et al., 1999).
VI Can tsunami and storm deposits be distinguished?
Tsunami deposits are distinctive in that they are frequently associated with the deposition of continuous and discontinuous sediments sheets across large areas of the coastal zone, reflecting the tendency for tsunamis to be constructive as they approach their land-ward limit (Nott, 1997; Morton et al., 2007). Also, they frequently consist of deposits of sand containing isolated boulders, and on occasions such boulders exhibit evidence of
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having been transported inland from the near-shore zone. Finally, microfossil assem-blages of diatoms and foraminifera within sand sheets may provide information of onshore transport of sediment from deeper water. But are these attributes diagnostic enough to distinguish tsunamiites from storm-related deposits, so called tempestites?
In fact, hurricane deposits are quite differ-ent. For example, Scoffin and Hendry (1984) used coral rubble stratigraphy on Jamaican reefs to identify past hurricane activity, while Perry (2004) used storm-induced coral rubble in reef facies from Barbados to identify epi-sodes of past hurricane activity. Similarly, in coastal Alabama, USA, a series of hurricanes during historical time has resulted in the deposition of multiple sand layers in low-lying coastal wetlands, but never as extensive as tsunami sediment sheets. By contrast, the overwash fans along the New England coastline used by Donnelly et al. (2001) to reconstruct a 700-year record of hurricane activity have analogues with similar tsunami-deposited fans (Andrade, 1992; de Lange and Moon, 2007; Nanayama et al., 2007). While it is accepted that storm surges result in the deposition of discrete sedimentary units, it is argued that tsunamis, in contrast to storm surges, generally result in deposition of sediment sheets, often continuous over relatively wide areas and considerable dis-tances inland. For example, sediment sheets in the Algarve, Portugal, produced by the AD 1755 tsunami occur in excess of 1 km inland (Dawson, 1994).
VII Tsunami marine processes
Whereas the majority of the literature concerned with tsunami sedimentation has focused attention on the coastal zone, relatively little attention has been given to tsunami depositional processes both in the near-shore zone and in the deep ocean (Dawson and Stewart, 2007). To a large ex-tent, this is because tsunami sediments are readily identifiable and easy to study along coastlines. Yet the fact that each incoming
tsunami wave is associated with strong back-wash flow from the coast into the sea high-lights the strong possibility that sediments picked up by tsunamis may also drape the sea floor, a consequence of the cumulative depositional effect of each backwash flow associated with the train of tsunami waves (Einsele et al., 1996). During this phase, pulses of tsunami backwash may generate turbidity currents that move seaward to-wards the abyssal zone via submarine gullies and canyons (Bralower et al., 1998).
The literature on marine tsunami deposits is surprisingly sparse. Bailey and Weir (1932) speculated that an enigmatic ‘boulder bed’ in the late Jurassic rocks of northern Scotland may have been produced as a result of tsu-nami activity, but thereafter there have been few published accounts of marine palaeo-tsunami deposits. Bailey (1940) highlighted the contemporary debate concerning pos-sible links between turbidity-current activity and the evolution of submarine canyons, recognizing that individual turbidity-current ‘events’ may have played an important role in canyon development. Coleman (1968: 269) noted that tsunamis have an ‘off-surge cap-acity’ of sediment transport and envisaged that this off-surge (backwash) ‘rolls in a turbid flow along the sea floor and as it loses energy drops its load progressively. The bulk of deposition may take place either in shallow water within a few kilometres of the coast or, paradoxically, in the deeper offshore, even bathyl water, beyond the shelf.’ Coleman stressed the important process links between tsunami sediment transport, turbidity-current flow and the long-term evolution of sub-marine canyons, adding that tsunami-driven sediment flows moving towards deeper water would be capable of transporting particles with a wide range of grain sizes resulting in the deposition of poorly sorted sediment ‘literally slopped forth in turbid masses relatively close to the shore’ (Coleman, 1968).
Although beyond the scope of this study, the possible relationships between submarine-canyon development and tsunami sediment
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transport point to such canyons as conduits through which tsunami-triggered turbidity currents transport and deposit sediment (Bailey, 1940). In so doing, such currents are likely to bring with them indicators of sedi-ment transport from shallow to deep water and thus produce sedimentary deposits ana-logous to turbidites (cf. Shiki and Yamazaki, 1996; Shiki et al., 2000; Fujiwara and Kamataki, 2007).
VIII Mechanisms of tsunami generation
Identification of recognizable tsunami de-posits in coastal sediment sequences has led to a new perspective on the past frequency and magnitude of tsunami events for differ-ent coastal regions of the world. In this sec-tion, we consider how tsunami geoscience has shed new light on the physical processes that generate such ocean disturbances.
1 Tsunamis generated by offshore earthquakes
Offshore earthquakes are by far the most common source of tsunamis (Murty, 1977). Thus there is a strong correlation between past tsunamis and zones of plate collision (eg, the Pacific ‘ring of fire’). For a tsunami to occur, however, the offshore earthquake must be associated with faulting across an area of seabed. In the case of the Indian Ocean tsunami of 2004, the global array of seismic monitoring stations was sufficient to enable the clear identification of a sub-marine earthquake linked to an episode of exceptional submarine faulting (Lay et al., 2005). Onshore measurement of tsunami run-up levels, in turn, allows the earthquake parameters to be determined together with the sea-floor rupture necessary to explain the observed run-up. In some cases (eg, the Nicaragua tsunami of 1992), the computed tsunami run-up values were considerably smaller than those observed at the coast – the discrepancy attributable to factors such as the computed earthquake focal plane solution or simply the grid size used in the computation of near-shore bathymetry
(cf. Satake, 2004). In many cases, large earthquakes offshore are not linked to sea-floor movements and, as a consequence, no tsunamis occur.
In some cases, sediments suspected of being tsunami deposits are the initial, and on occasion the only, evidence of a major earth-quake having struck a coastline in prehis-torical times. On the Pacific seaboard of Washington and Oregon states, USA, for example, a possible tsunami sand layer capp-ing of an abruptly submerged peaty soil were the first enigmatic traces of a convulsive dis-placement of the Cascadia coast. The precise date of the resulting transocean tsunami would come from historical tsunami records in Japan, which dated it to 26 January 1700 (Atwater and Yamaguchi, 1991; Yamaguchi et al., 1997), while subsequent sedimentary and microfossil investigations identified tsu-nami inundation extending from the shores of northern California to the coastal islands of British Columbia.
Although tsunami geoscience is now an integral part of seismic hazard assessments along many earthquake-prone shores, it is not always clear whether the coastal flooding has been solely due to offshore earthquakes. That is because earthquakes can trigger under-water sediment slides, which in turn can be tsunamigenic. For example, the December 1992 tsunami in Flores, Indonesia, is generally attributable to an offshore earthquake, but at the eastern end of the island anomal-ously high values for tsunami run-up appear to have been the result of seabed sediment mass movement triggered by the seismic event (Yeh et al., 1993; Shi et al., 1995). This is perhaps one of the most important issues facing tsunami researchers since, if the source mechanism for a particular tsunami is not clearly defined, significant errors are introduced into subsequent modelling ex-ercises (Dawson, 1996). It appears unlikely that tsunami deposits will provide direct information on whether or not an individual tsunami was principally produced by an
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offshore sediment slide or earthquake activity. One caveat is that while there is a physical limit to the size of the subsea fault displacement, which given the inherent strength of the rocks ensures that vertical displacement in even great seismic jolts cannot exceed ~10 m, there is no such limiting factor for subsea landslides. Consequently, palaeotsunamis whose reconstructed run-ups are exceptionally large may require a non-seismic generating mechanism, such as a submarine slope failure. The only diffi-culty is that there are other mechanisms that can produce such anomalously huge inundations.
2 Tsunamis generated by submarine slides
The advances in the exploration of the ocean floor have revealed a multiplicity of submarine slides that present a real tsunami risk (Driscoll et al., 2000; Ward and Day, 2001). For example, the Hawaiian island chain is flanked by at least 20 large underwater slides. Moore (2000) maintains that, on the island of Molokai, marine strata located on hillslopes up to c. 70 m above sea level are most likely to have been deposited by a single tsunami caused by one of these slides. Moore (2000) rules out deposition in association with higher sea levels since the oceanic islands associated with the hot-spot island chain of the Hawaiian islands are likely to have experienced long-term subsidence rather than uplift.
The debate concerning alleged marine sediments located at high altitudes (eg, >300 m above sea level) on the flanks of several ocean island hillslopes has been wide-ranging. Thus, whereas Moore and Moore (1988) interpreted such deposits on Lanai, Hawaii, as the product of an ancient tsunami, Keating and Helsey (2002) and Felton (2004) maintain that parts of the lithological assem-blage (which contains limestone clasts as well as basalt) are alluvial in origin. By contrast, others have argued that there is stratigraphic evidence for formerly higher sea levels on
Lanai up to 190 m above sea level and no evi-dence for past tsunamis at higher altitudes. McMurtry et al. (2004) maintain that there is good sedimentary evidence for a former tsunami dated to the last interglacial (marine isotope stage 5e) having flooded up to 60 m above sea level on the flank of the Kohala volcano on Hawaii.
That underwater slides are capable of producing large tsunamis is demonstrated by the 1998 tsunami that struck Papua New Guinea, causing widespread loss of life. Here a tsunami with run-up values in excess of 8 m was triggered by a landslide with a volume of 4 km3 located in a water depth of 550m (Heinrich et al., 2001).
Perhaps the most intensively studied tsunami triggered by a submarine slide is the well-known one that struck North Atlantic coastlines c. 8000 years ago as a result of one of the world’s largest underwater slides, the Storegga Slide, off western Norway (Figure 3). Along the coasts of the northern North Sea, Norwegian Sea and the north Atlantic Ocean, a very prominent sand layer, first thought to have been deposited by a storm surge, has more recently been attri-buted to a large tsunami (Dawson et al., 1988; Haflidason et al., 2004; Smith et al., 2004). The detail with which the tsunami is known is considerable. Several studies have exam-ined the sedimentology of the deposit, in-cluding its particle size, microfossil content and even the time of year it occurred. The diatom ecology of the layer has been exam-ined and over 100 14C dates on biogenic material both within the layer and from adjacent horizons have been obtained (Smith et al., 2004). Numerical modelling of the slide by Harbitz (1992) concluded that a landslide velocity of 30 m s–1 provided the closest approximation to the estimated run-up values based on geological data. How-ever, the weakness in this argument is that the geological data provide only minimum estimates of likely flood run-up and there-fore the related numerical models of the
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Figure 3 Section at Maggie Kettle’s Loch, Sullom Voe, Shetland Isles, showing tsunami deposits associated with the Storegga Slide tsunami of
c. 8000 yr BP. Note the large numbers of peat intraclasts ripped up from
the pre-tsunami peat surface and incorporated into the flood deposit
same tsunami will always underestimate the likely average value of the submarine slide velocity.
Giant submarine slides and their poten-tial to generate tsunamis are not restricted to the Norwegian Sea. Nisbet and Piper (1998) recognized a giant submarine slide occupying the majority of the sea floor of the western Mediterranean. The slide appears to have been generated in deep water adja-cent to western Sardinia and radiometric dates appear to indicate that it took place during the last glaciation of the Northern Hemisphere (probably c. 20 000–30 000 years BP). At present there is no geological evidence that this submarine slide generated a large tsunami. That such a large tsunami took place, however, can hardly be doubted. Palaeoenvironmental reconstruction of this event appeared to indicate that the slide took place at a time when sea level in the western Mediterranean may have been at c. –100 m below present and hence any geological record of the tsunami having taken place may lie below present sea level.
3 Tsunamis generated by oceanic island flank collapse
Recent years have witnessed a proliferation of papers describing the links between mega-tsunamis and the collapse of volcanic flanks (Scheffers and Kelletat, 2003). Large palaeo-tsunamis are envisaged as having occurred due to island flank failures and the downslope movement of material into the ocean. Our understanding of the flank failure mechan-isms is also linked to the contrasting views concerning the geological evolution of oce-anic islands where mass-wasting events are intimately linked to episodes of long-term hot-spot volcanism. Felton (2004), for ex-ample, has argued that, if megatsunamis are produced every time there is a sizeable failure of the flank of an oceanic island, then we ought to envisage tsunamis as a major agent of landscape change.
Is there evidence that major slope failures have occurred across the flanks of volcanoes in the past? Masson (1996) provided clear evidence that this may have indeed occurred across the slopes of the island of Hierro in the Canary Islands between c. 17–13 Kyr BP. Ontheotherhand,Pararas-Carayannis(2002) argues that the threat of megatsunami gen-eration from massive flank failures of island stratovolcanoes has been greatly overstated since (a) many hillslope mass movements are seismic in nature while (b) the known sudden flank failures are very unlikely to take place as massive sudden large-scale collapses and
(c) even large failures are likely to produce regional rather than transoceanic tsunamis.
Coastal and island volcanoes, however, have other ways in which to generate major tsunamis. Subsea explosive activity, the abrupt emplacement of major pyroclastic flows into the sea, and the paroxysmal im-plosion and collapse of volcanic edifices are all envisaged to be effective means of gen-erating highly destructive tsunamis. Here again, tsunami deposits can provide ‘forensic’ evidence to help reconstruct the chronology and character of major past volcanic blasts. Perhaps the best known of these is the
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Minoan Santorini volcanic eruption that took place in the eastern Mediterranean c. 3500 yr BP. Part of the key evidence for this tsunami is a mud-rich sedimentary unit (described as a ‘homogenite’) on the floor of the Aegean Sea, together with turbidites that are considered to have been deposited as a result of the passage of a large tsunami (cf. Cita et al., 1996; Cita and Rimoldi, 1997; Cita and Aliosi, 2000). The onshore legacy of this wave has been extensively sought in the surrounding coastal zones, and some sus-pected tsunami deposits of the appropriate age have been documented (eg, Minoura et al.. 2000). However, the general paucity of sedimentary deposits attributable to this apparently cataclysmic volcanic tsunami has led some workers (eg, Dominey-Howes et al., 1999; Dominey-Howes, 2004) to pro-pose that that the principal phase of caldera collapse may have taken place much earlier (during the last glacial maximum c. 20 Kyr BP) and that the Minoan tsunami may have been a far smaller event than generally believed.
4 Tsunamis generated by bolide impacts
Tsunamis triggered by bolide impacts pre-sent an almost separate set of issues. Several bolide impacts are believed to be associated with past tsunami activity, the oldest being an impact during the middle Devonian which is considered to have resulted in the depos-ition of an angular breccia deposit across NW Russia and Belarus (Masaitis, 2002). It has been argued that another impact, during the late Jurassic, generated a tsunami that deposited an erosional conglomerate across what is now northern France (Dypvik et al., 2003), while a Late Eocene impact is widely seen as being responsible for a breccia unit identified across Chesapeake Bay (Poag et al., 1992; Poag, 1997). For the Quaternary period, the only known bolide-tsunami is that associated with a south Pacific impact at 2.51 M yr and linked to possible tsunami deposits in Chile (Hartley et al., 2001). Per-haps the best-known bolide-tsunami is that associated with the K/T boundary event
(Takayama et al., 2000; Bourgeois et al., 1988). Smit et al. (1992), in a discussion of K/T tsunami deposits in Mexico, drew atten-tion to the presence within the deposit of wood debris and argued that sediment de-position occurred as a result of tsunami-induced gravity flows.
IX Constructing tsunami chronologies
For the majority of the geological record, lithological units attributable to tsunami (tsunamiites) are rare. Unfortunately, there-fore, the rock record provides no information on tsunami recurrence history. The same is true for the majority of the Quaternary, al-though in this case the dating of individual submarine slides together with ages obtained for individual tsunami deposits provide us with knowledge that several large tsunamis have taken place through the various glacial-interglacial cycles. The most information on tsunami history is for the Holocene, where the use of buried and uplifted sand layers, inferred to represent past episodes of extreme marine flooding, have been used to construct age-constrained tsunami chronologies. To date, the most complete records have come from Chile, Japan and Kamchatka and to a lesser extent the Pacific coast of the United States.
In Kamchatka, Pinegina and Bourgeois (2001) and Bourgeois et al. (2006) identified a series of peat sequences in which are re-presented up to 28 inferred tsunami sedi-ment sheets, the ages of which are partly constrained by well-defined tephra horizons. These authors attribute the majority of the tsunamis to offshore earthquake sources during the Holocene but are careful to recog-nize that some of the palaeotsunamis may have been generated by other mechanisms (eg, submarine slides, volcanic flank collapse). In Chile, Cisternas et al. (2005) have iden-tified a sequence of marine sand units attri-butable to past tsunamis intercalated with buried soil horizons, the position of the latter due to episodes of coseismic subsidence contemporary with palaeoearthquakes. Cisternas et al. (2005) identified for the Rio
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Maulin area a sequence of eight tsunamis that have struck this area over the last c. 2000 yr. Of these tsunamis, the only event that appears to be represented in the Kamchatka stratigraphy is the trans-Pacific tsunami of May 1960. In Japan, sediment sequences re-covered from coastal lakes and peat basins have revealed suites of high-energy marine deposits intercalated with organic sediments, each unit considered to represent a former tsunami (Sawai et al., 2007). More recently, Nanayama et al. (2003; 2007) have identified 13 tsunami sand units covering the last 4 Kyr along the coastline of eastern Hokkaido, northern Japan. Here 13 separate tsunami deposits together with a tsunami deposit considered as possibly left by the 1960 tsu-nami is the only one in the sequence with an equivalent elsewhere in the Pacific basin. It should be noted, however, that in each of these studies, the precision in dating indi-vidual prehistoric events might not be suf-ficient to identify (apart from the May 1960 event) synchronous trans-Pacific tsunami events – if they exist in the stratigraphic record.
X Defining tsunami-generating mechanisms
The above discussion serves to illustrate the exceptional difficulties faced in linking individual palaeotsunamis to specific sources (Dawson, 1999). Even for specific historical tsunamis known to be linked to particular earthquakes it is not a simple exercise to link earthquake to tsunami as cause–effect. This is because many earthquakes can simul-taneously trigger underwater slides with the consequence that the tsunami run-up at the coast is due to the combined influence of both processes. In the case of prehistoric tsunamis the linking of individual tsunami deposits to source mechanisms is even more difficult (Dawson and Stewart, 2007). In some cases, palaeo-earthquake activity can be demonstrated, for example where tsunami deposits occur in association with liquefaction structures or with indicators of coseismic
subsidence or uplift (Atwater, 1992; Nichol et al., 2007).
The occurrence of high-level marine de-posits on oceanic islands attributed by some to former tsunami flooding is puzzling. In general, oceanic island complexes that have experienced plate motion across hot spots are associated with long-term tectonic sub-sidence, while the Quaternary oxygen iso-tope record shows that global eustatic sea level during interglacials never reached more than a few metres above present. How, therefore, to explain high-level marine deposits except by tsunamis and, if so, where did the tsunamis come from?
XI Summary
The recognition of distinctive palaeotsunami deposits has been used to improve our under-standing of former offshore earthquakes and giant submarine slides. For the most part, how-ever, attention has been focused on traces of tsunami in the immediate coastal zone with the offshore zone and seabed scarcely stu-died. Despite the apparent growing body of sedimentary evidence, geological research on offshore tsunami deposits appears to have been largely characterized by a lack of knowledge on the key physical processes of tsunami marine erosion and sedimentation. To some extent, this no doubt reflects the paucity of studies of offshore hydraulics and sediment dynamics during modern tsunami events. Thus, although there have been a small number of studies undertaken on on-shore sedimentation following recent tsu-namis, there have been no equivalent studies of contemporaneous patterns of offshore deposition and transport.
Perhaps because of ease of study of tsunami sediments in coastal areas and also because many are still discovering that tsu-nami sediments exist, much of the recent research effort has focused on the identifi-cation and dating of tsunami sediments in areas where tsunami sediments were not known previously. A similar observation can be made in respect of tsunami boulder
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spreads, although many studies are still hin-dered by difficulties in understanding which boulders are storm deposits and which were transported by tsunamis. Real progress has been made in the construction of tsunami chronologies for specific regions, with the pioneering research of Atwater, Pinegina, Cisternas and Bourgeois providing us with new perspectives on the Holocene history of tsunamis across the Pacific basin.
In terms of understanding tsunami sedi-ment deposition at the coast, field meas-urements and records of stratigraphy have shown numerous ways in which tsunami sediments can be distinguished from those of storms. But we are still a long way away from understanding precisely how tsu-namis transport and deposit sediment across the coastal zone. There is a clear need for hydraulic modelling of tsunami sediment transport and deposition and how onshore surge and backwash currents lead to such distinctive massive sand and silt complexes. In addition, it needs to be borne in mind that many tsunamis do not strike the coastline at right angles; many tsunamis strike coastlines obliquely, some even moving subparallel to the coastal edge.
There has also been a rapid growth in the number of studies that address the relation-ships between submarine slides and tsunami generation, as well as the consideration of the process links between oceanic island hill-slope mass movement events and tsunami generation. Both of these research areas are likely to grow in future. In respect of under-water slides, new geological surveys of areas of ocean floor are likely to discover new slides and to date others. Similarly, the dating of tsunami deposits along the flanks of oceanic islands, as well as the identification and dating of relict slides that have moved from land into the ocean, are also likely to improve our understanding of tsunami history. In conclusion, the subject of tsunami geoscience is highly interdisciplinary, involving an under-standing of the interrelationships between
geology (onshore and offshore), sedimen-tology, seismology, tectonics, volcanology, geomorphology, oceanography and hydraul-ics. The most significant scientific advances in the future for tsunami geoscience will un-doubtedly lie at the various crossroads be-tween these disciplines.
Acknowledgements
This paper is a contribution from the Scottish Alliance for Geoscience, Environment and Society (SAGES). The authors would like to thank, for providing information, Genevieve Cain (Pacific Tsunami Warning Center, Hawaii), Alison Sandison and Jenny Johnston (for cartographic support).
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