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Linking tsunami deposits, submarine slides and offshore earthquakes
Quaternary International 60 (1999) 119}126
Linking tsunami deposits, submarine slides and o!shore earthquakes Alastair G. Dawson* Centre for Quaternary Science, William Morris Building, Coventry University, Coventry CV1 5FB, UK
Abstract The recognition that many tsunamis are associated with coastal sedimentation has been of great value in the study of tsunamis prehistorically. Geological investigation of such sediments has resulted in the identi"cation of a series of palaeotsunamis that appear to have taken place in di!erent areas of the world. In most cases, however, it has proved di$cult to link former tsunamis to speci"c source mechanisms. Studies of modern tsunamis have also faced di$culties in the recognition of the speci"c source mechanisms. For example, o!shore earthquakes may trigger submarine slides that combine to produce complex patterns of tsunami #ooding at the coast. ( 1999 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Geological investigations of former tsunamis is a relatively new research area. The recognition that many tsunamis deposit sediment in the coastal zone has only become an accepted idea during the last 5}10 years. Discussion of this concept has been accompanied by a proliferation of academic papers that have described a range of sediments that have been attributed to a series of former tsunamis (Minoura and Nakaya, 1991; Pasko!, 1991; Atwater, 1992; Minoura et al., 1994; Nishimura, 1994; Pinegina et al., 1997; Ranguelov, 1998). Unlike storm surges, tsunami runup across the coastal zone is frequently associated with the rapid lateral translation of water and suspended sediment. Thus, tsunami deposits can be used to provide an indirect record of former o!shore earthquakes and underwater landslides. It is exceptionally di$cult, however, if not impossible, to di!erentiate tsunami deposits attributable to former submarine slides or o!shore earthquakes (e.g. Perissoratis and Papadopoulos, 1998). In particular areas of the world, especially in areas of an active plate motion where an o!shore earthquake has taken place, it may be a gross over-simpli"cation to attribute the triggering mechanism solely to earthquake-induced sea bed faulting. Frequently, an o!shore earthquake may also generate local submarine slides thus leading to complex patterns of tsunami #ooding at the coast (e.g. Yeh et al., 1993). In
* Corresponding author. E-mail address: [email protected] (A.G. Dawson)
other areas (e.g. Hawaiian islands, Norwegian Sea) submarine sediment slides may be the dominant mechanism of tsunami generation (Moore and Moore, 1988; Bondevik et al., 1997). Tsunami deposits are distinctive (Dawson et al., 1996; Bourgeois and Minoura, 1997). They are frequently associated with the deposition of continuous and discontinuous sediments sheets across large areas of the coastal zone (Dawson et al., 1995). Frequently they consist of deposits of sand containing isolated boulders. On occasions such boulders exhibit evidence of having been transported inland from the nearshore zone. In addition microfossil assemblages of diatoms and foraminifera contained within sand sheets may provide information of onshore transport of sediment from deeper water (Hemphill-Haley, 1995a, b, 1996; Dominey-Howes, 1996).
2. Palaeotsunami deposits Detailed information from coastal Washington State indicate the former occurrence of a large tsunami that accompanied an episode of coseismic coastal submergence during a large earthquake which took place ca. 300 years ago (Atwater and Yamaguchi, 1991). Sedimentary evidence for this tsunami is widespread throughout the Paci"c west coast (Clague, 1997). For example, Clague and Bobrowsky (1994) described for the tidal marshes near To"no and Ucleuelet, Vancouver Island, British Columbia, salt marshes overlain by sand sheets containing marine foraminifera and vascular plant fossils
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demonstrating rapid submergence prior to burial by marine sands. Similar sheets of sand attributed to the 1964 great Alaska earthquake and tsunami have been described by Clague et al. (1994) for Port Alberni, British Columbia. Atwater and Moore (1992) described stratigraphical evidence from the Puget Sound, Washington, for a palaeotsunami that #ooded coastal areas ca. 1000 years ago. At Cultus Bay a sand sheet between 5 and 15 cm in thickness containing marine foraminifera is enclosed within peat. The deposits typically have a medium grain size of ca. 0.1 mm and exhibit a progressive "ning inland. Darienzo and Peterson (1990) provided evidence for palaeotsunami deposition across a series of salt marshes along the northern Oregon coastline. They described a series of sediment sheets occasionally containing clay/silt units, marine-brackish diatoms and generally massive structure. These authors argued that the sands were transported and deposited out of turbulent suspension rather than due to small-scale currents that produced ripples and dunes on the sea bed. However they also noted that the lateral extent of the sediment sheets indicated that the tsunami surges were capable of transporting "ne sands over distances greater than 0.75 km despite being associated with bottom shear stresses that were insu$cient to disturb/or remove the stems of plants rooted in the underlying marsh surfaces. Since it is well known that sand sheets are also deposited within the intertidal zone during episodes of channel migration, river #ood and storms, criteria are needed to distinguish tsunami deposits from similar types of sediment within tidal wetland sediment sequences. They also drew attention to the fact that in open coastal settings, deposits produced as a result of infrequent storm surges are often di$cult to distinguish from those due to tsunamis.
3. Submarine sediments attributed to palaeotsunamis Sedimentary evidence for palaeotsunamis is not solely restricted to the coastal zone. Submarine sediments attributed to palaeotsunamis have also been reported as having been produced as a result of bolide impact. The most well-known example has been described in respect of the Cretaceous-Tertiary impact of 65 My ago (Keller et al., 1994; Smit et al., 1992, 1994). Submarine sediments elsewhere in the world have been attributed to former tsunamis. In the western Mediterranean Sea a distinctive acoustically transparent horizon consisting of very "ne-grained structureless muds and silts that exhibit a distinctive "ning-upwards sequence have been described by Kastens and Cita (1981) and Cita et al. (1982). This sedimentary unit, described by them as a homogenite, was explained as the result of the passage
of a large tsunami that led to sediment transport across sea-bed slopes on the abyssal plain. Cita et al. (1984) and Heike (1984) related this homogenite to a tsunami generated by the collapse of the Santorini volcano in Minoan times.
4. Linkings palaeotsunami deposits to source mechanisms The dating of the Cascadia episode of coseismic substance and the related tsunami led Satake et al. (1996) to conclude on the basis of historical records that the ca. 300 yr BP palaeoearthquake described by Atwater and Yamaguchi (1991) may have generated a transPaci"c tsunami that struck the coastline of Japan on 26th January 1700. Satake et al. (1996) concluded that the reports of waves striking the Japan coast at this time were more characteristic of a far-"eld tsunami rather than a storm surge and noted that most storm surges in Japan are generated by typhoons that normally take place between August and October. This observation together with the fact that the historical reports appear to indicate an apparently uniform distribution of water heights along the Japan coastline at this time supported the notion of a palaeotsunami having taken place rather than a storm surge. In the majority of the scienti"c literature that describes tsunamis a!ecting individual stretches of coastline, it is not always clear whether the coastal #ooding has been solely due to o!shore earthquakes, underwater sediment slides or to a combination of processes. This is perhaps one of the most important issues facing tsunami researchers since if the source mechanism for a particular tsunami is not clearly de"ned then there are signi"cant errors introduced into subsequent modelling exercises (Dawson, 1996). For example, the December 1992 tsunami in Flores, Indonesia is generally attributable to an o!shore earthquake. However, in the eastern section of the Flores coastline it has been suggested that the exceptionally high values for tsunami runup recorded in this area are the result of seabed sediment mass movement and sliding triggered by the earthquake (Yeh et al., 1993; Shi et al., 1995). Thus, the tsunami deposits that occur along the Flores coastline, whilst mostly due to a tsunami generated by an o!shore earthquake, are partly also the result of underwater sediment sliding. Unfortunately, tsunami deposits cannot provide direct information on whether or not an individual tsunami was produced by an o!shore sediment slide or principally as a result of earthquake activity. Clearly, any numerical simulation of this tsunami would have to take into account both processes and it should be noted that a simple model produced on the basis of speci"c seabed fault characteristics may produce runup values quite di!erent to those observed at the coast.
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The Flores example illustrates some of the di$culties faced in interpreting the source mechanisms for a particular tsunami. However, the problem is much more widespread. For example, tsunamis generated in areas of plate collision and subduction may not always represent the simple e!ect of sea #oor fault displacement and hence tsunami prorogation. Areas of active plate collision are notoriously dynamic and sediments resting on the #anks of submarine trenches may be particularly susceptible to sediment sliding. For example, it has never been demonstrated that the Nicaragua tsunami of 1991 was solely the product of sea #oor faulting with no in#uence from o!shore sediment sliding (cf. Bourgeois et al., 1988; Bourgeois, 1993; Satake et al., 1993). How is it possible to tell therefore if a particular tsunami is attributable to an o!shore earthquake and no other in#uence? In the case of pre-historic tsunamis, however, it is a much more di$cult exercise to attribute individual tsunami deposits to source mechanisms. For example, on the island of Molokai, Hawaii, boulder accumulations considered to have been deposited by a palaeotsunami during the last interglacial, have been observed by Moore and Moore (1984, 1988). In this area accumulations of limestone and basal boulders together with fragments of coral and other reef fragments known as the Hulopoe Gravel and occurring up to 375 m above sea level have been interpreted as tsunami deposits. The Hulopoe Gravel is unique among deposits attributed to tsunamis owing to their great thickness (cf. Stearns, 1940). For example, Moore and Moore (1984, 1988) have described three sedimentary units ca. 2, 4 and 2 m in thickness attributed to successive waves within a tsunami wave train. Moore and Moore (1984) considered that the lower sediments were produced as a result of sedimentation during tsunami runup while the upper sub-units were deposited during episodes of backwash. The remarkably high run-up altitude was attributed by Moore et al. (1989, 1992) to a large submarine slide/debris avalanche (the Alika phase 2 event) that was considered to have taken place ca. 105,000 years ago on the western #ank of the Manua Loa volcano. This tsunami is also considered by Young and Bryant (1992) to have resulted in 20 m high tsunami waves that struck the coastline of New South Wales, Australia and led to the removal of large blocks of bedrock from pre-existing uplifted coastal rock platforms. It should be noted that the submerged slopes of the Hawaiian volcanic chain are characterised by large numbers of giant sediment slides. Most of these are presumed to be fossil and relatively few are dated. Indeed, an earlier slide (the Lanai slide) was considered to be the source mechanism for the Lanai tsunami deposits until the age of the slide was determined as ca. 400,000 yr BP. It would appear, therefore, that a large number of Paci"c tsunamis may have been generated during the Cenozoic as a result of o!shore sediment sliding and these may extend as far
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Fig. 1. Location of gigantic submarine slide deposited during the last (Late Weichselian) glaciation at a time when relative sea level in the western Mediterranean was low (ca.!100 m). The slide most probably generated an exceptionally large tsunami yet evidence for such an event has not to date been identi"ed (after Rothwell et al., 1998).
north as the limit of the Emperor Sea Mount Chain. Given the large number of relic submarine slides around the #anks of inactive Hawaiian volcanoes, it is surprising that the Lanai deposits are the only hillslope sediments have been attributed to a palaeotsunami. It might be anticipated, for example, that if the Lanai event is real there may have been a series of high-magnitude transPaci"c palaeotsunamis originating from Hawaiian sediment slides that traversed the Paci"c Ocean throughout the Cenozoic era. Similarly, the identi"cation of previously unknown giant submarine slides (e.g. western Mediterranean) point to the former occurrence of largescale prehistoric tsunamis in this area for which no information is presently known (Rothwell et al., 1998) (Fig. 1).
5. Estimating runup height from tsunami deposits Recent studies of coastal sediments deposited 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). Shi et al. (1993, 1995) demonstrated that the Flores tsunami of 12th December 1992 was associated with the deposition of extensive sheets of sediments up to 1 m thick, and these are continued landward by discontinued sediment accumulations. The highest of these sediment accumulations always occurs below the upper limit of tsunami runup. Similarly in the Algarve, Portugal, tsunami deposits produced during the great Lisbon earthquake of November 1st 1755 AD at Boca do Rio occur as a continuous sheet
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of sediment inland from the coast but farther inland are replaced by discontinuous and eventually sporadic sediment sheets until a point is reached when there is no sedimentary trace of the tsunami, despite historical observations that tsunami #ooding took place to considerably higher altitudes (Andrade, 1992; Dawson et al., 1995; Hindson et al., 1996). Thus, the upper limit of sediment deposition is invariably less than the upper limit of runup. Therefore, palaeotsunami deposits cannot provide useful information on former runup limits. However, study of the sedimentology of the palaeotsunami deposits may provide information on the characteristics of the various waves within tsunami wave trains that a!ected speci"c coastal zone. The interpretation of any tsunami deposit is made complicated by not only the occurrence of a series of long-period waves each of which may be responsible for localised littoral deposition but also by episodes of sediment erosion. Successive tsunami waves may result in the erosion of pre-existing tsunami deposits and, on occasions, their complete removal. The pattern and processes of tsunami sedimentation is additionally complicated by the occurrence of episodes of backwash #ow that follow the maximum landward inundation of individual tsunami waves. Under such circumstances strong currents may #ow from landward to seaward and may cause additional #uvial erosion and sediment re-deposition. Velocities of backwash #ow are, in turn, greatly in#uenced 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 terrestrial sediments including plant macrofossils into the sediment assemblage, thus complicating palaeoenvironmental interpretation. For example, Bondevik et al. (1997) noted within lake sediments in uplifted isolation basins in western Norway inundated by the tsunami associated with the Second Storrega Slide, large quantities of roots and twigs that were interpreted to have been deposited as a result of local tsunami backwash #ow. The rate of submarine sediment displacement is a crucial factor in determining the dimensions of the resultant tsunami. A useful illustration of this process is in respect of the palaeotsunami of ca. 7100 14C years ago generated in the Norwegian Sea as a result of the Second Storegga underwater slide (Bugge et al., 1987; Bugge, 1983; Bondevik et al., 1997). Geological investigations of tsunami deposits on the northern and eastern coastlines of Scotland as well as in uplifted lake basins along the west coast of Norway provided evidence of minimum tsunami runup (Dawson et al., 1988; Bondevik, 1996; Dawson and Smith, 1997). In eastern Scotland the estimated minimum value of runup associated with the tsunami is in the order of 4}6 m above contemporary high water mark (Dawson et al., 1988). However this
value, as stated above, should be treated with caution since tsunami #ooding to higher elevations may have taken place yet did not leave a sedimentary record. Harbitz (1991, 1992) attempted to develop a numerical model of the Second Storegga Submarine Slide. He noted that the scale of tsunami runup along the Scottish and Norwegian coastlines very much depended upon the average landslide velocity that was introduced into the model. For example he noted that an average slide velocity of 20 m/s resulted in runup values onto adjacent coastlines of between 1 and 2 m. By contrast a modelled landslide velocity of 50 m/s resulted in runup values of between 5 and 14 m, values signi"cantly in excess of the estimates for adjacent coastlines based on geological data. Harbitz (1992) concluded that a landslide velocity of 30 m/s provided the closest approximation to the estimated runup values based on geological data. However the weakness in this argument is that the geological data only provide minimum estimates of likely #ood runup and therefore the related numerical models of the same tsunami will always underestimate the likely average value of the submarine slide velocity.
6. Mechanisms of deposition Field observations of tsunami #ooding usually describe the rapid lateral translation of water across the coastal zone. Frequently, the lateral water motion associated with runup is in#uenced by local wave resonance. Thus, the tsunami waves as they strike the coast are unlike waves associated with storm surges since not only are they associated with considerably greater wavelengths and wave periods, but they are essentially constructive as they move inland across the coastal zone (Reinhardt and Bourgeois, 1989). The rapid water velocities (provided that there is an adequate supply of sediment in nearshore zone) are in most cases associated with the transport of a variety of grain size ranging from silt to boulders. Unlike storm surges individual tsunami waves reach a point of zero water velocity prior to backwash #ow. At this point large volumes of sediment may be deposited out of the water column onto the ground surface. Young and Bryant (1992) have made reference to isolated boulders in tsunami deposits in SE Australia. In this area, thicknesses of massive sands and silts include occasional isolated boulders, described by Young and Bryant (1992) as &boulder #oats'. In the absence of any other plausible mechanism of deposition, it is suggested here that the processes described above may provide the simplest explanation for their deposition. Isolated boulders contained within massive sandy deposits have been described for other palaeotsunami deposits (e.g. Shetland Islands).
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7. Boulder accumulations Empirical evidence of coastal boulder deposition during the tsunami was provided by Yeh et al. (1993) in their study of the Flores tsunami of December 1992. In this area sections of eroded coral reef were observed to have been transported by the tsunami and deposited farther inland. Bourrouilh-Le Jan and Talandier (1985) described areas of coral reef in the Tuamoto archipelago, SE Paci"c, where large numbers of giant boulders (up to 750 m3) appear to have been transported across the atoll rim and into the lagoon. However, although palaeotsunamis have been considered in this case as a possible depositional agent, similar boulders may also be deposited as a result of storm surges (Teissier, 1969; Pirazzoli et al., 1988). Boulder "elds attributable to tsunamis have been observed elsewhere in the world (e.g. Japan) (Ota et al., 1985; Kawana and Pirazzoli, 1990; Nakata and Kawana, 1993). In southern Portugal, Hindson and Andrade (pers. comm.) investigated boulders deposited by the tsunami caused by the Great Lisbon earthquake of 1755 AD. Hindson noted that at several locations on the Algarve coastline that the palaeotsunami was associated with the deposition of both continuous and discontinuous sediment sheets some of which contained boulders. The individual boulders were frequently pitted with bioerosional hollows in which were found marine mollusca. Since both the boulders and the sediment sheets were attributed to the former tsunami, it was concluded that the individual boulders were transported during the tsunami from the seabed o!shore (Fig. 2).
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world is how to be able to distinguish tsunami deposits from sediments deposited as a result of hurricane-induced storm surges. For example, Liu and Fearn (1993) have shown from coastal Alabama, USA, that a series of hurricanes during historical time have resulted in the deposition of multiple sand layers in low-lying coastal wetlands. Similarly, Davis et al. (1989) argued that hurricanes produced graded or homogenous deposits of sand, shell, gravel and mud found in the prominently clastic sediments in the coastal lagoons of Florida. While it is accepted that storm surges result in the deposition of discrete sedimentary units (Fig. 3), it is argued here that tsunamis in contrast to storm surges, generally result in deposition of continuous and discontinuous sediment sheets over relatively wide areas and considerable distances inland. For example, sediment sheets in the Algarve, Portugal associated with the Lisbon earthquake tsunami of 1755 AD occur in excess of 1 km inland (Hindson et al., 1996). Similarly, palaeotsunami deposits in Scotland associated with the Second Storegga Slide of ca. 7100 14C BP are frequently associated with extensive sediment sheets that occur many hundreds of metres inland (Smith et al., 1985; Dawson et al., 1988; Long et al., 1989).
9. Tsunami deposits and microfossil assemblages
One of the most awkward problems in reconstructing chronologies of palaeotsunamis for di!erent areas of the
Relatively few investigations have been undertaken on the microfossils associated with tsunami deposits. Hemphill-Haley (1995a, b, 1996) has described various diatom assemblages contained within tsunami deposits along the Paci"c coastline of Oregon, Washington and British Columbia. Hemphill-Haley described a variety of brackish-marine diatoms within tsunami deposits that could be used not only to identify tsunami deposits but also to estimate the greatest inland extent of tsunami
Fig. 2. Sub-rounded limestone boulder identi"ed within tsunami deposits at Boca do Rio, Algarve, Portugal, produced by the Great Lisbon tsunami of 1755 AD. Bioerosional hollows are characteristic of the boulder surface and locally contain marine mollusca (courtesy of R. Hindson).
Fig. 3. Storm surge sediments deposited by hurricane Hugo (September 1989) in western St Lucia, Windward Isles, Caribbean. Note that the deposits consist of a series of overlapping depositional fans that extend no more than a few metres inland.
8. Distinguishing storm surge and tsunami deposits
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inundation owing to the fact that certain assemblages occur beyond the landward limit of tsunami-deposited sands and silts. Furthermore, she noted that in certain coastal freshwater lakes located beyond the reach of storms, marine planktonic and neritic plankton can be used to identify former episodes of tsunami inundation. Studies of diatom assemblages contained within tsunami deposits in Scotland related to the Second Storegga Slide have shown the presence of exceptionally large numbers of the species Paralia sulcata with many individuals exhibiting evidence of breakage (Dawson et al., 1996b). Similarly, Dawson et al. (1996a) in an investigation of diatom assemblages contained within the Grand Banks tsunami deposit showed that the major diatom species present was Paralia sulcata and that the majority of individuals were broken. At "rst sight it may appear that large percentages of broken diatoms may be indicative of former tsunamis. However, this issue is made problematic since owing to the varying robustness of pennate (lenticular) and centric (circular) diatoms, some species are more able to withstand fracturing than others. Foraminifera also provide a useful means by which to identify former tsunami deposits. The most detailed investigation of foraminifera contained within tsunami deposits has been undertaken by Dominey-Howes (1996) and Dominey-Howes et al. (1999) in western Crete. Dominey-Howes (1996) showed that tsunami deposits dating to the "rst century AD contain a wide variety of species. In particular, the presence of certain species indicated transport from deep water o!shore and this observation was used to support the concept of a former tsunami having taken place. Studies of foraminifera contained within tsunami deposits are very much in their infancy and the value of this particular technique remains to be discovered.
10. Summary In recent years the recognition of distinctive palaeotsunami deposits has been used to improve our understanding of former o!shore earthquakes and giant submarine slides. In the Paci"c West Coast, palaeotsunami deposits have provided important information pointing to a relatively recent large-scale o!shore Cascadia earthquake. Owing to limited information of sea #oor geology, many former tsunamis cannot be backtraced to speci"c source mechanisms. Thus, it is almost impossible in certain cases to link individual events to speci"c generating mechanisms. This is particularly wellillustrated in the case of the trans-Paci"c Lanai tsunami in Hawaii. Despite recent advances, our understanding of tsunami deposits is relatively limited. For example, there never has been a detailed investigation undertaken of the
pattern and processes of sedimentation associated with a modern tsunami. Despite these di$culties, recent progress has been made as a result of microfossil investigations of tsunami deposits. These studies have provided relatively unambiguous information on the nature of former tsunamis. Progress has also been made in the recognition of distinctive tsunami deposits and how these can be di!erentiated from palaeostorm-surge deposits.
Acknowledgements Cartographic assistance was kindly provided by Erica Millwain and Michelle Walton and Gillian West kindly typed the manuscript. This paper is a contribution to Project GITEC-TWO &Genesis and Impact of Tsunamis on European Coasts', European Union Project ENV4CT96-0297-GITEC-TWO. It is also a contribution to IGCP Project 367 on &Rapid Coastal Changes' and was "rst presented to the International Tsunami Conference, Paris, May 1998.
References Andrade, C., 1992. Tsunami generated forms in the Algarve barrier islands. In: Dawson, A.G. (Ed.), European Geophysical Society 1992 Tsunami Meeting, Science of Tsunami Hazards, Vol. 10, pp. 21}34. Atwater, B.F., 1992. Geologic evidence for earthquakes during the past 2000 years along the Copalis River, Southern Coastal Washington. Journal of Geophysical Research 97 (B2), 1901}1919. Atwater, B.F., Moore, A.L., 1992. A tsunami about 1000 years ago in Opuget Sound, Washington. Science 258, 1614}1623. Atwater, B.F., Yamaguchi, D.K., 1991. Sudden, probably coseismic submergence of Holocene trees and grass in coastal Washington state. Geology 19, 706}709. Bondevik, S., 1996. The Storegga Tsunami deposits in Western Norway and postglacial sea-level changes on Svalbard. Thesis, University of Bergen, 107 pp. Bondevik, S., Svendsen, J.I., Johnsen, G., Mangerud, J., Kaland, P.E., 1997. The Storegga tsunami along the Norwegian coast, its age and runup. Boreas 26 (1), 29}54. Bourgeois, J., 1993. Tsunami deposits from 1992 Nicaragua event: implications for interpretation of palaeotsunami deposits * Eos Abstracts, O32C-7, 350, American Geophysical Union 1993 Fall Meeting, San Francisco. Bourgeois, J., Hansen, T.A., Wiberg, P.L., Kaufmann, E.G., 1988. A tsunami deposit at the Cretaceous-Tertiary Boundary in Texas. Science 241, 567}570. Bourgeois, J., Minoura, K., 1997. Palaeotsunami studies * contribution to mitigation and risk assessment. In: Gusiakov, V.K. (Ed.), Tsunami Mitigation and Risk Assessment, pp. 1}4, Novosibirsk. Bourrouilh-Le Jan, F.G., Talandier, J., 1985. Sedimentation et fracturation de hautes energie en milieu recifaltsunamis, ouragans et cyclones et leurs e!ects sur la sedimentologie et la geomorphologie d'un atoll: motu et hoa, a Rangiroa Tuamotu, Paci"que SE. Marine Geology 67, 263}333. Bugge, T., 1983. Submarine slides on the Norwegian continental margin, with special emphasis on the Storegga area. Continental Shelf and Petroleum Technology Research Institute, A/S publication, 110, 152 pp, Trondheim.
A.G. Dawson / Quaternary International 60 (1999) 119}126 Bugge, T., Befring, S., Belderson, R.H., Eidvin, T., Jansen, E., Kenyon, N., Holtedahl, H., Sejrup, H.-P., 1987. A giant three-stage submarine slide o! Norway. Geo-Marine Letters 7, 191}198. Cita, M.A., Maccagni, A., Pirovano, J., 1982. Tsunami as triggering mechanism of homogenites recorded in areas of the Eastern Mediterranean characterised by the cobblestone topography. In: Saxov, S.E., Niewenhuis, J.K. (Eds.), Marine Slides and other Mass Movements. Plenum Press, New York, pp. 233}260. Cita, M.A., Camerlenghi, A., Kastens, K.A., Mccoy, F.W., 1984. New "ndings of Bronze Age homogenities in the Ionian Sea: geodynamic implications for the Mediterranean. Marine Geology 55, 47}62. Clague, J.J., 1997. Evidence for large earthquakes at the Cascadia subduction zone. Reviews of Geophysics 35 (4), 439}460. Clague, J.J., Bobrowsky, P.T., 1994. Evidence for a Large Earthquake and Tsunami 100}400 Years Ago on Western Vancouver Island British Columbia. Quaternary Research 41, 176}184. Clague, J.J., Bobrowsky, P.T., Hamilton, T.S., 1994. A sand sheet deposited by the 1964 Alaska Tsunami at Port Alberni British Columbia. Estuarine, Coastal and Shelf Science 38, 413}421. Darienzo, M.E., Peterson, C.D., 1990. Episode tectonic subsidence of late Holocene salt marshes, northern Oregon, central Cascadia margin. Tectonics 9 (1), 1}22. Davis Jr., R.A., Knowles, S.C., Bland, M.J., 1989. Role of hurricanes in the Holocene stratigraphy of estuaries: examples from the Gulf Coast of Florida. Journal of Sedimentary Petrology 59, 1052}1061. Dawson, A.G., 1994. Geomorphological e!ects of tsunami run-up and backwash. Geomorphology 10, 83}94. Dawson, A.G., 1996. The Geological Signi"cance of Tsunamis. Zeitschrift Geomorphologie N.F., Suppl.-Bd. 102, 199}210. Dawson, A.G., Long, D., Smith, D.E., 1988. The Storegga Slides: evidence from eastern Scotland for a possible tsunami. Marine Geology 82, 271}276. Dawson, A.G., Hindson, R., Andrade, C., Freitas, C., Parish, R., Bateman, M., 1995. Tsunami sedimentation associated with the Lisbon earthquake of 1 November AD 1755: Boco do Rio, Algarve, Portugal. The Holocene 5 (2), 209}215. Dawson, A.G., Shi, S., Dawson, S., Takahashi, T., Shuto, N., 1996a. Coastal Sedimentation Associated with the June 2nd and 3rd, 1994 Tsunami in Rajegwesi Java. Quaternary Science Reviews 15, 901}912. Dawson, S., Smith, D.E., Ru!man, A., Shi, S., 1996b. The diatom biostratigraphy of tsunami sediments: examples from recent and middle Holocene events. Physical and Chemistry of the Earth 21 (12), 87}92. Dawson, S., Smith, D.E., 1997. Holocene relative sea-level changes on the margin of a glacio-isostatically uplifted area: an example from northern Caithness, Scotland. The Holocene 7 (1), 59}77. Dominey-Howes, D.T.M., 1996. The geomorphology and sedimentology of "ve tsunamis in the Aegean Sea Region, Greece. Unpublished Ph.D. thesis, Coventry University, September 1996. Dominey-Howes, D., Dawson, A.G., Smith, D.E., 1999. Late Holocene coastal tectonics at Falasarna, western Crete (Greece): a sedimentary contribution. Geological Society of London Special Publication, 146, 343}352. Harbitz, C.B., 1991. Model simulations of tsunami generated by the Storegga Slide. Institute of Mathematics, University of Oslo. Series No 5, 30 pp. Harbitz, C.B., 1992. Model simulations of tsunami generated by the Storegga Slides. Marine Geology 105, 1}21. Heike, W., 1984. A thick Holocene homogenite from the Ionian Abyssal Plain (eastern Mediterranean). Marine Geology 55, 63}78. Hemphill-Haley, E., 1995a. Diatom evidence for earthquakeinduced subsidence and tsunami 300 years ago in southern coastal Washington. Geological Society of America Bulletin 107, 367}378. Hemphill-Haley, E., 1995b. Intertidal diatoms from Willapa Bay, Washington: application to studies of small-scale sea-level changes. Northwest Science 69, 29}45.
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Hemphill-Haley, E., 1996. Diatoms as an aid in identifying late-Holocene tsunami deposits. The Holocene 6 (4), 439}448. Hindson, R.A., Andrade, C., Dawson, A.G., 1996. Sedimentary processes associated with the tsunami generated by the 1755 Lisbon earthquake on the Algarve Coast, Portugal. Physics and Chemistry of the Earth 21 (12), 57}63. Kastens, K.A., Cita, M.B., 1981. Tsunami-induced transport in the abyssal Mediterranean Sea. Geological Society of America Bulletin 92, 845}857. Kawana, T., Pirazzoli, P., 1990. Re-examination of the Holocene emerged shorelines in Orabu and Shimoji Islands, the South Ryukyus, Japan. Quaternary Research 28, 419}426. Keller, G., Stinnesbeck, W., Adatte, T., Macleod, N., Lowe, D.R., 1994. Field Guide to Cretaceous-Tertiary Boundary Sections in Northeastern Mexico * Lunar and Planetary Institute. 3600 Bay Area Boulevard, Houston TX 77058-1113, Contribution 827, 110 pp. Liu, K.-B, Fearn, M.L., 1993. Lake-sediment record of late Holocene hurricane activities from coastal Alabama. Geology 21, 793}796. Long, D., Smith, D.E., Dawson, A.G., 1989. A Holocene tsunami deposit in eastern Scotland. Journal Quaternary Science 4, 61}66. Minoura, K., Nakaya, S., 1991. Traces of tsunamis preserved in intertidal lacustrine and marsh deposits: some examples from Northeast Japan. Journal of Geology 99, 265}287. Minoura, K., Nakaya, S., Uchida, M., 1994. Tsunami deposits in a lacustrine sequence of the Sanriku coast, northern Japan. Sedimentary Geology 89, 25}31. Moore, G.W., Moore, J.G., 1988. Large-scale bedforms in boulder gravel produced by giant waves in Hawaii. In: Clifton, H.E. (Ed.), Sedimentologic Consequences of Convulsive Geologic Events. Geological Society of American Special Paper, Vol. 229, pp. 101}110. Moore, J.G., Moore, G.W., 1984. Deposit from a giant wave on the island of Lanai Hawaii. Science 226, 1312}1315. Moore, J.G., Clague, D.A., Holcom, R.T., Lipman, P.W., Normark, W.R., Torresan, M.E., 1989. Prodigious submarine landslides on the Hawaiian Ridge. Journal of Geophysical Research 94, 17465}17484. Moore, J.G., Normark, W.R., Gutmacher, C.E., 1992. Major landslides on the submarine #anks of the Manua Lao Volcano, Hawaii. Landslide News 13}16. Nakata, T., Kawana, T., 1993. Historical and prehistorical large tsunamis in the southern Ryukus, Japan. In: Tsunami '93, Proceedings of IUGG/IOC Uinst, Tsunami Symposium. Wakayama, Japan, August 23}27, pp. 297}307. Nishimura, Y., 1994. The 1640 Komagtake Tsunami runups as revealed by the tsunami deposits * Amer. Geophys. Union Western Paci"c Geophysics Meeting, Hong Kong. July, 25}29, Eos, Abstract, 63. Ota, Y., Pirazzoli, P.A., Kawana, T., Moriwaki, H., 1985. Late Holocene coastal morphology and sea-level records on three small islands, the South Ryukyus, Japan. Geographical Reviews of Japan 58B, 185}194. Pasko!, R., 1991. Likely occurrence of a mega-tsunami near Coqimbo, Chile. Reviews of Geology Chile 18, 87}91. Perissoratis, C., Papadopoulos, G., 1998. Sediment slumping in the South Aegean Sea and the case history of the 1956 Tsunami. In: Seventh International Conference on Natural and Man-Made Hazards. Chania, Crete, Greece, May 1998. Abstract, p. 120. Pinegina, T.K., Bazanova, L.I., Braitseva, O.A., Gusiakov, V.K., Melekestsev, I.V., Storcheus, A.V., 1997. East Kamchatka palaeotsunami traces. In: Gusiakov, V.K. (Ed.), Tsunami Mitigation and Risk Assessment, pp. 5}12. Pirazzoli, P.A., Montaggioni, L.F., Salvant, B., Faure, G., 1988. Late Holocene sea level indicators from twelve atolls in the central and eastern Tuamotos (Paci"c Ocean). Coral Reefs 7, 57}68. Ranguelov, B., 1998. Risk management of natural disasters and ecological catastrophes: a book for disaster education. In: Seventh International Conference on Natural and Man-Made Hazards, Chania, Crete, Greece, May 1998. Abstract, pp. 124}125.
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Reinhardt, M.A., Bourgeois, J., 1989. Tsunami favoured over storm or seiche for sand deposits overlying buried Holocene peat. Wilapa Bay, Washington. Eos, American Geophysical Union, October 24, p. 1331. Rothwell, R.G., Thomson, J., KaK hler, G., 1998. Low-sea-level emplacement of a very large Late Pleistocene &megaturbidite' in the western Mediterranean Sea. Nature 392, 377}380. Satake, K., Bourgeois, J., Abe, K., Abe, K., Tsuji, Y., Imamura, F., Iio, Y., Katao, H., Noguera, E., Estrade, F., 1993. Tsunami "eld survey of the 1992 Nicaragua earthquake. Eos, Transactions, American Geophysical Union 74(13), 145, 156}157. Satake, K., Shimazaki, K., Tsuji, Y., Udea, K., 1996. Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700. Nature 379 (6562), 246}249. Shi, S., Dawson, A.G., Smith, D.E., 1993. Geomorphological impact of the Flores tsunami of 12th December, 1992. In: `Tsunami '93a, Proceedings of the IUGG/IOC International Tsunami Symposium. Wakayama, Japan, August 23}27, pp. 689}696. Shi, S., Dawson, A.G., Smith, D.E., 1995. Coastal sedimentation associated with the December 12th 1992 Tsunami in Flores, Indonesia. In: Satake, K., Imamura, K. (Eds.), Recent Tsunamis, Pure and Applied Geophysics, Vol. 144, pp. 525}536. Smit, J., Montanari, A., Swinburne, N.H.M., Alavarez, W., Hilderbrand, A.R., Margolis, S.W., Claeys, P., Lowrie, W., Asaro, F., 1992.
Tektite-bearing deep-water clastic unit at the Cretaceous-Tertiary boundary in Northeastern Mexico. Geology 20, 99}103. Smit, S., Montanari, A., Alvarez, W., 1994. Tsunami-generated beds at the KT boundary in northeastern Mexico. In: Keller, G., Stinnesbeck, W., Adatte, T., MacLeod, N., Lowe, D.R. (Eds.), Field Guide to Cretaceous-Tertiary Boundary Sections in Northeastern Mexico. Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston TX 77058-1113, Contribution 827, pp. 95}110. Smith, D.E., Cullingford, R.A., Haggart, B.A., 1985. A major coastal #ood during the Holocene in eastern Scotland. Eiszeitalter und Gegenwart 35, 109}118. Stearns, H.T., 1940. Geology and ground-water resources of the islands of Lanai and Kahoolawe, Hawaii. Hawaii Division of Hydrography Bulletin 7, 177 pp. Teissier, R., 1969. Les Cyclones en Polynesie Francaise. Bulletin SocieteH Etudes Oceaniennes 14, 20}21. Yeh, H., Imamura, F., Synolakis, C., Ysuji, Y., Liu, P., Shi, S., 1993. The Flores Island Tsunamis. Eos Transactions, American Geophysical Union 74 (33), 369}373. Young, R.W., Bryant, T., 1992. Catastrophic wave erosion on the southeastern coast of Australia: impact of the Lanai Tsunami ca. 105ka? Geology 20, 199}202.
Linking tsunami deposits, submarine slides and o!shore earthquakes Alastair G. Dawson* Centre for Quaternary Science, William Morris Building, Coventry University, Coventry CV1 5FB, UK
Abstract The recognition that many tsunamis are associated with coastal sedimentation has been of great value in the study of tsunamis prehistorically. Geological investigation of such sediments has resulted in the identi"cation of a series of palaeotsunamis that appear to have taken place in di!erent areas of the world. In most cases, however, it has proved di$cult to link former tsunamis to speci"c source mechanisms. Studies of modern tsunamis have also faced di$culties in the recognition of the speci"c source mechanisms. For example, o!shore earthquakes may trigger submarine slides that combine to produce complex patterns of tsunami #ooding at the coast. ( 1999 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Geological investigations of former tsunamis is a relatively new research area. The recognition that many tsunamis deposit sediment in the coastal zone has only become an accepted idea during the last 5}10 years. Discussion of this concept has been accompanied by a proliferation of academic papers that have described a range of sediments that have been attributed to a series of former tsunamis (Minoura and Nakaya, 1991; Pasko!, 1991; Atwater, 1992; Minoura et al., 1994; Nishimura, 1994; Pinegina et al., 1997; Ranguelov, 1998). Unlike storm surges, tsunami runup across the coastal zone is frequently associated with the rapid lateral translation of water and suspended sediment. Thus, tsunami deposits can be used to provide an indirect record of former o!shore earthquakes and underwater landslides. It is exceptionally di$cult, however, if not impossible, to di!erentiate tsunami deposits attributable to former submarine slides or o!shore earthquakes (e.g. Perissoratis and Papadopoulos, 1998). In particular areas of the world, especially in areas of an active plate motion where an o!shore earthquake has taken place, it may be a gross over-simpli"cation to attribute the triggering mechanism solely to earthquake-induced sea bed faulting. Frequently, an o!shore earthquake may also generate local submarine slides thus leading to complex patterns of tsunami #ooding at the coast (e.g. Yeh et al., 1993). In
* Corresponding author. E-mail address: [email protected] (A.G. Dawson)
other areas (e.g. Hawaiian islands, Norwegian Sea) submarine sediment slides may be the dominant mechanism of tsunami generation (Moore and Moore, 1988; Bondevik et al., 1997). Tsunami deposits are distinctive (Dawson et al., 1996; Bourgeois and Minoura, 1997). They are frequently associated with the deposition of continuous and discontinuous sediments sheets across large areas of the coastal zone (Dawson et al., 1995). Frequently they consist of deposits of sand containing isolated boulders. On occasions such boulders exhibit evidence of having been transported inland from the nearshore zone. In addition microfossil assemblages of diatoms and foraminifera contained within sand sheets may provide information of onshore transport of sediment from deeper water (Hemphill-Haley, 1995a, b, 1996; Dominey-Howes, 1996).
2. Palaeotsunami deposits Detailed information from coastal Washington State indicate the former occurrence of a large tsunami that accompanied an episode of coseismic coastal submergence during a large earthquake which took place ca. 300 years ago (Atwater and Yamaguchi, 1991). Sedimentary evidence for this tsunami is widespread throughout the Paci"c west coast (Clague, 1997). For example, Clague and Bobrowsky (1994) described for the tidal marshes near To"no and Ucleuelet, Vancouver Island, British Columbia, salt marshes overlain by sand sheets containing marine foraminifera and vascular plant fossils
1040-6182/99/$20.00 ( 1999 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 9 ) 0 0 0 1 1 - 7
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demonstrating rapid submergence prior to burial by marine sands. Similar sheets of sand attributed to the 1964 great Alaska earthquake and tsunami have been described by Clague et al. (1994) for Port Alberni, British Columbia. Atwater and Moore (1992) described stratigraphical evidence from the Puget Sound, Washington, for a palaeotsunami that #ooded coastal areas ca. 1000 years ago. At Cultus Bay a sand sheet between 5 and 15 cm in thickness containing marine foraminifera is enclosed within peat. The deposits typically have a medium grain size of ca. 0.1 mm and exhibit a progressive "ning inland. Darienzo and Peterson (1990) provided evidence for palaeotsunami deposition across a series of salt marshes along the northern Oregon coastline. They described a series of sediment sheets occasionally containing clay/silt units, marine-brackish diatoms and generally massive structure. These authors argued that the sands were transported and deposited out of turbulent suspension rather than due to small-scale currents that produced ripples and dunes on the sea bed. However they also noted that the lateral extent of the sediment sheets indicated that the tsunami surges were capable of transporting "ne sands over distances greater than 0.75 km despite being associated with bottom shear stresses that were insu$cient to disturb/or remove the stems of plants rooted in the underlying marsh surfaces. Since it is well known that sand sheets are also deposited within the intertidal zone during episodes of channel migration, river #ood and storms, criteria are needed to distinguish tsunami deposits from similar types of sediment within tidal wetland sediment sequences. They also drew attention to the fact that in open coastal settings, deposits produced as a result of infrequent storm surges are often di$cult to distinguish from those due to tsunamis.
3. Submarine sediments attributed to palaeotsunamis Sedimentary evidence for palaeotsunamis is not solely restricted to the coastal zone. Submarine sediments attributed to palaeotsunamis have also been reported as having been produced as a result of bolide impact. The most well-known example has been described in respect of the Cretaceous-Tertiary impact of 65 My ago (Keller et al., 1994; Smit et al., 1992, 1994). Submarine sediments elsewhere in the world have been attributed to former tsunamis. In the western Mediterranean Sea a distinctive acoustically transparent horizon consisting of very "ne-grained structureless muds and silts that exhibit a distinctive "ning-upwards sequence have been described by Kastens and Cita (1981) and Cita et al. (1982). This sedimentary unit, described by them as a homogenite, was explained as the result of the passage
of a large tsunami that led to sediment transport across sea-bed slopes on the abyssal plain. Cita et al. (1984) and Heike (1984) related this homogenite to a tsunami generated by the collapse of the Santorini volcano in Minoan times.
4. Linkings palaeotsunami deposits to source mechanisms The dating of the Cascadia episode of coseismic substance and the related tsunami led Satake et al. (1996) to conclude on the basis of historical records that the ca. 300 yr BP palaeoearthquake described by Atwater and Yamaguchi (1991) may have generated a transPaci"c tsunami that struck the coastline of Japan on 26th January 1700. Satake et al. (1996) concluded that the reports of waves striking the Japan coast at this time were more characteristic of a far-"eld tsunami rather than a storm surge and noted that most storm surges in Japan are generated by typhoons that normally take place between August and October. This observation together with the fact that the historical reports appear to indicate an apparently uniform distribution of water heights along the Japan coastline at this time supported the notion of a palaeotsunami having taken place rather than a storm surge. In the majority of the scienti"c literature that describes tsunamis a!ecting individual stretches of coastline, it is not always clear whether the coastal #ooding has been solely due to o!shore earthquakes, underwater sediment slides or to a combination of processes. This is perhaps one of the most important issues facing tsunami researchers since if the source mechanism for a particular tsunami is not clearly de"ned then there are signi"cant errors introduced into subsequent modelling exercises (Dawson, 1996). For example, the December 1992 tsunami in Flores, Indonesia is generally attributable to an o!shore earthquake. However, in the eastern section of the Flores coastline it has been suggested that the exceptionally high values for tsunami runup recorded in this area are the result of seabed sediment mass movement and sliding triggered by the earthquake (Yeh et al., 1993; Shi et al., 1995). Thus, the tsunami deposits that occur along the Flores coastline, whilst mostly due to a tsunami generated by an o!shore earthquake, are partly also the result of underwater sediment sliding. Unfortunately, tsunami deposits cannot provide direct information on whether or not an individual tsunami was produced by an o!shore sediment slide or principally as a result of earthquake activity. Clearly, any numerical simulation of this tsunami would have to take into account both processes and it should be noted that a simple model produced on the basis of speci"c seabed fault characteristics may produce runup values quite di!erent to those observed at the coast.
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The Flores example illustrates some of the di$culties faced in interpreting the source mechanisms for a particular tsunami. However, the problem is much more widespread. For example, tsunamis generated in areas of plate collision and subduction may not always represent the simple e!ect of sea #oor fault displacement and hence tsunami prorogation. Areas of active plate collision are notoriously dynamic and sediments resting on the #anks of submarine trenches may be particularly susceptible to sediment sliding. For example, it has never been demonstrated that the Nicaragua tsunami of 1991 was solely the product of sea #oor faulting with no in#uence from o!shore sediment sliding (cf. Bourgeois et al., 1988; Bourgeois, 1993; Satake et al., 1993). How is it possible to tell therefore if a particular tsunami is attributable to an o!shore earthquake and no other in#uence? In the case of pre-historic tsunamis, however, it is a much more di$cult exercise to attribute individual tsunami deposits to source mechanisms. For example, on the island of Molokai, Hawaii, boulder accumulations considered to have been deposited by a palaeotsunami during the last interglacial, have been observed by Moore and Moore (1984, 1988). In this area accumulations of limestone and basal boulders together with fragments of coral and other reef fragments known as the Hulopoe Gravel and occurring up to 375 m above sea level have been interpreted as tsunami deposits. The Hulopoe Gravel is unique among deposits attributed to tsunamis owing to their great thickness (cf. Stearns, 1940). For example, Moore and Moore (1984, 1988) have described three sedimentary units ca. 2, 4 and 2 m in thickness attributed to successive waves within a tsunami wave train. Moore and Moore (1984) considered that the lower sediments were produced as a result of sedimentation during tsunami runup while the upper sub-units were deposited during episodes of backwash. The remarkably high run-up altitude was attributed by Moore et al. (1989, 1992) to a large submarine slide/debris avalanche (the Alika phase 2 event) that was considered to have taken place ca. 105,000 years ago on the western #ank of the Manua Loa volcano. This tsunami is also considered by Young and Bryant (1992) to have resulted in 20 m high tsunami waves that struck the coastline of New South Wales, Australia and led to the removal of large blocks of bedrock from pre-existing uplifted coastal rock platforms. It should be noted that the submerged slopes of the Hawaiian volcanic chain are characterised by large numbers of giant sediment slides. Most of these are presumed to be fossil and relatively few are dated. Indeed, an earlier slide (the Lanai slide) was considered to be the source mechanism for the Lanai tsunami deposits until the age of the slide was determined as ca. 400,000 yr BP. It would appear, therefore, that a large number of Paci"c tsunamis may have been generated during the Cenozoic as a result of o!shore sediment sliding and these may extend as far
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Fig. 1. Location of gigantic submarine slide deposited during the last (Late Weichselian) glaciation at a time when relative sea level in the western Mediterranean was low (ca.!100 m). The slide most probably generated an exceptionally large tsunami yet evidence for such an event has not to date been identi"ed (after Rothwell et al., 1998).
north as the limit of the Emperor Sea Mount Chain. Given the large number of relic submarine slides around the #anks of inactive Hawaiian volcanoes, it is surprising that the Lanai deposits are the only hillslope sediments have been attributed to a palaeotsunami. It might be anticipated, for example, that if the Lanai event is real there may have been a series of high-magnitude transPaci"c palaeotsunamis originating from Hawaiian sediment slides that traversed the Paci"c Ocean throughout the Cenozoic era. Similarly, the identi"cation of previously unknown giant submarine slides (e.g. western Mediterranean) point to the former occurrence of largescale prehistoric tsunamis in this area for which no information is presently known (Rothwell et al., 1998) (Fig. 1).
5. Estimating runup height from tsunami deposits Recent studies of coastal sediments deposited 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). Shi et al. (1993, 1995) demonstrated that the Flores tsunami of 12th December 1992 was associated with the deposition of extensive sheets of sediments up to 1 m thick, and these are continued landward by discontinued sediment accumulations. The highest of these sediment accumulations always occurs below the upper limit of tsunami runup. Similarly in the Algarve, Portugal, tsunami deposits produced during the great Lisbon earthquake of November 1st 1755 AD at Boca do Rio occur as a continuous sheet
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of sediment inland from the coast but farther inland are replaced by discontinuous and eventually sporadic sediment sheets until a point is reached when there is no sedimentary trace of the tsunami, despite historical observations that tsunami #ooding took place to considerably higher altitudes (Andrade, 1992; Dawson et al., 1995; Hindson et al., 1996). Thus, the upper limit of sediment deposition is invariably less than the upper limit of runup. Therefore, palaeotsunami deposits cannot provide useful information on former runup limits. However, study of the sedimentology of the palaeotsunami deposits may provide information on the characteristics of the various waves within tsunami wave trains that a!ected speci"c coastal zone. The interpretation of any tsunami deposit is made complicated by not only the occurrence of a series of long-period waves each of which may be responsible for localised littoral deposition but also by episodes of sediment erosion. Successive tsunami waves may result in the erosion of pre-existing tsunami deposits and, on occasions, their complete removal. The pattern and processes of tsunami sedimentation is additionally complicated by the occurrence of episodes of backwash #ow that follow the maximum landward inundation of individual tsunami waves. Under such circumstances strong currents may #ow from landward to seaward and may cause additional #uvial erosion and sediment re-deposition. Velocities of backwash #ow are, in turn, greatly in#uenced 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 terrestrial sediments including plant macrofossils into the sediment assemblage, thus complicating palaeoenvironmental interpretation. For example, Bondevik et al. (1997) noted within lake sediments in uplifted isolation basins in western Norway inundated by the tsunami associated with the Second Storrega Slide, large quantities of roots and twigs that were interpreted to have been deposited as a result of local tsunami backwash #ow. The rate of submarine sediment displacement is a crucial factor in determining the dimensions of the resultant tsunami. A useful illustration of this process is in respect of the palaeotsunami of ca. 7100 14C years ago generated in the Norwegian Sea as a result of the Second Storegga underwater slide (Bugge et al., 1987; Bugge, 1983; Bondevik et al., 1997). Geological investigations of tsunami deposits on the northern and eastern coastlines of Scotland as well as in uplifted lake basins along the west coast of Norway provided evidence of minimum tsunami runup (Dawson et al., 1988; Bondevik, 1996; Dawson and Smith, 1997). In eastern Scotland the estimated minimum value of runup associated with the tsunami is in the order of 4}6 m above contemporary high water mark (Dawson et al., 1988). However this
value, as stated above, should be treated with caution since tsunami #ooding to higher elevations may have taken place yet did not leave a sedimentary record. Harbitz (1991, 1992) attempted to develop a numerical model of the Second Storegga Submarine Slide. He noted that the scale of tsunami runup along the Scottish and Norwegian coastlines very much depended upon the average landslide velocity that was introduced into the model. For example he noted that an average slide velocity of 20 m/s resulted in runup values onto adjacent coastlines of between 1 and 2 m. By contrast a modelled landslide velocity of 50 m/s resulted in runup values of between 5 and 14 m, values signi"cantly in excess of the estimates for adjacent coastlines based on geological data. Harbitz (1992) concluded that a landslide velocity of 30 m/s provided the closest approximation to the estimated runup values based on geological data. However the weakness in this argument is that the geological data only provide minimum estimates of likely #ood runup and therefore the related numerical models of the same tsunami will always underestimate the likely average value of the submarine slide velocity.
6. Mechanisms of deposition Field observations of tsunami #ooding usually describe the rapid lateral translation of water across the coastal zone. Frequently, the lateral water motion associated with runup is in#uenced by local wave resonance. Thus, the tsunami waves as they strike the coast are unlike waves associated with storm surges since not only are they associated with considerably greater wavelengths and wave periods, but they are essentially constructive as they move inland across the coastal zone (Reinhardt and Bourgeois, 1989). The rapid water velocities (provided that there is an adequate supply of sediment in nearshore zone) are in most cases associated with the transport of a variety of grain size ranging from silt to boulders. Unlike storm surges individual tsunami waves reach a point of zero water velocity prior to backwash #ow. At this point large volumes of sediment may be deposited out of the water column onto the ground surface. Young and Bryant (1992) have made reference to isolated boulders in tsunami deposits in SE Australia. In this area, thicknesses of massive sands and silts include occasional isolated boulders, described by Young and Bryant (1992) as &boulder #oats'. In the absence of any other plausible mechanism of deposition, it is suggested here that the processes described above may provide the simplest explanation for their deposition. Isolated boulders contained within massive sandy deposits have been described for other palaeotsunami deposits (e.g. Shetland Islands).
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7. Boulder accumulations Empirical evidence of coastal boulder deposition during the tsunami was provided by Yeh et al. (1993) in their study of the Flores tsunami of December 1992. In this area sections of eroded coral reef were observed to have been transported by the tsunami and deposited farther inland. Bourrouilh-Le Jan and Talandier (1985) described areas of coral reef in the Tuamoto archipelago, SE Paci"c, where large numbers of giant boulders (up to 750 m3) appear to have been transported across the atoll rim and into the lagoon. However, although palaeotsunamis have been considered in this case as a possible depositional agent, similar boulders may also be deposited as a result of storm surges (Teissier, 1969; Pirazzoli et al., 1988). Boulder "elds attributable to tsunamis have been observed elsewhere in the world (e.g. Japan) (Ota et al., 1985; Kawana and Pirazzoli, 1990; Nakata and Kawana, 1993). In southern Portugal, Hindson and Andrade (pers. comm.) investigated boulders deposited by the tsunami caused by the Great Lisbon earthquake of 1755 AD. Hindson noted that at several locations on the Algarve coastline that the palaeotsunami was associated with the deposition of both continuous and discontinuous sediment sheets some of which contained boulders. The individual boulders were frequently pitted with bioerosional hollows in which were found marine mollusca. Since both the boulders and the sediment sheets were attributed to the former tsunami, it was concluded that the individual boulders were transported during the tsunami from the seabed o!shore (Fig. 2).
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world is how to be able to distinguish tsunami deposits from sediments deposited as a result of hurricane-induced storm surges. For example, Liu and Fearn (1993) have shown from coastal Alabama, USA, that a series of hurricanes during historical time have resulted in the deposition of multiple sand layers in low-lying coastal wetlands. Similarly, Davis et al. (1989) argued that hurricanes produced graded or homogenous deposits of sand, shell, gravel and mud found in the prominently clastic sediments in the coastal lagoons of Florida. While it is accepted that storm surges result in the deposition of discrete sedimentary units (Fig. 3), it is argued here that tsunamis in contrast to storm surges, generally result in deposition of continuous and discontinuous sediment sheets over relatively wide areas and considerable distances inland. For example, sediment sheets in the Algarve, Portugal associated with the Lisbon earthquake tsunami of 1755 AD occur in excess of 1 km inland (Hindson et al., 1996). Similarly, palaeotsunami deposits in Scotland associated with the Second Storegga Slide of ca. 7100 14C BP are frequently associated with extensive sediment sheets that occur many hundreds of metres inland (Smith et al., 1985; Dawson et al., 1988; Long et al., 1989).
9. Tsunami deposits and microfossil assemblages
One of the most awkward problems in reconstructing chronologies of palaeotsunamis for di!erent areas of the
Relatively few investigations have been undertaken on the microfossils associated with tsunami deposits. Hemphill-Haley (1995a, b, 1996) has described various diatom assemblages contained within tsunami deposits along the Paci"c coastline of Oregon, Washington and British Columbia. Hemphill-Haley described a variety of brackish-marine diatoms within tsunami deposits that could be used not only to identify tsunami deposits but also to estimate the greatest inland extent of tsunami
Fig. 2. Sub-rounded limestone boulder identi"ed within tsunami deposits at Boca do Rio, Algarve, Portugal, produced by the Great Lisbon tsunami of 1755 AD. Bioerosional hollows are characteristic of the boulder surface and locally contain marine mollusca (courtesy of R. Hindson).
Fig. 3. Storm surge sediments deposited by hurricane Hugo (September 1989) in western St Lucia, Windward Isles, Caribbean. Note that the deposits consist of a series of overlapping depositional fans that extend no more than a few metres inland.
8. Distinguishing storm surge and tsunami deposits
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inundation owing to the fact that certain assemblages occur beyond the landward limit of tsunami-deposited sands and silts. Furthermore, she noted that in certain coastal freshwater lakes located beyond the reach of storms, marine planktonic and neritic plankton can be used to identify former episodes of tsunami inundation. Studies of diatom assemblages contained within tsunami deposits in Scotland related to the Second Storegga Slide have shown the presence of exceptionally large numbers of the species Paralia sulcata with many individuals exhibiting evidence of breakage (Dawson et al., 1996b). Similarly, Dawson et al. (1996a) in an investigation of diatom assemblages contained within the Grand Banks tsunami deposit showed that the major diatom species present was Paralia sulcata and that the majority of individuals were broken. At "rst sight it may appear that large percentages of broken diatoms may be indicative of former tsunamis. However, this issue is made problematic since owing to the varying robustness of pennate (lenticular) and centric (circular) diatoms, some species are more able to withstand fracturing than others. Foraminifera also provide a useful means by which to identify former tsunami deposits. The most detailed investigation of foraminifera contained within tsunami deposits has been undertaken by Dominey-Howes (1996) and Dominey-Howes et al. (1999) in western Crete. Dominey-Howes (1996) showed that tsunami deposits dating to the "rst century AD contain a wide variety of species. In particular, the presence of certain species indicated transport from deep water o!shore and this observation was used to support the concept of a former tsunami having taken place. Studies of foraminifera contained within tsunami deposits are very much in their infancy and the value of this particular technique remains to be discovered.
10. Summary In recent years the recognition of distinctive palaeotsunami deposits has been used to improve our understanding of former o!shore earthquakes and giant submarine slides. In the Paci"c West Coast, palaeotsunami deposits have provided important information pointing to a relatively recent large-scale o!shore Cascadia earthquake. Owing to limited information of sea #oor geology, many former tsunamis cannot be backtraced to speci"c source mechanisms. Thus, it is almost impossible in certain cases to link individual events to speci"c generating mechanisms. This is particularly wellillustrated in the case of the trans-Paci"c Lanai tsunami in Hawaii. Despite recent advances, our understanding of tsunami deposits is relatively limited. For example, there never has been a detailed investigation undertaken of the
pattern and processes of sedimentation associated with a modern tsunami. Despite these di$culties, recent progress has been made as a result of microfossil investigations of tsunami deposits. These studies have provided relatively unambiguous information on the nature of former tsunamis. Progress has also been made in the recognition of distinctive tsunami deposits and how these can be di!erentiated from palaeostorm-surge deposits.
Acknowledgements Cartographic assistance was kindly provided by Erica Millwain and Michelle Walton and Gillian West kindly typed the manuscript. This paper is a contribution to Project GITEC-TWO &Genesis and Impact of Tsunamis on European Coasts', European Union Project ENV4CT96-0297-GITEC-TWO. It is also a contribution to IGCP Project 367 on &Rapid Coastal Changes' and was "rst presented to the International Tsunami Conference, Paris, May 1998.
References Andrade, C., 1992. Tsunami generated forms in the Algarve barrier islands. In: Dawson, A.G. (Ed.), European Geophysical Society 1992 Tsunami Meeting, Science of Tsunami Hazards, Vol. 10, pp. 21}34. Atwater, B.F., 1992. Geologic evidence for earthquakes during the past 2000 years along the Copalis River, Southern Coastal Washington. Journal of Geophysical Research 97 (B2), 1901}1919. Atwater, B.F., Moore, A.L., 1992. A tsunami about 1000 years ago in Opuget Sound, Washington. Science 258, 1614}1623. Atwater, B.F., Yamaguchi, D.K., 1991. Sudden, probably coseismic submergence of Holocene trees and grass in coastal Washington state. Geology 19, 706}709. Bondevik, S., 1996. The Storegga Tsunami deposits in Western Norway and postglacial sea-level changes on Svalbard. Thesis, University of Bergen, 107 pp. Bondevik, S., Svendsen, J.I., Johnsen, G., Mangerud, J., Kaland, P.E., 1997. The Storegga tsunami along the Norwegian coast, its age and runup. Boreas 26 (1), 29}54. Bourgeois, J., 1993. Tsunami deposits from 1992 Nicaragua event: implications for interpretation of palaeotsunami deposits * Eos Abstracts, O32C-7, 350, American Geophysical Union 1993 Fall Meeting, San Francisco. Bourgeois, J., Hansen, T.A., Wiberg, P.L., Kaufmann, E.G., 1988. A tsunami deposit at the Cretaceous-Tertiary Boundary in Texas. Science 241, 567}570. Bourgeois, J., Minoura, K., 1997. Palaeotsunami studies * contribution to mitigation and risk assessment. In: Gusiakov, V.K. (Ed.), Tsunami Mitigation and Risk Assessment, pp. 1}4, Novosibirsk. Bourrouilh-Le Jan, F.G., Talandier, J., 1985. Sedimentation et fracturation de hautes energie en milieu recifaltsunamis, ouragans et cyclones et leurs e!ects sur la sedimentologie et la geomorphologie d'un atoll: motu et hoa, a Rangiroa Tuamotu, Paci"que SE. Marine Geology 67, 263}333. Bugge, T., 1983. Submarine slides on the Norwegian continental margin, with special emphasis on the Storegga area. Continental Shelf and Petroleum Technology Research Institute, A/S publication, 110, 152 pp, Trondheim.
A.G. Dawson / Quaternary International 60 (1999) 119}126 Bugge, T., Befring, S., Belderson, R.H., Eidvin, T., Jansen, E., Kenyon, N., Holtedahl, H., Sejrup, H.-P., 1987. A giant three-stage submarine slide o! Norway. Geo-Marine Letters 7, 191}198. Cita, M.A., Maccagni, A., Pirovano, J., 1982. Tsunami as triggering mechanism of homogenites recorded in areas of the Eastern Mediterranean characterised by the cobblestone topography. In: Saxov, S.E., Niewenhuis, J.K. (Eds.), Marine Slides and other Mass Movements. Plenum Press, New York, pp. 233}260. Cita, M.A., Camerlenghi, A., Kastens, K.A., Mccoy, F.W., 1984. New "ndings of Bronze Age homogenities in the Ionian Sea: geodynamic implications for the Mediterranean. Marine Geology 55, 47}62. Clague, J.J., 1997. Evidence for large earthquakes at the Cascadia subduction zone. Reviews of Geophysics 35 (4), 439}460. Clague, J.J., Bobrowsky, P.T., 1994. Evidence for a Large Earthquake and Tsunami 100}400 Years Ago on Western Vancouver Island British Columbia. Quaternary Research 41, 176}184. Clague, J.J., Bobrowsky, P.T., Hamilton, T.S., 1994. A sand sheet deposited by the 1964 Alaska Tsunami at Port Alberni British Columbia. Estuarine, Coastal and Shelf Science 38, 413}421. Darienzo, M.E., Peterson, C.D., 1990. Episode tectonic subsidence of late Holocene salt marshes, northern Oregon, central Cascadia margin. Tectonics 9 (1), 1}22. Davis Jr., R.A., Knowles, S.C., Bland, M.J., 1989. Role of hurricanes in the Holocene stratigraphy of estuaries: examples from the Gulf Coast of Florida. Journal of Sedimentary Petrology 59, 1052}1061. Dawson, A.G., 1994. Geomorphological e!ects of tsunami run-up and backwash. Geomorphology 10, 83}94. Dawson, A.G., 1996. The Geological Signi"cance of Tsunamis. Zeitschrift Geomorphologie N.F., Suppl.-Bd. 102, 199}210. Dawson, A.G., Long, D., Smith, D.E., 1988. The Storegga Slides: evidence from eastern Scotland for a possible tsunami. Marine Geology 82, 271}276. Dawson, A.G., Hindson, R., Andrade, C., Freitas, C., Parish, R., Bateman, M., 1995. Tsunami sedimentation associated with the Lisbon earthquake of 1 November AD 1755: Boco do Rio, Algarve, Portugal. The Holocene 5 (2), 209}215. Dawson, A.G., Shi, S., Dawson, S., Takahashi, T., Shuto, N., 1996a. Coastal Sedimentation Associated with the June 2nd and 3rd, 1994 Tsunami in Rajegwesi Java. Quaternary Science Reviews 15, 901}912. Dawson, S., Smith, D.E., Ru!man, A., Shi, S., 1996b. The diatom biostratigraphy of tsunami sediments: examples from recent and middle Holocene events. Physical and Chemistry of the Earth 21 (12), 87}92. Dawson, S., Smith, D.E., 1997. Holocene relative sea-level changes on the margin of a glacio-isostatically uplifted area: an example from northern Caithness, Scotland. The Holocene 7 (1), 59}77. Dominey-Howes, D.T.M., 1996. The geomorphology and sedimentology of "ve tsunamis in the Aegean Sea Region, Greece. Unpublished Ph.D. thesis, Coventry University, September 1996. Dominey-Howes, D., Dawson, A.G., Smith, D.E., 1999. Late Holocene coastal tectonics at Falasarna, western Crete (Greece): a sedimentary contribution. Geological Society of London Special Publication, 146, 343}352. Harbitz, C.B., 1991. Model simulations of tsunami generated by the Storegga Slide. Institute of Mathematics, University of Oslo. Series No 5, 30 pp. Harbitz, C.B., 1992. Model simulations of tsunami generated by the Storegga Slides. Marine Geology 105, 1}21. Heike, W., 1984. A thick Holocene homogenite from the Ionian Abyssal Plain (eastern Mediterranean). Marine Geology 55, 63}78. Hemphill-Haley, E., 1995a. Diatom evidence for earthquakeinduced subsidence and tsunami 300 years ago in southern coastal Washington. Geological Society of America Bulletin 107, 367}378. Hemphill-Haley, E., 1995b. Intertidal diatoms from Willapa Bay, Washington: application to studies of small-scale sea-level changes. Northwest Science 69, 29}45.
125
Hemphill-Haley, E., 1996. Diatoms as an aid in identifying late-Holocene tsunami deposits. The Holocene 6 (4), 439}448. Hindson, R.A., Andrade, C., Dawson, A.G., 1996. Sedimentary processes associated with the tsunami generated by the 1755 Lisbon earthquake on the Algarve Coast, Portugal. Physics and Chemistry of the Earth 21 (12), 57}63. Kastens, K.A., Cita, M.B., 1981. Tsunami-induced transport in the abyssal Mediterranean Sea. Geological Society of America Bulletin 92, 845}857. Kawana, T., Pirazzoli, P., 1990. Re-examination of the Holocene emerged shorelines in Orabu and Shimoji Islands, the South Ryukyus, Japan. Quaternary Research 28, 419}426. Keller, G., Stinnesbeck, W., Adatte, T., Macleod, N., Lowe, D.R., 1994. Field Guide to Cretaceous-Tertiary Boundary Sections in Northeastern Mexico * Lunar and Planetary Institute. 3600 Bay Area Boulevard, Houston TX 77058-1113, Contribution 827, 110 pp. Liu, K.-B, Fearn, M.L., 1993. Lake-sediment record of late Holocene hurricane activities from coastal Alabama. Geology 21, 793}796. Long, D., Smith, D.E., Dawson, A.G., 1989. A Holocene tsunami deposit in eastern Scotland. Journal Quaternary Science 4, 61}66. Minoura, K., Nakaya, S., 1991. Traces of tsunamis preserved in intertidal lacustrine and marsh deposits: some examples from Northeast Japan. Journal of Geology 99, 265}287. Minoura, K., Nakaya, S., Uchida, M., 1994. Tsunami deposits in a lacustrine sequence of the Sanriku coast, northern Japan. Sedimentary Geology 89, 25}31. Moore, G.W., Moore, J.G., 1988. Large-scale bedforms in boulder gravel produced by giant waves in Hawaii. In: Clifton, H.E. (Ed.), Sedimentologic Consequences of Convulsive Geologic Events. Geological Society of American Special Paper, Vol. 229, pp. 101}110. Moore, J.G., Moore, G.W., 1984. Deposit from a giant wave on the island of Lanai Hawaii. Science 226, 1312}1315. Moore, J.G., Clague, D.A., Holcom, R.T., Lipman, P.W., Normark, W.R., Torresan, M.E., 1989. Prodigious submarine landslides on the Hawaiian Ridge. Journal of Geophysical Research 94, 17465}17484. Moore, J.G., Normark, W.R., Gutmacher, C.E., 1992. Major landslides on the submarine #anks of the Manua Lao Volcano, Hawaii. Landslide News 13}16. Nakata, T., Kawana, T., 1993. Historical and prehistorical large tsunamis in the southern Ryukus, Japan. In: Tsunami '93, Proceedings of IUGG/IOC Uinst, Tsunami Symposium. Wakayama, Japan, August 23}27, pp. 297}307. Nishimura, Y., 1994. The 1640 Komagtake Tsunami runups as revealed by the tsunami deposits * Amer. Geophys. Union Western Paci"c Geophysics Meeting, Hong Kong. July, 25}29, Eos, Abstract, 63. Ota, Y., Pirazzoli, P.A., Kawana, T., Moriwaki, H., 1985. Late Holocene coastal morphology and sea-level records on three small islands, the South Ryukyus, Japan. Geographical Reviews of Japan 58B, 185}194. Pasko!, R., 1991. Likely occurrence of a mega-tsunami near Coqimbo, Chile. Reviews of Geology Chile 18, 87}91. Perissoratis, C., Papadopoulos, G., 1998. Sediment slumping in the South Aegean Sea and the case history of the 1956 Tsunami. In: Seventh International Conference on Natural and Man-Made Hazards. Chania, Crete, Greece, May 1998. Abstract, p. 120. Pinegina, T.K., Bazanova, L.I., Braitseva, O.A., Gusiakov, V.K., Melekestsev, I.V., Storcheus, A.V., 1997. East Kamchatka palaeotsunami traces. In: Gusiakov, V.K. (Ed.), Tsunami Mitigation and Risk Assessment, pp. 5}12. Pirazzoli, P.A., Montaggioni, L.F., Salvant, B., Faure, G., 1988. Late Holocene sea level indicators from twelve atolls in the central and eastern Tuamotos (Paci"c Ocean). Coral Reefs 7, 57}68. Ranguelov, B., 1998. Risk management of natural disasters and ecological catastrophes: a book for disaster education. In: Seventh International Conference on Natural and Man-Made Hazards, Chania, Crete, Greece, May 1998. Abstract, pp. 124}125.
126
A.G. Dawson / Quaternary International 60 (1999) 119}126
Reinhardt, M.A., Bourgeois, J., 1989. Tsunami favoured over storm or seiche for sand deposits overlying buried Holocene peat. Wilapa Bay, Washington. Eos, American Geophysical Union, October 24, p. 1331. Rothwell, R.G., Thomson, J., KaK hler, G., 1998. Low-sea-level emplacement of a very large Late Pleistocene &megaturbidite' in the western Mediterranean Sea. Nature 392, 377}380. Satake, K., Bourgeois, J., Abe, K., Abe, K., Tsuji, Y., Imamura, F., Iio, Y., Katao, H., Noguera, E., Estrade, F., 1993. Tsunami "eld survey of the 1992 Nicaragua earthquake. Eos, Transactions, American Geophysical Union 74(13), 145, 156}157. Satake, K., Shimazaki, K., Tsuji, Y., Udea, K., 1996. Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700. Nature 379 (6562), 246}249. Shi, S., Dawson, A.G., Smith, D.E., 1993. Geomorphological impact of the Flores tsunami of 12th December, 1992. In: `Tsunami '93a, Proceedings of the IUGG/IOC International Tsunami Symposium. Wakayama, Japan, August 23}27, pp. 689}696. Shi, S., Dawson, A.G., Smith, D.E., 1995. Coastal sedimentation associated with the December 12th 1992 Tsunami in Flores, Indonesia. In: Satake, K., Imamura, K. (Eds.), Recent Tsunamis, Pure and Applied Geophysics, Vol. 144, pp. 525}536. Smit, J., Montanari, A., Swinburne, N.H.M., Alavarez, W., Hilderbrand, A.R., Margolis, S.W., Claeys, P., Lowrie, W., Asaro, F., 1992.
Tektite-bearing deep-water clastic unit at the Cretaceous-Tertiary boundary in Northeastern Mexico. Geology 20, 99}103. Smit, S., Montanari, A., Alvarez, W., 1994. Tsunami-generated beds at the KT boundary in northeastern Mexico. In: Keller, G., Stinnesbeck, W., Adatte, T., MacLeod, N., Lowe, D.R. (Eds.), Field Guide to Cretaceous-Tertiary Boundary Sections in Northeastern Mexico. Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston TX 77058-1113, Contribution 827, pp. 95}110. Smith, D.E., Cullingford, R.A., Haggart, B.A., 1985. A major coastal #ood during the Holocene in eastern Scotland. Eiszeitalter und Gegenwart 35, 109}118. Stearns, H.T., 1940. Geology and ground-water resources of the islands of Lanai and Kahoolawe, Hawaii. Hawaii Division of Hydrography Bulletin 7, 177 pp. Teissier, R., 1969. Les Cyclones en Polynesie Francaise. Bulletin SocieteH Etudes Oceaniennes 14, 20}21. Yeh, H., Imamura, F., Synolakis, C., Ysuji, Y., Liu, P., Shi, S., 1993. The Flores Island Tsunamis. Eos Transactions, American Geophysical Union 74 (33), 369}373. Young, R.W., Bryant, T., 1992. Catastrophic wave erosion on the southeastern coast of Australia: impact of the Lanai Tsunami ca. 105ka? Geology 20, 199}202.