Alastair G. Dawson

Coastal Sedimentation Associated with the December 12th, 1992 Tsunami in Flores, Indonesia

This paper presents the result of a detailed granulometric investigation of sediments deposited by a modern tsunami, the 1992 tsunami in Flores, Indonesia. Eyewitness accounts indicate that sediments were deposited upon coastal lowlands over wide areas as a result of the tsunami inundation. Distinctive vertical and lateral variations in particle size composition are characteristic features of the tsunami deposits and these are intimately related to sedimentary processes associated with flood inundation. The geomorphological and sedimentary evidence is used here to establish a preliminary model of tsunami sedimentation. This information is believed to be of great value in understanding sedimentary processes associated with tsunami flooding and in the interpretation of palaeo-tsunami deposits.

Coastal sedimentation associated with the June 2nd and 3rd, 1994 tsunami in Rajegwesi, Java

Abstract This paper presents the second detailed study of sediments deposited by modern tusnamis, the first being that of the Flores (Indonesia) tsunami of December 1992 (Shi et al., 1995). Sediment cores were collected from areas in which eyewitnesses reported sediment deposition. Grain size analysis shows pronounced vertical variations in grain size as well as changes in standard deviation, skewness and kurtosis that appear to be indicative of complex tsunami flooding. Vertical variations in grain size in individual cores are greater than spatial variations between cores taken along a transect completed perpendicular to the coastline. The Java tsunami-deposited sediments do not show unequivocal evidence of local erosion but instead evidence for sediment transport and deposition is clear and is characterised by dominantly unimodal sediments with fine-tailed distributions.

Tsunami sedimentation associated with the Lisbon earthquake of 1 November AD 1755: Boca do Rio, Algarve, Portugal

A description is given of a distinctive marine deposit at Boca do Rio on the Algarve coast of Portugal. It is proposed here that the sediment accumulation was deposited by the tsunami generated by the Lisbon earthquake of 1 November, AD 1755. The deposit exhibits sedimentary characteristics quite unlike other coastal sediment accumulations that are deposited by more moderate wave régimes. These include laterally continuous sand layers, chaotic pebble horizons, large amounts of gravel-sized shell debris and distinctive assemblages of marine microfossils. A preliminary attempt is made here to use the empirical field and laboratory data to define more clearly the processes of coastal sedimentation that characterize tsunami runup. The use of tsunami sediments as time synchronous marker horizons is also discussed.

Record‐breaking height for 8000‐year‐old tsunami in the North Atlantic

One of the largest Holocene sub-marine slides mapped on Earth is the Storegga slide offshore Norway [Bugge, 1987] (Figure 1). Approximately 3500 km3 material slid out and generated a huge tsunami dated to about 7300 14C yr BP [Bondevik et al., 1997a], or ca 8150 calendar years BP. The tsunami is known from onshore deposits in Norway [Bondevik et al., 1997a], on the Faroe Islands [Grauert et al., 2001], and in Scotland [Dawson et al., 1993]. Of these, the tsunami deposits in western Norway reaches the highest elevation, indicating a runup of 10–12 m. In this article, we demonstrate that at the Shetland Islands between Norway and Scotland (Figure 1), this tsunami reached onshore heights at least 20 m above the sea level of that time.

Sedimentary processes associated with the tsunami generated by the 1755 Lisbon earthquake on the Algarve coast, Portugal

Abstract A distinctive marine deposit at Boca do Rio on the Algarve coast of Portugal occurs up to 1km inland, and exhibits sedimentological and palaeontological characteristics which suggest deposition under high energy conditions. The inland extent of the deposit precludes deposition by even the most extreme storms. The characteristics of the boulders and cobbles in the deposit suggest transport onshore from the zone of wave shoaling into an estuarine environment by long period waves. It is proposed here that the sediment accumulation was deposited by the tsunami generated by the Lisbon earthquake of November 1st, 1755AD.

Linking tsunami deposits, submarine slides and offshore earthquakes

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 identification of a series of palaeotsunamis that appear to have taken place in different areas of the world. In most cases, however, it has proved difficult to link former tsunamis to specific source mechanisms. Studies of modern tsunamis have also faced difficulties in the recognition of the specific source mechanisms. For example, offshore earthquakes may trigger submarine slides that combine to produce complex patterns of tsunami flooding at the coast.

Coastal wetlands as recorders of earthquake subsidence in the Aegean: a case study of the 1894 Gulf of Atalantiearthquakes, central Greece

Earthquakes may have a major impact on the morphology and evolution of coasts in tectonically active areas. In the area of the Gulf of Atalanti, central Greece, a damaging earthquake sequence in 1894 is reported to have caused nearshore slumping, tsunami-induced flooding, and decimetre-to metre-scale coastal subsidence. The earthquakes caused major changes to coastal configuration, in particular the separation of the Gaiduronisi peninsula from the mainland. Detailed stratigraphical, microfossil and radiometric (210Pb and 137Cs) analyses of coastal wetlands around the Gulf of Atalanti show that these environments preserve evidence of sudden, lasting subsidence contemporaneous with the 1894 events, with foraminiferal assemblages at a site on the mainland opposite the island indicating an abrupt change from supratidal soil to intertidal marine conditions. The elevation change indicated by this terrestrial to marine transition is 30–80 cm. The coastal stratigraphy in the Atalanti area however lacks laterally extensive sharp soil–mud contacts or tsunami deposits of the type used in identifying past earthquake subsidence events at plate–boundary settings. Despite the major earthquake-induced changes in coastal configuration, the stratigraphical signature of the 1894 earthquakes is relatively subtle, and is consequently difficult to distinguish from other rapid coastal changes (e.g. storm events or barrier breaching) without high-resolution dating control and reference to documentary records. For this reason, coastal wetland stratigraphies are only likely to be useful in identifying pre-historical or poorly documented earthquakes in the Aegean at sites where metre-scale coseismic elevation changes and/or major tsunami inundation have occurred. In addition, the resolution of the biostratigraphic data reported here is insufficient to distinguish between the contrasting fault rupture models for the 1894 events. The coastal elevation changes recorded in these wetlands, however, are best accounted for by a slip of 1 m on a multi-segment fault.

A 200-year record of gale frequency, Edinburgh, Scotland: possible link with high-magnitude volcanic eruptions

Most research concerned with the relationship between volcanic activity and global climate change has focused on the impacts that volcanoes have on atmospheric temperature. Very little attention, however, has been given to the effect of volcanic eruptions on patterns of storminess. Here we present a historical record of gale-day frequency for Edinburgh, Scotland, extending from AD 1780 to 1988 (the Mossman-Hickey chronology), which we believe represents the longest historical record of gales in Europe. Calculation of gale- day frequency for this time interval shows three clear peaks in storminess that follow the volcanic eruptions of Tambora (1815), Krakatoa (1883) and El Chichon (1982). It appears that the greatest periods of storminess evident in the Edinburgh record during the last 200 years have taken place during relatively short intervals following major episodes of volcanism. If correct, the processes linking high-magnitude volcanic eruptions to storminess in the North Atlantic need to be included in IPCC research that presently focuses most attention on linkages between increased storminess and the effects of global warming.

Lateglacial sea-level changes and ice-limits in Islay, Jura and Scarba, Scottish Inner Hebrides

Synopsis Ice-marginal landforms produced at the edge of the last (Late-Devensian) ice-sheet in Islay, Scottish Inner Hebrides are described. A glacial stillstand is indicated by the central Islay moraine and is associated with a fall in the marine limit of c. 12 m. Deglaciation occurred initially in western and central Islay and was associated with the formation of a series of raised coastal terrace fragments. Slightly later, western Jura was deglaciated. This was followed by the development of two prominent Lateglacial shorelines (LI and L2). The latter features decline in altitude SW from c. 40 m in NW Jura to c. 15 m in central Islay and possess regional gradients of 0.59 m/km and 0.56 m/km. The relative regression of the Lateglacial sea was accompanied in western Jura by the widespread development of beach ridges. Measurement of the ridge altitudes shows that in this area the relative retreat of the Lateglacial sea was uninterrupted by any major transgressions, relative stillstands or renewed periods of glacio-isostatic downwarping.

Geomorphological Impact of the Flores Tsunami of 12th December, 1992

This paper presents a description of geomorphological change associated with the Flores tsunami of 12th December 1992. Evidence for severe coastal erosion during the tsunami is indicated by extensive eroded coral boulder complexes, considerable lowering of coastal’ land surfaces and the formation of ephemeral cliff lines.