Yuki Sawai

Medieval forewarning of the 2004 Indian Ocean tsunami in Thailand

Recent centuries provide no precedent for the 2004 Indian Ocean tsunami, either on the coasts it devastated or within its source area. The tsunami claimed nearly all of its victims on shores that had gone 200 years or more without a tsunami disaster. The associated earthquake of magnitude 9.2 defied a Sumatra–Andaman catalogue that contains no nineteenth-century or twentieth-century earthquake larger than magnitude 7.9 (ref. 2). The tsunami and the earthquake together resulted from a fault rupture 1,500 km long that expended centuries’ worth of plate convergence. Here, using sedimentary evidence for tsunamis, we identify probable precedents for the 2004 tsunami at a grassy beach-ridge plain 125 km north of Phuket. The 2004 tsunami, running 2 km across this plain, coated the ridges and intervening swales with a sheet of sand commonly 5–20 cm thick. The peaty soils of two marshy swales preserve the remains of several earlier sand sheets less than 2,800 years old. If responsible for the youngest of these pre-2004 sand sheets, the most recent full-size predecessor to the 2004 tsunami occurred about 550–700 years ago.

Marine incursions of the past 1500 years and evidence of tsunamis at Suijin-numa, a coastal lake facing the Japan Trench

Sandy deposits of marine origin underlie the floor of Suijin-numa, a coastal lake midway along the subduction zone marked by the Japan Trench. The deposits form three units that are interbedded with lacustrine peat and mud above a foundation of marine, probably littoral sand. Unlike the lacustrine deposits, all three sandy units contain marine and brackish diatoms. The middle unit (B) contains, in addition, graded beds suggestive of multiple waves of long wavelength and period. The uppermost unit (C) probably dates to a time in the areas written history when the lake was separated from the sea by a beach-ridge plain at least 0.5 km wide and several metres high. Units A and B postdate AD 540—870, and unit C postdates AD 1030—1640 as judged from radiocarbon dating of leaves and seeds. Unit B pre-dates AD 915 and unit C postdates that year as judged from a tephra within the peat that separates units B and C. The age constraints permit correlation of unit B with a tsunami in AD 869 that reportedly devastated at least 100 km of coast approximately centred on Sendai. Unit C may represent a later catastrophic tsunami in 1611, or perhaps a storm surge that inundated much of Sendai. The lake lacks obvious signs of tsunamis from the regions largest twentieth-century earthquakes, which were centred to the north in 1933 (M 8.1) and directly offshore in 1936 (M 7.5), and 1978 (Mw 7.6).

Evidence for 17th-century tsunamis generated on the Kuril–Kamchatka subduction zone, Lake Tokotan, Hokkaido, Japan

Abstract In the seventeenth century, two tsunamis that were generated by earthquakes on the Kuril–Kamchatka subduction zone inundated the eastern coast of Hokkaido, northern Japan. Stratigraphic evidence for these two tsunamis and related land-level change in coastal Hokkaido consists of two landward-thinning sand layers in the sediments of Lake Tokotan, a coastal lagoon on the Hokkaido coast. The marine origin of these sand layers is indicated by the presence of brackish–marine diatoms. The rarity and high degree of fragmentation of diatom valves suggests that the sands were transported in a short time over a considerable distance. Tsunamis at this site were probably generated by great earthquakes along the Kuril–Kamchatka Trench. Volcanic ash deposits lying just above the sands suggest that tsunamis occurred in the late 17th century. Tsunamis during the historic period are not recorded in Lake Tokotan, which suggests that the sand layers were deposited by tsunamis substantially larger than historic tsunamis.

Tsunami heights and damage along the Myanmar coast from the December 2004 Sumatra-Andaman earthquake

The tsunami heights from the 2004 Sumatra-Andaman earthquake were between 0.4 and 2.9 m along the Myanmar coast, according to our post tsunami survey at 22 sites in Ayeyarwaddy Delta and the Taninthayi coast. Interviews to coastal residents indicate that the tsunami heights were lower than high tide level in rainy season, probably by storm surge. They also testified that the arrival times were between 2 and 5.5 hours after the earthquake but the reliability may be low because nobody felt ground shaking. Much smaller tsunami than the neighboring Thai coast, where the tsunami heights were 5 to 20 m, explains relatively slighter tsunami damage in Myanmar; the casualties were reported as 71, compared to about 8300 in Thailand. The smaller tsunami was probably due to the fact that the main tsunami source did not extend to Andaman Islands. The tsunami travel times and maximum heights computed from a 700 km long source are basically consistent with the observations. For a nearby tsunami source, the tsunami hazard would be more significant in Myanmar, because coastal houses are unprotected for tsunamis and no infrastructure exists to disseminate tsunami warning information.

Distribution of living and dead diatoms in tidal wetlands of northern Japan: relations to taphonomy

Abstract This paper presents the results of a survey of the contemporary diatom distributions in four tidal marshes of eastern Hokkaido, northern Japan. Distribution of living diatom cells, as defined by cluster analysis, have relations to vegetation and substratum, and clearly to tide level. Comparison between living and dead diatoms in each cluster provides synecological information relating to diatom taphonomy in the tidal wetland environments. Four assemblages are recognized and they are summarized as follows. (1) An upland assemblage consisting of autochthonous freshwater diatoms adapted to relatively dry conditions. (2) A high marsh assemblage formed at or slightly above mean high tide level (MHTL) with only few allochthonous components. (3) A low marsh assemblage formed below MHTL. Dead valves within this assemblage are repeated widely redistributed by tidal processes. This assemblage is therefore a ‘mixed assemblage’ comprising a combination of allochthonous and autochthonous diatoms. (4) A tidal flat assemblage. On death diatom valves in tidal flat are immediately redistributed by tidal currents, and the dead diatom assemblage here has the characteristics of an ‘autochthonous’ or ‘residual’ assemblage. Autecological perspectives based on the distribution patterns can advance these interpretations. Dead valves of the epiphytic diatom Cocconeis scutellum, which possesses a raphid valve (R-valve) and a rapheless valve (P-valve), are common on two transects but only P-valves are widely distributed beyond the seaweed zone which represents the actual habitat of this species. Such selective processes are related mainly to the life form characteristics of monoraphid species: only R-valves remain attached to their substratum on death. The tychoplankton Paralia sp. is also a widely distributed species. In the case of Paralia, their long chains float more readily than other specimens are transported easily by tidal currents, and then trapped in entire tidal environments. These observations suggest it is likely that concentrations of only P-valves of C. scutellum or Paralia in the tidal marsh deposits should be treated as allochthonous components in the diatom assemblage.

Shorter intervals between great earthquakes near Sendai: Scour ponds and a sand layer attributable to A.D. 1454 overwash

A sparsely documented tsunami in 1454 may subdivide the recurrence interval between the 869 and 2011 tsunamis near Sendai, as judged from geomorphic, stratigraphic, and archival evidence. Pond-filled breaches cut across beach ridges on century-old topographic maps. The basal pond deposit in one of these breaches postdates 1454. Stratigraphy on Sendai Plain includes a sand sheet that contains marine and brackish diatoms. Radiocarbon ages suggest that the sheet dates to 1406–1615 (2σ), and written records for this interval in Tohoku mention a tsunami in 1454. The inferred inundation extended 1.0–2.5 km inland from an approximate medieval shoreline. Simulated tsunamis that best account for the sand sheet require a thrust earthquake of moment magnitude 8.4 or larger. If the sand sheet represents the 1454 tsunami, the two most recent intervals between great thrust earthquakes in Sendai region spanned 585 and 557 years.

Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India

Coastal stratigraphy near Port Blair, Andaman Islands, where the A.D. 2004 Sumatra-Andaman earthquake was accompanied by ∼1 m of subsidence, provides evidence for two prior earthquakes, perhaps both from the past 400 yr. The first of these (event I) is marked by an abrupt mud-over-peat contact best explained by subsidence similar to that in 2004. Event II is evidenced by an overlying chaotic layer composed of mud clasts in a sandy matrix that is connected with feeder dikes. These mud clasts, probably produced by liquefaction, are capped by laminated sand and mud that we ascribe to an event II tsunami. Radiocarbon ages of plant remains in the peat give discordant ages in the range 100 B.C. to A.D. 1950. Event I probably resembled the 2004 Sumatra-Andaman earthquake in that it was accompanied by subsidence (as much as 1 m) but not by strong shaking near Port Blair. If event II was the A.D. 1762 Arakan earthquake, the laminated sand and mud provide the first evidence that this earthquake was associated with a tsunami.

Three extant species of Paralia (Bacillariophyceae) along the coast of Japan

Y. Sawai, T. Nagumo and K. Toyoda. 2005. Three extant species of Paralia (Bacillariophyceae) along the coast of Japan. Phycologia 44: 517–529. Three extant species of the genus Paralia (Bacillariophyceae) occur along the coast of Japan, two of them are new species. Observations of living populations suggested that significant characters for separating Paralia species are (1) structure of the mantle of the separation valve; (2) shape of the linking spines; (3) topography and morphology of valve face on the separation valve, and possibly (4) cingulum structures; and (5) rimoportula morphology. Some features vary but the structure of the separation valves in girdle view was stable in all specimens. Based on these criteria, we identified Paralia sulcata (Ehrenberg) Cleve and two new species, P. fenestrata Sawai & Nagumo sp. nov., and P. capitata Sawai & Nagumo sp. nov. from coast of Japan.

Sand Sheets on a Beach-Ridge Plain in Thailand: Identification and Dating of Tsunami Deposits in a Far-Field Tropical Setting

Kruawun Jankaew1, Maria E. Martin2, Yuki Sawai3 and Amy L. Prendergast4 1Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330 2Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310 3Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8567 4Department of Archeology, University of Cambridge, Downing Street, Cambridge CB2 3DZ 1Thailand 2USA 3Japan 4UK

14.25 Microfossils in Tidal Settings as Indicators of Sea-Level Change, Paleoearthquakes, Tsunamis, and Tropical Cyclones

Fine-grained sediments deposited in low-energy, inter-tidal settings are an archive of sea-level change and the occurrence of paleoearthquakes, tsunamis, and tropical cyclones. Some of the best reconstructions of these coastal processes have been derived from microfossils, such as pollen, diatoms and foraminifera that accumulate in salt-marsh and estuarine environments. Early microfossil work in the coastal zone employed pollen as an indicator of vegetation and as a chronostratigraphic marker. Use of diatoms and foraminifera has become increasingly widespread because their distribution is closely linked to tidal elevation. In this chapter, we discuss the use of microfossils in estuarine and salt-marsh sediments to reconstruct sea level along subsiding coastlines in temperate regions. We also describe how microfossils from isolation basins are used to reconstruct sea level along coastlines undergoing uplift. Microfossils can also estimate land-level changes along tectonically active coasts associated with paleoearthquakes. We explain the use of transfer functions for calculating quantitative estimates of past environmental conditions from microfossil data. Finally, we reveal how microfossils are used to reconstruct the recurrence of tsunamis and tropical cyclones from the sedimentary deposits these high-energy events leave behind.