Medieval forewarning of the 2004 Indian Ocean tsunami in Thailand

Abstract

Nothing known from written history gave reason to expect the Indian Ocean tsunami that took nearly a quarter million lives on 26 December 2004. That tsunami entered geological history by laying down centimetres of sand on the coastal plains that it overran. Jankaew et al. have now found such sedimentary records of earlier tsunamis preserved in the dark soils of marshy swales at Phra Thong, a barrier island in western Thailand. The cover shows an example from a pit dug there in 2007: the topmost light-coloured layer represents the 2004 tsunami, while a similar layer below records a tsunami in the fourteenth or fifteenth century AD. The ruler divisions are 10 cm long. In a separate study in Aceh, Indonesia, Monecke et al. found the 2004 sand sheet preceded by the deposits of three tsunamis from the past 1,200 years. One of these earlier deposits may match the medieval one found in Thailand. The combined findings suggest that the 2004 tsunami is neither the first nor the last of its kind. 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 disaster1. 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 convergence2,3,4,5. Here, using sedimentary evidence for tsunamis6, 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.