A 1,000-year sediment record of tsunami recurrence in northern Sumatra

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. The Indian Ocean tsunami of 26 December 2004 reached maximum wave heights of 35 m in Aceh, the northernmost province of Sumatra1,2. Both the tsunami and the associated Sumatra–Andaman earthquake were unprecedented in Acehnese history3,4. Here we use sand sheets to extend tsunami history 1,000 years into Aceh’s past. The 2004 tsunami deposited a sand sheet up to 1.8 km inland on a marshy beach ridge plain. Sediment cores from these coastal marshes revealed two older extensive sand sheets with similar sediment characteristics. These sheets, deposited soon after ad 1290–1400 and ad 780–990, probably resulted from earlier tsunamis. An additional sand sheet of limited extent might correlate with a documented smaller tsunami of ad 1907. These findings, a first step towards a palaeotsunami record for northern Sumatra, suggest that damage-causing tsunamis in Aceh recur infrequently enough for entire human lifetimes to typically elapse between them. Such recurrence adds to the challenge of preparing communities along the northern Indian Ocean shorelines for future tsunamis.