Distribution and significance of the 2004 Indian Ocean tsunami deposits: Initial results from Thailand and Sri Lanka

The Indian Ocean tsunami of December 26, 2004 left tsunami deposits in the coastal areas of the surrounding countries. We conducted a preliminary investigation of the tsunami deposits on the coasts of Thailand and Sri Lanka 3 months after the event to clarify their distribution and significance. We observed abundant tsunami-transported reef blocks, megaripples, and selective erosion and sedimentation in the backshore of Pakarang Cape in Thailand. Our data indicate eastward inundation of the west side of the cape by the tsunami. We also investigated sheet-like tsunami deposits at Bang Sak beach in Thailand and at Garanduwa in Sri Lanka, where the tsunami direction was recorded by bent grasses and trees. The thickness and grain size of the tsunami deposits are dependent on the local topography, but generally the deposits thin and fine landward. The deposits consist of silicate minerals, shell fragments, and coral skeletal fragments transported from the beach or the offshore sea bottom by the tsunami waves. In the studied sites, the tsunami deposits are restricted to the lower half of the inundated zone, reflecting the low sediment transport energy of the waves.