PRELIMINARY RESULTS OF FIELD SURVEY ON THE INDIAN OCEAN TSUNAMI DEPOSITS IN THAILAND AND SRI LANKA

The Sumatra-Andaman islands earthquake on 26th December 2004 generated one of the largest tsunamis (the Indian Ocean tsunamis) in the human history, causing the worst casualties. Such large tsunamis could have transported a great amount of sediments toward the land and/or sea and formed tsunami deposits. The magnitude and mode of the 2004 Indian Ocean tsunamis have been well understood by the effort of international tsunami survey groups. Thus, this tsunami event gives us a very rare opportunity to clarify the formation process of tsunami deposits and its relation to the magnitude and inundation process of tsunamis. Therefore, we investigated the distribution and significances of the Indian Ocean tsunami deposits. In this paper, we report preliminary results of our field survey. Geological survey was conducted mainly in the Pakarang Cape and the Bang Sak beach, Thailand and Garanduwa, Sri Lanka 3 months after the 2004 Indian Ocean tsunami event. Abundant tsunami stones, sand dunes, and selective erosion behind the landward side of the trees are observed along the eastern shoreline in the Pakarang cape, Thailand. These evidences would imply the occurrence of eastward run up wave in western coast of the Pakarang cape. The thickness and grain size of the tsunami deposits in Bang Sak beach and Garanduwa are variable depending on the local topographic change, but generally show landward thinning and fining characters. Current direction is recorded as bent grasses and trees, cross lamination, and wave ripples. These information will also be useful to estimate the magnitude of tsunamis and to clarify the process of the tsunami sedimentation.