Horizontal and vertical variation of 2004 Indian tsunami deposits: An example of two transects along the western coast of Thailand

We investigated horizontal and vertical variations in modern tsunami deposits along two transects at Nam Khem and Khao Lak, western coast of Thailand, deposited by the large tsunami associated with the earthquake (magnitude 9.0) of 26 December 2004 off Sumatra, Indonesia. Tsunami waves 6–10 m high struck the area approximately 2 h after the earthquake. Tsunami deposits cover the low-lying coastal plains and extend more than 1 km inland from the shoreline. No landward decrease in sediment thicknesses was found clearly at either transect. Terrace scarps and a steep slope behind the coastal plain probably stopped tsunami deposition further inland, causing substantial sediments to be deposited in front of these features. Very clear vertical variations in grain size and multiple layers are found in the deposits to about 600 m inland at Nam Khem. Fine-grained sediments overlie the coarse-grained sediments of the basal layer of the tsunami deposits. At some sites, the fine-grained sediments are overlain by another layer of coarse-grained sediments, suggesting deposits laid down in succession by multiple waves. The basal coarse-grained sediments at Nam Khem fine landward. The up-flow waned inland based on the assumption that the grain size of the basal deposits relates to the strength of the up-flow. These results are potentially useful in disaster prevention and coastal environmental change management as well as for interpreting paleotsunami deposits in geological records.