MULTIPLE LAYER IDENTIFICATION AND TRANSPORTATION PATTERN ANALYSIS FOR ONSHORE TSUNAMI DEPOSIT AS THE EXTENDING TSUNAMI DATA – A CASE STUDY FROM THE THAI ANDAMAN COAST

On 26thDecember 2004, a strong Indian Ocean earthquake of moment magnitude 9 generated a deadly tsunami that hit the west coast of southern Thailand and many coastal nations of the Indian Ocean. Two tsunami-affected areas on the Thai Andaman coast (Ao Kheuy beach and Khuk Khak beach) were investigated. Multiple sediment layers in the tsunami deposits are identified and are analyzed. The sediment transportation patterns are also determined. Tsunami deposits consist of graded sand layers overlying the pre-existing soil. The particle size profile of the tsunami sediment and the plot of grain-size standard deviation with depth are used to identify major layers in tsunami deposit. There are three major sediment layers in the tsunami deposit in the study areas. They reflect three depositional sequences created by three tsunami run-ups. The mean grain-size of tsunami deposit and the results of sediment trend analysis show that the tsunami deposit is generally fining upwards and landwards. Each major sediment layer is created by sediments settled from suspension in a set of run-up and backwash. The percentage by weight of sediment settled from suspension during the backwash is small when it is compared to the percentage by weight of sediment settled from suspension during the run-up. The 1stdepositional sequence has higher quantity of coarse grain particles than the following depositional sequences. At a mild slope shore face, sediments are transported and deposited on land far from their origins. The number of major sediment layers in tsunami deposit can be used as the extending data for reconstructing individual tsunami run-up by using numerical and/or simple models.