Features and formation processes of multiple deposition layers from the 2004 Indian Ocean Tsunami at Ban Nam Kem, southern Thailand

Multiple-layered tsunami deposits have been frequently reported from coastal stratigraphic sequences, but the formation processes of these layers remain uncertain. A terrestrial sandy deposit formed by the 2004 Indian Ocean Tsunami was investigated at Ban Nam Kem, southern Thailand. Four internal layers induced by two tsunami waves were identified in the tsunami deposit. Sedimentary structures indicated that two units were formed by run-up currents caused by the tsunami and the other two units were deposited by the backwash flows. Graded bedding was common in the layers, but inverse grading was observed at limited intervals on the surveyed transects. The characteristics of the multiple-layered tsunami deposit vary remarkably over a very short distance (< 1 m) in response to the local topography. Remarkable asymmetries in thickness and grain-size distribution are recognized between the run-up and backwash flow deposits. On the basis of the interpretation of sedimentary structures, the formation process of the multiple-layered tsunami deposit observed in this study can be explained in a schematic model as the modification of the ideal tsunami sequence by local erosion and the asymmetric hydraulic properties of tsunami waves, such as the maximum shear velocity and the heterogeneity of the flow velocity field.