The post-depositional changes of the onshore 2004 tsunami deposits on the Andaman Sea coast of Thailand

Abstract

The Indian Ocean tsunami flooded the coastal zone of the Andaman Sea and left tsunami deposits with a thickness of a few millimetres to tens of centimetres over a roughly one-kilometre-wide tsunami inundation zone. The preservation potential and the post-depositional changes of the onshore tsunami deposits in the coastal plain setting, under conditions of a tropical climate with high seasonal rainfall, were assessed by reinvestigating trenches located along 13 shore-perpendicular transects; the trenches were documented shortly after the tsunami and after 1, 2, 3 and 4 years. The tsunami deposits were found preserved after 4 years at only half of the studied sites. In about 30% of the sites, the tsunami deposits were not preserved due to human activity; in a further 20% of the sites, the thin tsunami deposits were eroded or not recognised due to new soil formation. The most significant changes took place during the first rainy season when the relief of the tsunami deposits was levelled; moderate sediment redeposition took place, and fine surface sediments were washed away, which frequently left a residual layer of coarse sand and gravel. The fast recovery of new plant cover stabilised the tsunami deposits and protected them against further remobilisation during the subsequent years. After five rainy seasons, tsunami deposits with a thickness of at least a few centimetres were relatively well preserved; however, their internal structures were often significantly blurred by roots and animal bioturbation. Moreover, soil formation within the deposits caused alterations, and in the case of thin layers, it was not possible to recognise them anymore. Tsunami boulders were only slightly weathered but not moved. Among the various factors influencing the preservation potential, the thickness of the original tsunami deposits is the most important. A comparison between the first post-tsunami survey and the preserved record suggests that tsunamis with a run-up smaller than three metres are not likely to be preserved; for larger tsunamis, only about 50% of their inundation area is likely to be presented by the preserved extent of the tsunami deposits. Any modelling of paleotsunamis from their deposits must take into account post-depositional changes.

Historical and paleo-tsunami deposits during the last 4000 years and their correlations with historical tsunami events in Koyadori on the Sanriku Coast, northeastern Japan

“…Before considering the approximate age intervals of tsunami events, we need to discuss the preservation potential of tsunami deposits at this site. Szczucinski (2012) and Spiske et al (2013) mentioned the preservation potential of tsunami deposits in tropical and temperate climate regions, respectively, and showed that the characteristics of tsunami deposits (thickness and sedimentary structure) degrade over time. Spiske et al (2013) emphasized the significance of the preservation potential in assessing the intervals and frequencies of tsunamis, because tsunami deposits are not necessarily preserved in whole inundated areas.…”

26 December 2004 tsunami deposits left in areas of various tsunami run up in coastal zone of Thailand

“…Several studies were also conducted one or more years after a tsunami. However, they are not discussed here because they have not assessed the post-depositional changes that were found to be important in the case of the onshore tsunami deposits in Thailand (Szczuciński et al, , 2007Goto et al, 2008Goto et al, , 2012Jankaew et al, 2008;Szczuciński, 2012), as well as in other parts of the Indian Ocean basin (Nichol and Kench, 2008). This paper has four objectives: -to provide a new documentation of the recent (i.e., not altered by post-depositional processes) onshore tsunami deposits (spatial extent, thickness, sediment structure and texture) from areas flooded by the 2004 IOT that experienced various runup heights, -to interpret the possible tsunami-related sedimentation processes, -to examine if variations in the tsunami runup heights are reflected in the tsunami deposit properties -and to compare the new data with the existing common set of diagnostic features for tsunami deposits.…”

Local inundation distances and regional tsunami recurrence in the Indian Ocean inferred from luminescence dating of sandy deposits in Thailand

“…The uppermost sand sheet (S1) was formed by the IOT in December 2004. All older layers (S2-5) revealed similar sedimentary characteristics as the IOT deposit; differences in geochemistry (zero instead of 8-15 % CaCO 3 ), fossil content (no fossils in the older layers) and geometry (discontinuous extent) can be addressed to post-depositional changes (Szczuciński, 2012). Since the prehistoric sand sheets are interpreted to have the same origin as those described by Jankaew et al (2008) and Fujino et al (2009), they are referred to as tsunamites of unknown age (see Sect.…”