TSUNAMI DEPOSITS, THEIR POSSIBLE APPLICATION FOR RISK ASSESSMENT AND DIFFERENTIATION FROM STORM DEPOSITS

Onshore tsunami deposits can provide useful information on the palaeotsunami history of a coastal region, extending far beyond the range which is covered by historical records. In uninhabited areas such deposits often provide the only information on the local tsunami hazard. The main issue when studying (palaeo)tsunami deposits is their differentiation from storm surge deposits. Similar to tsunami, storm surges are also capable of transporting marine and coastal sediments to considerable distances inland and tend to form similar deposits (mostly sandy sediment layers and boulder accumulations). However, reconstruction of the palaeotsunami (and also palaeostorm) history for a coastal region essentially requires the unambiguous differentiation of tsunami and storm surge deposits.

Our studies include detailed observations of deposits of recent tsunami events (silty-sandy sediment layers and boulders of uncommonly large size deposited by the Sumatra-Andaman Tsunami in the area of Khao Lak, Thailand) as well as the detection of historical and prehistorical tsunami deposits in the Dominican Republic. There, sandy sediment layers attributed to historically known tsunami (NE-coast: August 1946; SW-coast: October 1751) were observed. Concerning the fine-grained sediment layers, characteristics which definitely prove deposition by a tsunami were found in all cases. Sedimentary petrography and benthic foraminifers showed the influence of sediments from water depths which could not have been eroded and redeposited by storm waves. Additionally, distinct sedimentary structures (syn-sedimentary flame structure and rip-up clasts), for which a formation by storm waves is very unlikely, were observed in some cases.

Giant boulders detected along the southern coast of the Dominican Republic are interpreted as tsunami deposits. Considering topographic position (height a.s.l., distance to the sea), morphological characteristics (dimensions; overturning, fragmentation during deposition and occasional stacking above each other in considerable distance from the shore) and hydrodynamic calculations, many of the boulders appear to be beyond the transport capability of storm waves.