Improving Tsunami Hazard Assessment: Lessons from Deposits from Seven Modern Tsunamis

Abstract:

Terrestrial paleotsunami deposits are studied to understand the timing and inundation extent of past tsunamis. Developing techniques to extract more information, such as flow depth, from paleotsunami deposits will improve understanding of the tsunami history in vulnerable areas and provide data for tsunami hazard assessment and planning. The analysis of deposits from recent tsunamis enables the comparison between deposit characteristics and event processes because data on the source and hydrodynamics of the tsunami can be collected.

We compile and analyze data on tsunami hydrodynamics, deposits, and depositional environments collected in field studies over the last 16 years from 50 study sites from 7 tsunami events, including Papua New Guinea 1998, Peru 2001, Indian Ocean 2004, Sumatra 2005, Samoa 2009, Chile 2010, and Japan 2011. This approach allows comparisons of tsunami deposit characteristics across events and depositional environments. We find a moderate positive correlation between the mean grain size of a deposit and the flow depth of the tsunami, likely because more coarse material is transported by faster tsunami flows that are typical of greater flow depths. We also find that the local deposit thickness is not primarily controlled by flow depth and is strongly influenced by the contributions of bedload, variations in sediment supply, and local topography. We find that deposits often thicken when local topographic slope decreases and vice versa.

Further study will investigate if the thicknesses of individual suspension graded layers within deposits are controlled by tsunami flow depth. By ignoring the portions of the deposit formed by bedload transport or spatial flow deceleration, the parts of the deposit formed from sediment falling out of suspension during the temporal flow deceleration of each tsunami wave may be identified. Focusing on individual layers of tsunami deposits, hypothetically formed by a single process and wave, may reveal relationships between deposit characteristics and hydrodynamic conditions that are not apparent when analyzing deposits as a whole. Refining the understanding of depositional processes occurring in modern tsunami events provides a foundation for developing techniques to reconstruct flow conditions from paleotsunami deposits.