Do Tsunami Deposits Thin Landward? Observations from the 11 March 2011 Tohoku-oki Tsunami on the Sendai Coastal Plain

Case studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on the spatial distribution of deposit thickness resulting from the March, 2011 Tohoku-oki tsunami along a nearly 4.5 km shore-normal sampling transect on the coastal plain near Sendai, Japan. The study area consisted of a broad, low-relief prograding coastal plain which comprised a broad sand beach backed by low (~3 m) sand dunes and forest; a wetland and a shore-parallel engineered drainage canal several meters deep and ~40 m wide; agricultural rice fields marked by low-lying rectangular dyke systems with occasional buildings, canals, and roads; and an elevated highway embankment which generally marked the landward extent of inundation except where gaps in the structure allowed flow to penetrate further inland. Field observations in May 2011 documented a tsunami deposit that generally thinned landward from an average maximum ~30 cm thick sand deposit in the coastal forest to a thin mud drape several mm thick near the inundation limit. Although there was an overall thinning of the deposit, this trend was often interrupted by localized features that led to variability in deposit thickness (from 0 to 20+ cm) over short distances (<10 m) along transect and adjacent to the main transect. The variability in both the along- and transverse flow directions is related to micro-topography, sediment source availability, and gradients in flow speed and duration. Although there is an overall landward thinning of the deposit, localized thinning and thickening occurred in numerous locations along our transect. This information is critical for accurately identifying paleotsunami deposits and for improving tsunami hazard assessment from the geologic record.