Tsunami Inundation: detection beyond sand layers - The 2011 Tohoku-oki Tsunami Example

The 2011 Tohoku-oki tsunami caused massive damages along coastal areas of Northern Japan. In the Aomori Prefecture waves of 6-10 m inundating up to 550 m inland, were recorded. Sandy deposits, however, reach only ~250-350 m inland. These clearly identifiable sandy remains of up to 10 cm thickness were detected together with other tsunami indicators inundation (such as barrels, fishing nets, buoys, etc.) in the coastal forest at the small fishing port of Misawa. In tsunami studies these sand layers are used to identify the maximal inundation of tsunamis and for a hazard assessment of coastal areas.

The tsunami inundating the coastal forest eroded subsequently the organic-rich topsoil, including loose plant material and wood. This organic-rich layer emerges as the tsunami sand thins out and was observed with a maximum thickness of 5 cm. Swimmable plastic particles were transported inland and incorporated in the organic- and plant-rich tsunami deposit. Additionally, do organic-geochemical anthropogenic markers and biomarkers suggest this layer to be of tsunami origin. As the harbor suffered severe damages environmental pollutants, such as polycyclic aromatic hydrocarbons, pesticides and anthropogenic compounds (e.g., fuels, fats, tarmac, plastics) were released during the 2011 Tohoku-oki tsunami and distributed with the inflowing water-masses. Environment specific natural biomarkers (such as n-alkanes, fatty acids, n-aldehydes and steroids), which can be detected within both, the sandy and organic-rich tsunami deposits, indicate on the one hand the marine origin of the event deposit but represent on the other hand all during run-up crossed and eroded facies.

Concluding it can be stated, that the 2011 Tohoku-oki tsunami can serves as a blue-print showcasing advantages of organic geochemistry in tsunami studies, extending the inundation detection-limit and help in creation of improved hazard maps and evacuation plans. Most studies solely rely on sand deposits for the detection of inundation distances, however, the lightweight, floating organic- and plant-rich sediments reach further inland and are preserved as "invisible" tsunami layers but can be recognized by organic-geochemistry.