Geological and geochemical field survey on the Sendai Plain following the 11 March 2011 Tohoku-Oki Tsunami

A post-tsunami survey was carried out in May 2011 by members of a UNESCO-IOC International Tsunami Survey Team. The geological and geochemical survey was carried out along a transect extending 4.5 km inland north of Sendai airport, and focused on tsunami flow characteristics, sedimentation and erosion, as well as assessing the impact of saltwater contamination on the paddy fields . Tsunami inundation in this area reached c. 4.5 km inland, and the limit was marked by the elevated Tobu Highway, except where underpass structures allowed inundation further inland. The tsunami deposit generally thinned and fined inland, with the sandy deposit thinning landward from about 30 cm thickness in the coastal forest to less than 0.5 cm c. 2.8 km inland. Rip-up clasts were observed mostly near the base of the sandy deposits. Further inland, the deposit was dominated by mud, although it contained thin sand laminae one to a few grain-thick up to the limit of inundation near Tobu Highway. The thickness of the tsunami deposit was found to show large variability over short distances. Erosion and liquefaction features were also commonly observed. Ponded water was reported between the coastal forest and up to 2.6 km inland, while salt crusts were observed on numerous rice paddy fields up to the limit of tsunami inundation, where the water had evaporated. Conductivity measurements of ponded water, canals, irrigation and drainage channels revealed that the water was still saline to brackish, despite >60 mm of precipitation in the two months since the tsunami. Elevated concentrations of water-leachable chloride (salt) were measured both in mud and sand deposits, where seawater had stagnated and evaporated.

Keywords

Natural disasters, Exceptional natural disaster, Japan, Tsunamis, Sedimentation, Geologic surveys, Erosion, Seawater

Citation

Chagué-Goff, C., Goto, K., Fujino, S., Nishimura, Y., Sugawara, D., Szczuciński, W.;, Richmond, B. M.; Tappin, D. R., Jaffe, B. E., Witter, R. C., Yulianto, E., Goff, J. R. (2011). Geological and geochemical field survey on the Sendai Plain following the 11 March 2011 Tohoku-Oki Tsunami. Paper presented at the AGU Fall Meeting, San Francisco, California, USA, 5-9 December 2011.