2023 Volume 132 Issue 4 Pages 341-352
This paper provides an overview of the sedimentological, geochemical, mineralogical and microfossil characteristics of the mud caps overlying the tsunami sandy deposits and mud units extending beyond the limit of the visible sandy deposit left behind by the 2011 Tohoku-oki tsunami. This review is based on findings from studies on the Sendai Plain that were carried out during post-tsunami surveys or within 2-3 years after the event, and from studies on the Aomari coast, where traces of the tsunami could still be found 7-9 years after the event. Much emphasis is given to the geochemical indicators, as they were extensively investigated and they allowed the positive identification of the mud units as tsunami deposits, although the combined proxies showed that the mud was mostly sourced from eroded paddy fields or forest soil and vegetation and transported inland by the tsunami waves. Research revealed that the tsunami mud units extended to up to 98% of the tsunami inundation limit, while the visible sand deposit often extended only to about 60-80% in areas where the tsunami inundated more than 2.5 km inland. This has significant implications for tsunami hazard assessment, as using the extent of the visible sandy tsunami deposits has led to an underestimation of the magnitude of earlier tsunamis and associated earthquakes. While water-leachable marine markers are affected by post-depositional changes, they might be better preserved in organic-rich fine sediments. Recent studies show that organic compounds exhibit a higher preservation potential and can be used as markers of the tsunami inundation limit, even in the absence of clear sedimentological evidence. However, small or very thin samples might present technical challenges.
