South of Tofugaura Park, Noda Village, Iwate Prefecture. Outcrops along the coast near Route 45 (cars can be seen). Exposed outcrop is from the Cretaceous. The grassy outcrop in front of the prefabricated hut at the right side of the exposed outcrop shows layers of tsunami deposits dating back about 6500 years. This valley topography has miraculously preserved tsunami deposits for many years without river inflows from the landward side (Onodera, 2023). Shells from the tsunami caused by the 2011 Off the Pacific Coast of Tohoku Earthquake can be seen at the top of the exposed outcrops. The maximum run-up height in this area is about 40 meters. The image was taken by a drone from the sea (July 2021).

(Photograph & Explanation: Noriyoshi TSUCHIYA)

 Identifying invisible tsunami deposits in the geological record is challenging and requires multiproxy analyses. In particular, geochemical signatures provide useful information for identifying paleo-tsunami deposits, as well as for reconstructing the paleoenvironment history, even when other proxies are equivocal. In addition, geochemical proxies help to provide an understanding of environmental changes that occur post-tsunami. As a result, they are now frequently used in paleotsunami research. Tsunami deposits are identified on the Sendai Plain, Japan, and environmental changes over time are reconstructed based mainly on geochemical data. Using a high-resolution elemental analysis with XRF core scanning, evidence of seawater inundation is discovered that is difficult to recognize with a naked eye. Based on the combined results of multiproxy analyses, together with radiocarbon dating, not only a 1611 Keicho tsunami deposit but also a minor signal of a potential 1454 Kyotoku tsunami deposit are identified.

 A practical lesson using tsunami deposits along the coast of Iwate prefecture is reviewed, as well as a practical lesson using tsunami deposits in schools. How they should be used in the future is examined. Noda Village Noda's event layer is stripped off to prepare specimens, and practical training is conducted to determine the cycle of tsunamis associated with trench-type earthquakes. Evacuation behavior is examined and efforts are made to raise awareness of disaster prevention related to tsunamis. Descriptions related to the tsunami narrative have appeared in textbooks since the Tohoku-Pacific Ocean Earthquake, and are repeatedly studied in elementary schools, junior high schools, and high schools. Classes utilizing tsunami deposits distributed in Aramakihama, Miyako City are conducted in collaboration with junior high schools, high schools, the government, and Sanriku Geopark. Historical earthquakes and tsunamis are identified, and tsunami inundation areas and evacuation behavior are verified. It is confirmed that 93.7% of students have improved awareness. It is hoped that improving the accuracy of tsunami deposit certification not only in lowlands but also in small valleys and hills will strengthen descriptions of tsunami in textbooks and increase the number of practical lessons.

 Array coring survey at the site of Yonezu Pond, which is depicted on a 1680s map, revealed a sand bed deposited by a tsunami caused by the 1498 CE Meio Earthquake occurred in the eastern Nankai Trough. This sand bed consists mainly of medium-grained sand, 10-15 cm thick, forming large ripples or dunes that record the reversal of tsunami inundation and return flows. Our age model based on radiocarbon dating limits the depositional timing of the sand bed to c. 1440-1600 CE. Only a tsunami could have generated a flow fast enough and long enough in duration to deposit a large amount of sand in Yonezu Pond, which at that time was more than 1.2 km inland from the coast and river. The facies change from peat to clay and pollen composition before and after the Meio tsunami suggest that the tsunami had a significant impact on the vegetation around the pond, especially herbaceous vegetation. Plant opal analysis revealed that paddy field devastation occurred with the formation of the Meio tsunami deposit.

 The physical and chemical properties of tsunami deposits of the Great East Japan Earthquake on March 11, 2011, are investigated. In addition, environmental impact assessments are conducted based on estimations of tsunami deposit areas. The contents of heavy metals in tsunami deposits are similar to those in common soils. The leaching values of arsenic and lead are slightly higher than those of general soils. Large volumes of silt and clayey tsunami deposits were deposited from Kamaishi City in Iwate Prefecture to the area south of Minamisoma City in Fukushima Prefecture. Ignition losses are high in these areas, and a positive correlation is observed between ignition loss and silt/clay content. In addition, arsenic content in tsunami deposits is positively correlated with silt/clay contents, and the fraction of silt/clay is more than 30% at many sites where Japanese environmental criteria for leaching values of arsenic are exceeded. These results indicate that the environmental impacts of arsenic might need to be evaluated in areas where fine-grained tsunami deposits are deposited, because humans might ingest arsenic in tsunami deposits, or it might be leached and transferred to the groundwater and agricultural products.

 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.

 U–Pb dating of detrital zircons was performed to characterize the paleotsunami deposits of the 1662 CE Kanbun-Hyuganada sea earthquake in the SY05 core from the Miyazaki plain on the Pacific coast of southwest Japan. Twenty-five large-sized zircon grains (> ∼50 μm) were separated from the paleotsunami deposits by chemical abrasion. In the present work, ²³⁸U–²⁰⁶Pb ages were measured by a laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-ICPMS) system at Tono Geoscience Center, Japan Atomic Energy Agency. ²³⁸U–²⁰⁶Pb ages of the detrital zircons ranged from ∼14 Ma to ∼2405 Ma, and the age spectrum revealed high relative probability values around ∼100 Ma and ∼2000 Ma. Detrital zircons with the age of ∼100 Ma in the 1662 CE paleotsunami deposits could be partly originated from the Shimanto Supergroup (Cretaceous–Paleogene) in Kyusyu Island, southwest Japan. Moreover, the U–Pb age spectrum of detrital zircons with Precambrian grains from the paleotsunami deposits was similar with those of fore-arc sandstones from central Kyushu Island.