岩手県野田村十府ヶ浦公園の南方，国道45号線（画面右奥）沿いの海岸近傍露頭．正面の露頭には灰白色の白亜系が露出する．右手のプレハブ小屋の陸側の草の生えた斜面には最近6500年間の複数の津波堆積物層が観察される．ここには，陸側からの河川の流入もなく，壊されやすい津波堆積物が奇跡的に保存されている（本特集，小野寺, 2023参照）．さらに白亜系露頭頂部には，2011年の東北地方太平洋沖地震の津波でうち上げられた貝殻が観察される．この付近での最大遡上高さは約40 mであった．ドローンにより海上から撮影（2021年7月）．

（写真・説明：土屋範芳）

 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.

 本論文では，2011年東北沖津波により東北地方の陸上に形成された堆積物のなかでもとくに，砂質津波堆積物を覆う泥層（= mud cap）と砂質津波堆積物の堆積限界よりも内陸に堆積した泥質津波堆積物の堆積学的，地球化学的，鉱物学的そして微古生物学的特徴を総括した。この論文は，仙台平野で津波直後および津波発生後2-3年以内に行われた調査と，青森で津波発生から7-9年後に行われた調査から得られた知見に基づいている。地球化学的な指標は広範囲にわたって検討され，泥質堆積物が津波由来であることを認定することができた。また，さまざまなプロキシから，泥質津波堆積物のほとんどが侵食された水田や森林の土壌，植生を起源としており，津波によって内陸に運ばれたことが示された。調査の結果，津波が2.5 km以上内陸に浸水した地域では視認可能な砂質堆積物は60-80％程度までしか広がっていないのに対し，泥質堆積物は津波浸水限界の最大98％まで広がっていたことが明らかになった。このことは津波リスク評価にとって重要な意味をもつ。なぜなら，砂質津波堆積物に基づき津波浸水範囲を決めてしまうと，津波やそれを生起した地震の規模を過小評価してしまうためである。水溶性の海洋指標は時間経過によって変化を受けやすいが，有機物に富む微細な堆積物中では保存性が高い可能性がある。最近の研究で，生物由来の有機化合物はより高い保存性をもつことがわかってきており，堆積学的に明確な痕跡がない場合でも，有機化合物が津波浸水限界のマーカーとして利用できる可能性が示されている。しかし，泥質・有機質津波堆積物は粒径が小さいことや層が薄いために識別が難しく，依然として技術的課題を考えなければならない。

 1662年寛文日向灘地震による宮崎平野の津波堆積物の特徴を把握するため，日本原子力研究開発機構 東濃地科学センターのレーザーアブレーション付き誘導結合プラズマ質量分析装置（LA-ICPMS）を用いて津波堆積物中の砕屑性ジルコンのU–Pb年代測定を実施した。本研究では，試料の化学的前処理手法としてchemical abrasion法を適用させ，U–Pb年代測定に必要な粒子径約50 μm以上のジルコン粒子25点を回収した。LA-ICPMS測定の結果，津波堆積物中の砕屑性ジルコンのU–Pb年代の範囲は約14 Maから約2405 Maであり，特に100 Maと2000 Ma付近の年代値を示すジルコン粒子が多く見られた。今後，測定実績をさらに蓄積していくことが重要であるが，本研究で得られた100 Ma付近の年代値を示す津波堆積物中のジルコン粒子は白亜紀から古第三紀に相当する四万十層群に由来する可能性が考えられる。また，1662年寛文日向灘地震による津波堆積物中の砕屑性ジルコンのU–Pb年代の分布は，九州中央部の前弧砂岩から得られている結果と類似していることが示された。