はじめに：

　2011年3月11日に発生した東北地方太平洋沖地震の際，東京湾岸埋立地北部では斑状の液状化－流動化に伴う地盤の沈下が多数発生した．この中には，被害から10年経た現在でも地盤の沈下が継続し，地表の変形が進んでいる部分が存在する．その中の浦安市高洲9丁目にて地質調査を行った結果を述べる．

　調査地付近では，東日本大震災直後2011年3月～4月の地表調査により，液状化－流動化に伴い45～80cmの建築物の抜け上がりがみられていた（千葉県環境研究センター，2011）．その後，グーグルアースの時系列航空写真によれば，2012年4月にはそれらの敷地全体は平らに補修されたが，2016年12月には再び抜け上がりがみられるようになった．2022年3月の実測では，17～18cmにも及んでいる．

　オールコアボーリングは抜け上がりがみられた施設から約6m離れた場所（北緯35度37分46秒，東経139度55分3秒，標高3.0m）で深度22.45mまで行った．また，この周囲で4mから8m間隔に動的コーン貫入試験（斜面調査用簡易貫入試験）を深度約9～10mまで行った．

地層構成：

　深度9.58mに人自不整合があり，これより上位は人工地層，下位は沖積層である．

沖積層は，層相上，下部（深度17.93m以深）・中部（深度17.93～11.24m）・上部（11.24～9.58m）に細分され，下部は厚さ0.5～7cmの極細粒砂～細粒砂層をまれに挟む黒褐～灰

　中部の厚さ0.2m以上の砂層の一部では斜交葉理が不明瞭ないし変形がみられ，これ以外の砂層の多くは斜交葉理が明瞭にみられる．

人工地層は，深度0.69mを境にこの上位が盛土アソシエーション，下位が埋立アソシエーションである．

埋立アソシエーションは，砂層からなる最下部バンドル（深度9.58～8.50m），泥層からなる下部バンドル（深度8.50～7.67m），細粒砂層主体の中部バンドル（深度7.67～4.58m），泥層主体で砂層を挟む上部バンドル（深度4.58～2.00m），貝殻質砂層主体の最上部バンドル（深度2.00～0.69m）から構成される．

　最下部バンドルは，オリーブ黒色の塊状の中粒砂層から構成され，硬さはNc=15～30（簡易貫入試験値を以後「Nc=」と略す）とゆるい～中位である．下部バンドルは，灰

盛土アソシエーションは，灰色の砕石層と黄褐～にぶい黄色の細粒砂層ないし極粗粒砂層との互層である．硬さはNc=10～25とゆるい～中位である．

液状化－流動化に関して：

　液状化－流動化の判定は，風岡ほか（1994）・風岡（2003）に基づき，地層断面における初生的な堆積構造の状態により判断した．埋立アソシエーションの最下部・中部・上部バンドルの砂層の大部分では，葉理が不明瞭ないし消失していることから，この部分が液状化－流動化したものと考えられる．特に，中部バンドルの下半部の砂層は現在でも非常にゆるく，この上位の厚い泥層である難透水層により，地震時に上昇した間隙水圧の減衰速度が規制され，沈下が継続している可能性が考えられる．

引用文献：

千葉県環境研究センター，2011，千葉県環境研究センター報告，G-8, 3-1～3-25．

風岡 修・楠田 隆・香村一夫・楡井 久・佐藤賢司・原 雄・古野邦雄・香川 淳・森崎正昭，1994，日本地質学会第101年総会･討論会 講演要旨，125-126．

風岡 修，2003，液状化・流動化の地層断面．アーバンクボタ40号，5-13．

Sooty blotch was found on Gynura bicolor seedlings in Okinawa Prefecture in 2017. Fungal-like isolates, mainly obtained from the lower leaves, caused similar symptoms on plants sprayed with individual isolates, which were identified as Zasmidium gynurae based on morphological characteristics and sequences of rDNA ITS regions. This is the first report of a disease of G. bicolor caused by Z. gynurae in Japan, and we propose the Japanese name “susuhan-byou” (sooty blotch) of Gynura bicolor.

The type of seasonal change of water color in the ponds under consideration are classified into two categories from the chromatological point of view. I compared the ponds of type A with those of type B on the basis of seasonal change of phytoplankton-flora and its population. This classification of the types A and B was made on the basis of whether the locate point x, y, of the water color approaches to the Illuminant C in June. The plankton flora and its population in the period when the locate point of water color approaches to the Illuminant C is discussed below. This period is June and December in the ponds of type A and December in type B. (1) In both types of ponds, the population of Chlorophyceae was large and that of Cyanophyceae was small during the winter, and the plankton-flora consisting chiefly of Scenedesmus occurred continuously during this period. In the pond of type A, Chlorophyceae were dominant not only in winter but also in the middle or the end of June. It is, therefore, considered that the phenomenon, on which the chromaticity coordinates of water color approaches to the Illuminant C, was caused by the large increase of Chlorophyceae and the decrease of Cyanophyceae in all seasons. In such occasions, the water color becomes generally dark olive and the values of brightness and excitation purity of water color are always low ; this will be explained by the small plankton population in this period. After December, the water colors in both types of ponds become gradually bright olive and their locate points x, y, leave the Illuminant C accompanied with the rich production of Chlorophyceae. This fact that the locus of water color runs almost parallel with the isodominant wave-length's line on the C.I.E. Chromaticity diagram is, I think, caused by the change of quantity of Chlorophyceae, especially of Scenedesmus. When Chlorophyceae increases, the water color becomes bright olive ; when it decreases, the water color becomes dark olive. This phenomenon means that the seasonal change of water color in this period is caused by the change of its brightness and excitation purity within certain limits of the hue. (2) The locus of water color on the C.I.E. Chromaticity diagram during the period, with the exception of certain months mentioned in the above section (1), runs almost parallel with the spectralocus. This phenomenon is found during the period from May to November except June in the ponds of type A, and is found through the period from April to November in the ponds of type B. In this period, Aphanocapsa, Microcystis, Merismopedia, Lyngbya and Anabaena become dominant in all the gold-fish ponds, and populations of such blue-green algae are extremely large, and many of them become also neuston, floating in the upper layer of the water. There fore, the water color in this period is caused chiefly by the reflected light by neuston, so that the brightness and the excitation purity of water color in this period is always high. On the other hand, the hue of the water color in this period changes between yellowish green and olive by the change of quantity of Cyanophyceae. The above results show the following facts concerning the mechanism of the appearance of water color in the gold-fish ponds. (a) When Scenedesmus (Chlorophyceae) increases and becomes dominant, the locus of water color on the C.I.E. Chromaticity diagram runs almost parallel with the isodominant wavelength's line accompanied with the variability of its population. (b) When Cyanophyceae increases and Chlorophyceae decreases, the locus of the water color runs almost parallel with the spectralocus accompanied with the variability of Cyanophyceae population. (c) As the algal population increases in the ponds, the brightness and the excitation purity of water color become generally high. This tendency is remarkable in the period when Chlorophyceae increases. From both the chromatological a

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