地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
論説
2011年東北地方太平洋沖地震による液状化層のボーリングおよびトレンチ調査の結果
小松原 琢水野 清秀齋藤 勝細矢 卓志卜部 厚志宮地 良典風岡 修
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2018 年 127 巻 3 号 p. 361-389

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 The 2011 off the Pacific coast of the Tohoku earthquake caused severe liquefaction events in the Kanto region, which is 300-400 km south of the earthquake's epicenter. All-core drillings and trenching surveys were carried out at Yodaura and Mukoya, which are situated in the lowland along the lower Tone River in the central Kanto region. The Yodaura site is on reclaimed land where a former lake was filled in by sand pumping from 1969 to 1974. The sediments at Yodaura consisted of silts and clays of the former Lake Yodaura deposits (natural sediments) and sandy strata composed of artificial fill. No evidence was found of liquefaction in the natural deposits, but the artificial-fill deposits suffered severe liquefaction. Three distinct sand dikes (yS1 sand dyke–yS3 sand dyke) cut the artificial strata at Yodaura: yS1 sand dyke, composed of gray, unoxidized sand, reached the ground surface and cut yS2 sand dyke, which was composed of light-brown, oxidized sand. Therefore, two liquefaction events occurred at the same point: the older event, which produced yS2 sand dyke, was probably induced by the 1987 off the east coast of Chiba Prefecture earthquake (Mj = 6.7), and the newer one, which produced yS1 sand dyke, was induced by the 2011 earthquake. The third sand dike (yS3 sand dyke) originated in a bed of fine to medium sand containing shell fragments in the artificial strata and contained fragments of asphalt from the ground surface. This dike is consistent with eyewitness accounts of sand gushing during the 2011 event. These accounts report that the ground pulsated at intervals of several seconds, and water and sand spouted from the ground simultaneously with each ground motion pulse. The presence at Yodaura of massive un-stratified sand beds within well-stratified sandy layers, especially near the sand dikes, indicates that liquefaction destroyed the original structure of the sediments. The Mukoya site is on reclaimed land where an abandoned channel of the Tone River was artificially filled in after 1956 by sand pumping and sediment dredging. The surface sediments at the Mukoya site are composed of Holocene floodplain deposits, abandoned channel sediments deposited between 1626 and 1956, and artificial strata. Two distinct deformation structures (mS1 and mS2) were observed in a trench wall. mS1 was a sand dyke that originated in the upper member of the abandoned channel sediments and reached the ground surface. The mS1 sand dyke consisted of liquefied materials derived from the dredged fine to medium sand deposits of the lower part of the artificial strata, and where it was ejected, a “shoulder-like” point formed on the upper surface of the sandy dredged deposits. Structure mS2 was a depression structure in the lower part of the artificial strata that displaced the sandy dredged deposits and an underlying buried soil layer downward. A “sill-like” horizontal sand dike extended from the sandy deposits into the buried soil. In the muddy upper part of the artificial strata, there were many fractures parallel to sand dyke mS1 and the “shoulder” of the sandy dredged deposits. The presence of these fractures indicates that minor subsurface geological structures affected the ground motion and location of liquefied sites caused by the earthquake.

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© 2018 公益社団法人 東京地学協会
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