2015 年 124 巻 2 号 p. 273-286
The 2011 off the Pacific coast of Tohoku Earthquake liquefied large areas of the Kanto region. At the reclaimed land of Urayasu and Mihama, Chiba prefecture, liquefaction tended to concentrate in sandy land-fill layers covering thick alluvium. Alluvial plains along the lower reaches are identified as depositional surfaces of the coastal prism (CP). The CP is sandwiched between the present and the last glacial river-profiles. In Japan, the last glacial rivers developed basal gravel layers (BG) along the bottom of the CP. A thick CP with BG lengthens the secondary seismic wave period and its duration because of a slow s-wave velocity and multi-reflection, resulting in increased internal water pressure and liquefaction of the upper sandy layer of the CP. Historic liquefaction sites show close relations with the distribution of the CP. Subduction-zone large earthquakes caused repeated liquefaction in an alluvial plain where the CP was more than 30 m thick. The inland limit of the liquefaction area roughly coincides with the upstream edge of the CP. The three largest rivers in the Kanto regionof Kinu River, Ara River, and Naka River (Furutone River) have the longest CP in Japan. As a result, the Great East Japan Earthquake liquefied large areas of the Kanto region. The Earthquake also liquefied inland basins such as the Koriyama basin with late Quaternary lacustrine sediments. This demonstrates that a mega-thrust earthquake has the potential to liquefy inland sediment-fill basins beyond the inland limit of the CP.