Leveling of long-term settlement of Holocene clay ground induced by the 2011 off the Pacific coast of Tohoku earthquake

The aim of the study reported in this paper is to reveal the long-term settlement of a Holocene clay ground after the 2011 off the Pacific coast of Tohoku earthquake. The authors took level readings six times across Urayasu City, Chiba Prefecture, during the period from April 2011 to April 2013. The results are a chronological record of changes in settlement of first-order benchmarks and survey points showing that long-term ground settlement was taking place due to the earthquake. In Phase I reclaimed land (which was constructed between 1968 and 1975), the settlement rate is shown to be approximately 6 mm/year, more than twice the rate of the settlement before the earthquake. In Phase II reclaimed land (which was constructed between 1978 and 1980), the settlement behavior is found to differ according to whether the reclaimed land had been improved or not. Furthermore, the natural alluvial lowland has not settled since the earthquake.