Ground failures on reclaimed land during the 2011 Tohoku earthquake: A case study in Urayasu City, Japan

Abstract

The 2011 Tohoku earthquake caused ground failures over a wide area around Tokyo Bay, especially soil liquefaction in areas of reclaimed land (umetate-chi in Japanese). These areas, which were constructed largely by using sediment dredged from the seafloor of Tokyo Bay, suffered the greatest damage during the earthquake; no comparable damage has been reported in Holocene lowlands within our survey area. To investigate the relationship of the near-surface sediment profile in Urayasu City to ground failures caused there by the earthquake, we used aerial photographs and historical records of reclamation work, collected and analyzed hand-auger samples, and conducted dynamic cone penetration tests and micro-tremor observations. Two large cracks that formed in the grounds of Urayasu High School during the earthquake coincided with rows of piles used during reclamation work. Hand-auger samples and dynamic cone penetration tests revealed distinct differences in the lithology of the sediments on either side of both cracks; these differences allowed us to define three areas with different geological and seismological characteristics. The surface traces of the two cracks were coincident with the boundaries between these three areas. Micro-tremor observations in the three areas and in a nearby area of Holocene lowland revealed that the major ground failures of the 2011 Tohoku earthquake were closely related to subsurface interfaces between different lithologies that were created by land reclamation. This case study shows that understanding both Holocene geological processes and knowledge of the history and methods of land reclamation are important components of planning for the prevention or mitigation of earthquake disasters in reclaimed coastal areas.