Yo Fukutani

Building damage characteristics based on surveyed data and fragility curves of the 2011 Great East Japan tsunami

A large amount of buildings was damaged or destroyed by the 2011 Great East Japan tsunami. Numerous field surveys were conducted in order to collect the tsunami inundation extents and building damage data in the affected areas. Therefore, this event provides us with one of the most complete data set among tsunami events in history. In this study, fragility functions are derived using data provided by the Ministry of Land, Infrastructure and Transportation of Japan, with more than 250,000 structures surveyed. The set of data has details on damage level, structural material, number of stories per building and location (town). This information is crucial to the understanding of the causes of building damage, as differences in structural characteristics and building location can be taken into account in the damage probability analysis. Using least squares regression, different sets of fragility curves are derived to demonstrate the influence of structural material, number of stories and coastal topography on building damage levels. The results show a better resistant performance of reinforced concrete and steel buildings over wood or masonry buildings. Also, buildings taller than two stories were confirmed to be much stronger than the buildings of one or two stories. The damage characteristic due to the coastal topography based on limited number of data in town locations is also shortly discussed here. At the same tsunami inundation depth, buildings along the Sanriku ria coast were much greater damaged than buildings from the plain coast in Sendai. The difference in damage states can be explained by the faster flow velocities in the ria coast at the same inundation depth. These findings are key to support better future building damage assessments, land use management and disaster planning.

Relocation After Tsunamis in the Sanriku Area and the Condition of Fishing Villages Two Years After the 2011 Great East Japan Tsunami

The 2011 Great East Japan tsunami severely damaged or destroyed most of the fishing ports and facilities along the Sanriku coast. Reconstruction is ongoing, and a relocation plan has already been enacted. Interviews with fishermen in three fishing villages were performed to obtain reports on current situations as well as opinions and problems. For each village, information regarding reconstruction after historical tsunamis and the 2011 tsunami were obtained, and comparisons were made amongst the target villages. A land ownership problem was found in Tadakoshi village. Moving to high ground was proposed for the first time after the 2011 tsunami in Niranohama village. Housing relocation occurred in some parts of Tadakoshi and Yagawa after historical tsunamis in the Sanriku area, but the whole village will be relocated to high ground in the future, as lessons from the 2011 tsunami revealed that the tsunami inundation area was much larger. In general, all of the villages are still facing problems resulting from land subsidence where the ports are partly submerged during high tide. Although there are some small differences in detail, the three villagers have the same desire to move the entire community to high ground, making high seawalls unnecessary because there will be no more houses on the low land area. Some disagreement regarding the height of seawalls remains between coastal residents and local governments.