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 and building damage data in the affected areas. Therefore, this event provides us with one of the most complete datasets amongst tsunami events in history. In this study, fragility function are derived using data provided by the Ministry of Land, Infrastructure and Transportation of Japan (MLIT), with more than 250,000 surveyed structures. The dataset have details on damage level, structural material, number of stories per building and location (town). This informtaion 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. Firstly, the results show a resistant performance of RC (reinforced concrete) and steel buildings over wood or brick buildings, which is consistent with previous studies. For example, the probability that a 2 m inundation depth damages RC, steel, brick and wood buildings as level 5 (collapsed and washed away) is approximately 0.1, 0.2, 0.25 and 0.45 respectively. While all (100%) of wood buildings and most lightweight buildings were washed away when the inundation depth was greater than 10 m, only 50% or less were washed away for steel and RC structures. Secondly, three (or more) storey buildings were much stronger than one or two storey buildings. For both RC and wood structures, there are no major differences in damage probability between one and two-storey buildings; whereas the damage probability is significantly lower for multistorey structures (3 storeys or more). Finally, the effects of coastal topography on damage probability are also discussed here. At the same tsunami inundation depth, buildings along the Sanriku ria coast suffered greater damage than buildings from the plain coast in Miyagi prefecture. For example in Ishinomaki city, the damage probability for "washed away" at 2 m inundation depth is less than 0.05 in plain coast while it reaches approximately 0.4 in ria coast. For similar inundation depths, such differences can potentially be explained by the higher flow velocities in the ria coast, which was hit by the most energetic waves. The findings presented in this paper are key to support better building damage assessments, land use management and disaster planning in the future.