Investigation of the Effect of Debris-Induced Damage for Constructing Tsunami Fragility Curves for Buildings

Tsunami fragility curves are statistical models which form a key component of tsunami risk models, as they provide a probabilistic link between a Tsunami Intensity Measure (TIM) and building damage. Building damage due to tsunamis can occur due to fluid effects (e.g. drag) and debris impact, two effects which have different implications for building damage levels and mechanisms. However, existing studies often pool all available damage data for a location regardless of whether damage was caused by fluid or debris effects, and so it is not clear whether the inclusion of debris-induced damage introduces bias in existing fragility curves. This paper uses a detailed disaggregated damage dataset from the 2011 Great East Japan Earthquake and Tsunami together with several advanced statistical methods in order to identify the effect that debris-induced damage has on fragility function derivation. Buildings are identified which are most likely to have sustained significant debris damage, based on the proportion of nearby buildings which have been designated as “washed away” in their post-tsunami survey. Fragility curves are then constructed for observed inundation depth and simulated force, and fragility curves with/without debris impact are compared for each damage state. Finally complex models which include all buildings and additional parameters corresponding to debris impact are considered. The influence of debris model parameters on determining building damage was shown to be significant for all but the lowest damage state (“minor damage”), and more complex fragility functions which incorporate debris model parameters were shown to have a statistically significant better fit to the observed damage data than models which omitted debris information