Fragility curves for structural damage by the 1993 Okushiri tsunami

In 1993, the tsunami accompanied with the M7.8 earthquake off the south-west coast of Hokkaido, Japan attacked Okushiri island, which locates 30 km west of Hokkaido, within 5 minutes after the quake and caused more then 200 casualties. Especially, Aonae district in the southern most area of Okushiri island suffered devastating damage by approximately 11 m tsunami attacking from the west coast of the island and the fire caused during and after the tsunami attack. After the event occurred, extensive field survey was conducted to measure the tsunami run-up height, extent of inundation zone and damage. The tsunami damage, structural damage and casualty, and tsunami run-up heights in Aonae district are summarized by the survey teams, e.g. Shuto and Matsutomi (1995). However, the relations between the tsunami hazard and structural damage have never been discussed. Recently, we developed a methodology to identify the relationships between tsunami hazard and the damage occurred in the 2004 Sumatra-Andaman earthquake tsunami by using the advances of tsunami numerical modeling, remote sensing and GIS analysis, to construct fragility curves to identify structural damage probabilities with particular regard to the tsunami hydrodynamics (Koshimura et al., 2009). Following that procedure, the present research aims to revisit the 1993 Okushiri tsunami disaster and identify the relationships between the tsunami hazard and the structural damage, that has never been discussed in the earlier studies. First, we reproduce the 1993 Okushiri tsunami by the numerical modeling with high-resolution topography data obtained through the recent Realtime-Kinematic GPS measurement to reproduce the pre-tsunami topographic condition. Second, we interpret the structural damage using visual inspection of aerial photos acquired before and after the tsunami. Third, both of tsunami hazard and damage information are correlated by GIS analysis. As a result, the relations between tsunami hazard and structural damage were identified with the form of tsunami fragility curves as structural damage probability with particular regard to the tsunami hydrodynamics such as inundation depth, current velocity and hydrodynamic force, which increases the capability to assess the structural damage by tsunami inundation flow in a quantitative manner (see Figure). Tsunami fragility curves for house damage by the 1993 Okushiri tsunami, in terms of (a) the inundation depth, (b) the current velocity and (c) the hydrodynamic force. The solid lines are the best-fitted curves of the plot by lognormal cumulative distribution function.