Assistant Professor Xin Wang of the Hazard and Risk Evaluation Research Division in IRIDeS, calculated damage and collapse ratios of wooden houses according to different building ages, and has pointed out there is a high possibility that old wooden houses may collapsed the most in the Kumamoto Earthquake. And it is necessary to consider the risk of old wooden houses for disaster prevention in future earthquakes.

Assistant Professor Xin Wang calculated the damage and collapse ratios of wooden houses in 10 most affected areas based on vulnerability functions which were extracted from the statistics of damaged buildings in past earthquakes. Based on the vulnerability functions of the 1995 Hyogoken-Nanbu Earthquake (Murao, Yamazaki, 2000), as for old wooden houses built before 1951, damage ratios are estimated to be almost 90% in Mashiki Town, 50% in Uto City, and 1% in Kumamoto City. As for buildings constructed between 1982 and 1994, the estimated damage ratios are about 50% in Mashiki Town, 20% in Kumamoto City, and 2% percent in Uto City. Furthermore, damage ratios also estimated based on the vulnerability functions extracted from the earthquakes after the 1995 Hyogoken-Nanbu Earthquake, such as Chuestu Earthquake etc. (Midorikawa et al., 2011). It has been estimated that damage ratios of wooden houses built before 1981 under the old aseismic design may be 25% in Mashiki Town, 6% in Kumamoto City, and 0.2% in Uto City; the same percentage for wooden houses built after 1981 under the new building code was about 12% in Mashiki Town, 3% in Kumamoto City, and 0% percent in Uto City.

As a result of this analyzation, for all cases, Mashiki region had the highest estimated damage ratios, next the Kumamoto region, then the Uto region. The calculations identified concentration of building damage especially in the Mashiki region. Old wooden houses collapsed the most. The risk of old wooden houses should be considered seriously in future earthquakes. Though new buildings performed well, the dangerous of accumulative damages due to continuous aftershocks should be emphasized.

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