Wooden Building Damage Analysis in Mashiki Town for the 2016 Kumamoto Earthquakes on April 14 and 16

The 2016 Kumamoto earthquakes caused serious building damage in Mashiki town. Because two large earthquakes occurred within an interval of 28 hours, it is difficult to separate the damage caused by each of these earthquakes. We analyzed aerial photos of the center of Mashiki town taken before and after the second event, which allow us to separate the damage caused by the two earthquakes. Our analysis shows that building damage was concentrated especially on the river terrace of the Akitsu river, and there were almost no collapsed buildings in the south of the damaged area. The pattern of damage distribution of the two events was similar, which suggests that the damage to the wooden buildings was caused by local conditions. The analysis of past aerial photos showed that the heterogeneity of the damage distribution is difficult to explain by only the building age. The cause of this heterogeneity was found to be not because of an earthquake faulting effect, but because of a combination of building seismic performance and local site conditions.