Estimation of earthquake ground motions and correlation with house damages for the 2011 off the Pacific coast of Tohoku Earthquake

The 2011 off the Pacific coast of Tohoku Earthquake caused local house damages in Monou Town, Ishinomaki City, Miyagi Prefecture. To understand the mechanism of the earthquake damage, we conducted an earthquake damage survey and microtremor observation in Monou Town three months after the main shock. Then we estimated earthquake ground motions using microtremor H/V and the strong ground motion of the main shock observed in Monou Town. After earthquake damage survey, 310 houses were classified into 6 damage ranks. As the result, only about 3 percent of them were seriously damaged (collapsed). However, the locations of collapsed houses were limited around the Town Hall of Monou Town (Oda and Toda, 2011). We carried out microtremor observation and estimated strong ground motion of the main shock at each microtremor observation point using a method developed by Harada et. al. 2009). We classified estimated earthquake spectrums into 2 types according to predominant frequency (have a range of 3.0-4.0Hz or others). The result shows that locations of collapsed houses correspond to the locations where spectrum of estimated strong ground motion has predominant frequency of 3.0-4.0Hz. Therefore, these results present that the predominant frequency of 3.0-4.0Hz has an effect causing major house damages.