Abstract
The 2011 off the Pacific coast Tohoku Earthquake triggered powerful tsunami that caused major damage of the coast of from Kanto through Tohoku. However, house damages caused by strong ground motion of this earthquake was not so heavy relatively to its magnitude and seismic intensities. Although house damages was not so heavy on the whole, there are houses seriously damaged by the earthquake in Monou-Cho, Ishinomaki City, Miyagi Prefecture, Japan. The locations of damaged houses are limited in Monou-Cho. To figure out the cause of such local earthquake damage, we conducted investigation of house damages and microtremor measurements three months after the earthquake. We classified each house into 6 damage ranks according to the house damage investigation. The number of seriously damaged houses classified to rank A and B is 10 (approx. 3%) and its location is limited around the City Hall of Monou-Cho. Microtremor measurements was conducted at 46 points and predominant frequency of H/V spectrum ratio varied from 1 to 4Hz. Locations of seriously damaged houses classified to A and B correspond to points with predominant frequency over 2.5Hz. Housed would collapsed because of resonance phenomenon between strong motion and surface geology with predominant frequency over 2.5Hz.
