The effects of subsurface structure on the concentration of structural damage during the 2007 Noto-hanto Earthquake

During the 2007 Noto-hanto Earthquake in the central part of Japan, the town of Anamizu experienced severe concentrated damage, where more than 780 Gal was recorded at the K-NET station in horizontal motion. The predominant frequency of the record is about 1.0 Hz, which is widely known as causative of wooden house damages. More than seventy wooden structures were completely destroyed in the area of 500m square; especially high damage ratio was observed in the zone of 100 m square in that area. This distinguished damage concentration can be explained by constructive interference of waves due to subsurface topography. Extensive micro-tremor measurements were carried out to infer subsurface structure by H/V spectra. The predominant frequency at the station was about 1.5 Hz, which may have been lowered from 1.5 Hz to 1.0 Hz by the nonlinear effects of soil layers. The area with completely destroyed houses exhibits predominant frequency between 1.2 Hz and 3.0 Hz. A two-layered subsurface structure model was constructed by simulating H/V spectrum with frequency dependent particle motion of Rayleigh wave fundamental mode. The damage concentrated zone may be located at the mouth of an open-bay-like configuration basin with surface layer of 12 m thick. This subsurface topography was modeled by two-dimensional FEM, and ground motion during the main shock was evaluated as about 10 % larger than that of the K-NET station in terms of acceleration response spectrum around 1 Hz, which is due to constructive interference of reverberation and Rayleigh waves.