The Effects of Topography on Damage Concentration in a Mountainous Area

Abstract eng:
The 2014 Nagano prefecture Kamishiro fault Earthquake of magnitude 6.7 occurred on November 22, which jolted central mountainous area in Japan. The maximum seismic intensity was lower 6 on the Japanese scale of 7. Fortunately there was no dead or missing, but a lot of houses were damaged in northern part of Nagano prefecture, including Hakuba village. According to the local government statistics, damaged houses in the village were remarkably concentrated in two areas namely Horinouchi and Mikkaichiba. There found 39 houses collapsed and 51 houses partially destroyed, while damaged houses were scarcely observed in the surrounding areas of several km expanse. On the cause of the damage concentration in the village, general conclusions have not been reached so far. Our study is aiming at contribution of shallow subsurface structure to the local damage concentration. We carried out high density micro-tremor observation in the area at sixty locations using portable miniature array of radius 0.9 m in the summer of 2015. K-NET and KiK-net earthquake observation stations, which are outside of damage concentrated area, were included since they are with PS logging data. We extracted dispersion curves and H/V spectra from the micro-tremor observation, deduced average shear wave velocity from Rayleigh wave velocity at reference wavelengths, and inferred surface layer thickness from predominant frequency of H/V spectra by the law of quarter wavelength. We conclude that the areas with thick soft layer encircled by relatively hard soil were seriously damaged.

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