[S16P-05] Spectral amplification of ambient vibrations due to topography: Instrumental evidence from Chiba and Yamanashi Prefectures, Japan
Amplification of earthquake ground motions due to topography has been observed during past earthquakes. This phenomenon could lead to highly damaging landslides in the mountainous areas. However, as the availability of ground motion data in mountainous regions is limited, the topographical amplification is still poorly understood. Numerical simulations drive most research related to topographical amplification. Recently, ambient vibration studies have been employed to understand this phenomenon. In the absence of earthquake ground motion data, these studies help instrumentally determine the natural frequency of topographical features. In this study, ambient vibration studies have been conducted in two mountains. The first mountain is in Chiba prefecture and has a prominence of 73 m. The second mountain is in Yamanashi prefecture and has a prominence of 327 m. These mountains are selected such that they are free-standing as much as possible. It is easier to make discussions with free-standing mountains so that the effect of extended features need not be included. However, free-standing mountains are very rare. Accelerometers are temporarily employed at the base and summit of the mountains, and spectral amplifications are estimated from the corresponding ratio of Fourier acceleration spectra. The spectral amplification plots in both cases show the clear presence of a peak. The role of resonance in explaining the peak of these spectral amplification plots has been investigated. The results are significant from the point of view of assessing the potential of earthquake-induced landslides when earthquake scenario ground motions are available.