Amplification of seismic response of a large deep-seated landslide in Tokushima, Japan

Abstract

To better understand seismic site responses of deep-seated landslides, we carried out long-term seismic monitoring of a target landslide reactivated by a rainstorm in 2004 accompanying Typhoon Namtheun in Naka Town, Tokushima Prefecture, Japan. Five seismometers with high sensitivity were installed at different locations on landslide areas with different elevations. By examining numerous records of earthquake events, we distinguished the effects of material contrast and topography on these localized amplifications for different areas, and summarized their features, with help of detailed geological and geophysical surveys. To analyze differences in amplification, the HVSR method (calculating the horizontal to vertical spectral ratio of ground motion) was applied. The amplification on a landslide block with an HVSR peak amplitude smaller than 5 and resulting from topography is predominantly perpendicular to the direction of elongation of the mountain ridge. However, the amplification (revealed by HVSR peak amplitude of 3 to 5) resulting from material contrasts in multiple strata showed differing amplification directions. By examining the relationship between the HVSR peak amplitude and shear velocity profiles, we found that the amplifications at different frequencies are greatly affected by the extent of material contrasts. This may explain why the toe part of the slope, with ancient landslide deposits, generally shows the greatest ground motion (characterized by HVSR peak amplitude normally >10). This type of strong amplification due to material contrasts may result in severe damage to residential areas in mountainous areas during an earthquake and should be considered for the construction of buildings on landslide areas.