1983 年 36 巻 3 号 p. 383-392
An observation was carried out at Osakayama hill, situated in the southwestern part of Shiga Prefecture, to obtain data needed for studying the topographic effects on seismic ground motion. The Osakayama hill exhibits a simple topography extending nearly from north to south and the linearity of the ridge lends itself readily to two-dimensional modeling. Three-component seismometers were installed both at the ridge crest and in the disused railway tunnel running through below the hill, and many records were obtained simultaneously at both observation points from local earthquakes occurring in the Kinki district.
Analyzing the records frequency-dependent amplification of the motion at the ridge crest relative to the motion in the tunnel was observed clearly for the component perpendicular to the trend of the ridge. All the spectral ratio curves estimated have a peak commonly over a narrow range of frequency round 5Hz, with the maximum amplification of about 15. On the other hand, no clear peak was found for the component parallel to the trend of the ridge and the amplification was relatively small. Taking into account these phenomena, the horizontal particle motions were examined with the use of bandpass filtered seismograms, and then a specific feature reflecting the effect of topography was discerned. It was found that the motions at the crest are polarized in the direction perpendicular to the trend of the ridge compared with those in the tunnel, especially in the range of frequency 4-6Hz corresponding to the peak amplification. It was also found that the linearlity of particle motion increase gradually with time. Judging from the fact that all the phenomena mentioned above are observed independently of either azimuth or wave type, they were considered to be caused by resonance of the hill.