An Observed Effect of Topography on Seismic Ground Motions

Abstract

An observation of local earthquakes occurring in the Kinki district was carried out at the Osakayama hill, situated in the southwestern part of Shiga Prefecture, in order to obtain data pertaining to effects of topography on seismic ground motion. Two sets of 3-component seismometers were installed respectively at the ridge crest of the hill and in a disused railway tunnel running through below the hill. Analyzing some good quality records obtained simultaneously at the both observation points, a frequency-dependent amplification of motion at the crest relative to motion in the tunnel was clearly discerned for the component perpendicular to ridge strike. Horizontal particle motions and temporal variation of amplitude ratios were examined in detail with the use of bandpass filtered seismograms, and then it was found that the motions at the crest are polarized in the direction perpendicular to ridge strike compared with those in the tunnel. It was also found that the linearity of particle motion increase gradually with time. Judging from the fact that all the above mentioned phenomena are observed independently of either azimuth or wave type, they were attributed to resonance of the hill.