Buried Channel Delineation Using A Passive Surface-Wave Method In Urban Area

A passive surface wave method has been applied to delineate buried channels in urban area of<br>Japan. S-wave velocity structure down to 100m is very important in the local site effect of strong<br>ground motion caused by earthquake. Especially, the buried channels filled with alluvial deposits<br>intensify seismic waves and cause strong ground shaking. We have tried to delineate threedimensional<br>S-wave velocity structure down to 100m on the basis of the passive surface wave method. The test site is in Soka city, Saitama prefecture, Japan and the size of the site is about 3 km<br>square. Sixty-two passive surface wave methods were carried out to delineate buried channels filled<br>with alluvial deposits, which is embedded about 50m beneath surface of this area. Array size is<br>about 50 to 100m and triangular or L shaped arrays with 10 or 11 receivers were deployed. A spatial<br>auto correlation method was applied to the approximately ten minutes vertical component of microtremors<br>data. Phase velocity curves were calculated in the frequency range of between 2 and 10 Hz.<br>Fundamental mode of phase-velocity curves are clearly obtained in all observation points. A one<br>dimensional inversion using a non-linear least square method has been applied to the phase-velocity<br>curves and one-dimensional S-wave velocity structures were obtained. The resultant onedimensional<br>structures were interpolated into a three-dimensional structure. We succeeded to map<br>the shape of buried channel and the depths of the channel agree very well with the borehole data.