抄録
Experimental studies of surface waves are carried out, for a model which is shown in Fig. 1, by means of ultra-sonic techniques.
The relation between periods and amplitudes, when the layer thickness alone is varied, are investigated in the experiments.
In all cases, two wave trains are evident in the waves observed. One of two wave trains contains the waves having the maximum amplitude in all wave trains, its phase velocity is nealy equal to the shear velocity in the layer and its group velocity is smaller than that of RAYLEIGH waves in the layer. These wave trains have the periods that are shorter than that correspond to the minimum group velocity, the period correspond to the maximum amplitude is expressed by TVpl/H≈2.5, where T, Vpl and H indicate the period, the longitudinal velocity in the layer and the layer thickness respectively.
Another wave trains appear at the time expected by refracted P waves, its phase velocity is almost equal to the longitudinal velocity in the lower medium. This wave trains have the period expressed by TVpl/H≈3.
The waves, of which the phase velocity is equal to the shear velocity in the layer, correspond to the fundamental mode of RAYLEIGH waves.
Another waves, of which the phase velocity is almost equal to longitudinal velocity in the lower medium, is suspected to be complex roots in case of the normal mode solution.
The trains of these two waves are observed to lengthen as the epicentral distance increases.
The two wave trains do not change in phase velocity as well as group velocity, whereas they change in periods in proportion to the thickness of the layer when layer thickness varies.
