With the object of investigating influences of earthquake motion upon landslide, the observation of earthquakes and microtremors was enforced at the landslide area of Yui in the Sizuoka Prefecture. The ratio of spectra was gotten from the data of two sets of seismographs which had been placed so that a surface layer might be put between them, and under the ratio the density, rigidity and Q value of the surface layer were presumed by the aid of the multiple reflection theory. Next, by the aid of

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... their constants and the multiple reflection theory, acceleration and shearing stress which occur in the surface layer were calculated when a certain seismic wave enters from the base layer to the surface layer, and the shearing stress was compared with shearing strength that was gotten by soill tests. Further, by introducing acceleration and shearing stress, which were gotten according to the above-stated technique, to the calculation for stability of some sections in the Yui landslide are, influences of seismic force upon safety factor were examined.