Abstract
Observations have been made of microtremors in an area around the landslide which occurred on the northern slope of Mt. Iizuna in Kinasa-mura, Nagano-ken, April 1973. The spectral curves have a clear appearance with one dominant peak at a period of 0.2 to 0.3 sec. at the locations, where the landslide produces very little effect. While the amplitude spectra near the landslide region spread over a period range of 0.3 to 1.0 sec., and for this reason, it becomes difficult to evaluate accurrately the predominant period. The effect of viscosity may be considered in the superficial layer to account for such spectra. By comparison with the calculations of the amplitude due to multiple reflections of SH waves in two layered half space, the viscosity coefficient is deduced to be about 5×106 to 107 CGS near the landslide region This value corresponds to the Q value of about 1 to 2.
After about one and a half year from the occurrence of landslide, observations were carried out again near the landslide region. From these observations it was concluded that the amplitude spectra vary so that the predominant period appears clearly in the period range of 0.3 to 0.4 sec. Because of a decrease of water content in the superficial layer, the elastic properties may be considered to acquire a stable state perior to the occurrence of landslide.
