Mitsuo Nogoshi

On the Attenuation Characteristics of Microtremors

The attenuation characteristics as the fundamental nature of microtremors are discussed at the present paper. The experimental field is Hakodate City in Hokkaido. Recently, the attenuations of Rayleigh waves and Love waves were investigated. However, the attenuations of microtremors have not yet been investigated. As the first step, the present paper described the discussion on the attenuation characteristics of microtremors by means of energy of wave motion. Energy ratio contours agree very well with the structure of the streets in Hakodate City. Namely, the higher level places of artificial disturbance ; factories and streets having larger traffic noises, show larger values of the energy ratios. This fact agrees well with the propagation directions which were obtained by tripartite observations of microtremors. Therefore, it is suggested that the source of microtremors may have a special frequency characteristics as a source function. Next, the attenuation of microtremors was obtained by calculating the distance-energy relation along an observation line. Fig. 15 and Fig. 16 show these results. Therefore, it is clearly found that both the vertical and the horizontal components of microtromors are attenuated as the invese first power of distance. These attenuation characteristics of microtremors seem to indicate a nature of surface waves. Attenuation coefficients were obtained as reasonable values.