Attenuation of Short Period Seismic Waves in the Upper Mantle and Its Regional Difference

Abstract

A spectral analysis is applied to the seismograms recorded by short period seismometer at Mt. Tsukuba to compare the attenuation of the seismic waves in the upper mantle both for the north-eastern and south-western parts of Japan. The epicenters of the earthquakes used on analysis are located in Hokkaido, Kurile, Sakhalin, Kyushu, Ryukyu and Taiwan.The Q values range from 700 to 5000 in the north-eastern part and from 200 to 1200 in the south-western part of Japan. The average of these values is 2100 in the former and 600 in the latter respectively. The ratio of the averaged Q in the north-eastern part to that in the south-eastern part of Japan is about 3.5 and its maximum is 10. The Q depends on the magnitude of the earthquakes occurring in Hokkaido and Kurile but not on that in Kyushu region. It should be explained by the fact that the geological structure is different in both regions and that the source spectrum might have a different scaling law with respect to the magnitude.