On the Attenuation of S-waves and the Structure of the Upper Mantle in the Region of Japanese Islands

Abstract

S-waves of 51 deep earthquakes observed during the past 30 years around Japan were examined for the purpose of studying their attenuation caused by passing through different geographical locations and depths.
First, log A/T-Δ curves were prepared and their relation to the respective seismic paths was investigated. Corrections were made to obtain A, taking into consideration the effects from geometrical dispersion in space, azimuthal distribution by the mechanism of earthquake occurrence, and reflection at the surface.
A close relation between the attenua t ion along a seismic path and seismic active space was found, and various types of observed seismic motions on the ground were interpreted.
Seismic active space ( SA-zone) is composed of shallow and deep ones (SSA- and D SA-zone) and they meet under the MS (Morioka-Shirakawa) line in East Japan and the BK (Beppu-Kagoshima) line in Kyushu.
Vertical distance of the gap of these two zones is about 100 km near the line of joint, and that part corresponds to the portion of the largest attenuation. This portion is located under the zone of active volcanoes. The wider the gap, the weaker is attenuation with increasing horizontal distance towards the west of MS and BK lines. The well-known phenomenon about the abnormal distribution of seismic intensities in Japan was explained from the results of this investigation.