The structure of the seismic wave attenuation zone beneath the Yakedake Volcano in the Hida Mountain range of central Japan was disclosed by a temporary seismic observation array across the mountain range. The amplitudes of P waves from four distant offset explosions showed abrupt decay of seismic wave amplitude under the volcano. However, the amount of decay was dependent strongly on the azimuth. This suggests the existence of a very localized attenuation zone at a shallow depth. To explain consistently these observed data, a vertical wall-like attenuation zone that extends beneath the volcano in the strike of N33°W is suggested. The attenuation ratio that equals the product of the attenuation coefficient and the width of the zone is 0.6. The Q derived from this value is estimated as 30 at 15 Hz if we assume the width of the zone as 2 km. The estimated depth of the top of the zone is about 1 to 2.5 km below the surface. The location coincides well with not only a high geothermal belt near the volcano but also with the active fault that stretches southeast from the volcano.

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An Indirect Boundary Element Method that uses the full-space Green's function is proposed for the calculation of in-plane seismic response of irregularly stratified media with buried interfaces extended infinitely. The method uses the solution for horizontally stratified media obtained by the Thomson-Haskel propagator matrix method in the wavenumber domain as the reference solution. A recursive matrix operation is introduced to solve the boundary integral equations, which in turn facilitates the calculation of multi-layer problems, preventing excessive requirements of computer main memory. The proposed formulation efficiently eliminates non-physical waves due to numerical truncation.

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Two-dimensional SH-wavefields in laterally heterogeneous multi-layered media are calculated by using a hybrid method in which the boundary element method (BEM) is combined with the thin layer finite element method (TLFEM) by matching the boundary conditions on vertical boundaries. The traction operators defined on vertical boundaries are used to match the boundary conditions. The traction operators can be approximately calculated from eigenvalues and eigenfunctions for the multi-layered media, which are obtained by using the TLFEM. This method is efficient to calculate scattering of surface waves in laterally heterogeneous multi-layered media. Scattering of Love waves in multi-layered slope structure is investigated. For the case in which a source is in the shallower side, Love waves mainly propagate in the forward direction. The dispersion of Love waves shows an averaged structure of both sides. For the case in which a source is in the deeper side, the amplitude of scattered waves is large, and scattered body waves and back-scattered Love waves exist.

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