Three-dimensional attenuation structure of intrinsic absorption and wide-angle scattering of S waves in northeastern Japan

Abstract

SUMMARY Intrinsic absorption and random velocity inhomogeneities are important medium properties for studies on wave propagation at high frequency (>1 Hz) in the lithosphere. Recent studies have shown that the spatial distribution of random velocity inhomogeneities can be estimated by an inversion analysis of peak delay times, where the peak delay time is the time lag between direct wave onset and its maximal amplitude arrival. This study estimates the 3-D structure of the S-wave attenuation quality factor Q−1, after we corrected the maximal amplitude attenuation due to multiple forward scattering on the basis of the Markov approximation for parabolic wave equation. Even though this estimate of Q−1 includes both intrinsic absorption and wide-angle scattering, the contribution of wide-angle scattering can be identified by taking into account random inhomogeneities estimated in the peak delay time analysis. The estimated Q−1 structure in northeastern Japan shows weak attenuation (Q−1(f) ∼ 10−2.5f−0.5) on the forearc side of the volcanic front, and strong attenuation (Q−1(f) ∼ 10−2.3f−0.3) beneath the Quaternary volcanoes and near the collision zone between the Honshu and Kuril arcs. According to the Born approximation, the estimated Q−1 at 0–40 km depth consists mainly of the intrinsic absorption if the characteristic scale of random inhomogeneities is 5 km or longer. Beneath the Quaternary volcanoes of northeastern Japan, Q−1(f) shows weak frequency dependence and the power spectral density functions of random velocity inhomogeneities are characterized by a weak spectral gradient that means rich in small-scale inhomogeneities. These results demonstrate the importance of the frequency dependence of wave propagation in the lithosphere. That is, the scale dependence of velocity fluctuation or small-scale inhomogeneities is an important property of the lithosphere underlying Quaternary volcanoes.