Three-dimensional anelasticity (Q_s^-1) structure of the upper mantle beneath the northeastern Japan arc is obtained using spectral amplitude ratio of P and S waves observed by the seismic network of Tohoku University. An S-to-P spectral ratio technique is adopted to avoid dependance on the source spectra of the earthquakes used here, assumingQ_p/Q_s to be 2.25. The crust and upper mantle are divided into many rectangular blocks, and an anelasticity parameter of each block is deduced by using the damped least squares method.
The anelasticity structure of the crust and upper mantle beneath the northeastern Japan arc consists of an extremely high-Q (Q_s≅1500) inclined oceanic plate and a relatively low-Q land-side region which can be divided into two zones: an intermediate-Q (Q_s≅500-800) land-side mantle between the Pacific coast and the volcanic front, and a low-Q (Q_s≅150) land-side mantle between the volcanic front and the coast of the Japan Sea. Moreover extremely low-Q zones exist in the crust beneath the active volcanoes.