[1] We apply a three-step approach to estimate three-dimensional (3-D) P wave attenuation (Q p À1 ) structure beneath northeastern Japan. First, corner frequencies of earthquakes are determined using the spectral-ratio method for S-coda waves. Then, whole-path attenuation terms, t*, and site-amplification factors are simultaneously estimated by a joint inversion. The set of t* is finally inverted for 3-D attenuation structure. The results show that the mantle wedge has low attenuation in the fore arc and high attenuation in the back arc. A depth profile of Q p À1 in the back-arc mantle is explained by attenuation expected for a two-dimensional (2-D) thermal model with Q p /Q s = 2 and grain sizes of 1 and 3 cm. However, an inclined high-attenuation zone observed in the back-arc mantle wedge, which is interpreted as an upwelling flow, shows higher attenuation than that calculated from the 2-D thermal model. The higher seismic attenuation is probably caused by the concentration of partial melt in the upwelling flow. A combined interpretation of seismic attenuation and velocity structures further suggests that the degree of partial melt in the upwelling flow varies along the arc and that volcanoes are clustered transverse to the arc, below which the upwelling flow contains a higher degree of melt. These observations indicate that magmatism is controlled by a mantle-wedge process that depends strongly on spatial variations in the degree of partial melt in the upwelling flow. Our results further imply the breakdown of hydrous minerals in a hydrous layer above the Pacific plate at a depth of~120 km.