We estimated both P (Vp) and S wave velocity (Vs) structures beneath northeastern Japan by applying a tomographic method to 169,712 P and 103,993 S wave arrival time data from 4338 local events. The average value of Vp/Vs ratio is ~1.69 in the upper crust, ~1.75 in the lower crust, and ~1.77 in the uppermost mantle. These differences in Vp/Vs ratio may mainly reflect the lithological variations with depth. Low-Vp, low-Vs, and high Vp/Vs zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge. Here the low-Vp, low-Vs, and high Vp/Vs values are interpreted because of partial melting materials, which imply the presence of a vast amount of melt in the uppermost mantle. In the lower crust, low-Vp, low-Vs, and high Vp/Vs zones are not continuously visible along the volcanic front but are confined to individual volcanic areas, which suggests that melting of lower crustal materials occurs just beneath each active volcano. In contrast, the upper crust beneath active volcanoes exhibits low Vp, low Vs, and low Vp/Vs. These results suggest that the partial melting zones under northeastern Japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes. The low-Vp, low-Vs, and low Vp/Vs features in the upper crust suggest the existence of H2O (rather than melt) right beneath active volcanoes. ¿ 2001 American Geophysical Union