Three-dimensional structure of V_p, V_s and V_p/V_s beneath northeastern Japan: Implications for arc magmatism and fluids

We estimated both P (V_p) and S wave velocity (V_s) 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 V_p/V_s ratio is ∼1.69 in the upper crust, ∼1.75 in the lower crust, and ∼1.77 in the uppermost mantle. These differences in V_p/V_s ratio may mainly reflect the lithological variations with depth. Low-V_p, low-V_s, and high V_p/V_s 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-V_p, low-V_s, and high V_p/V_s 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-V_p, low-V_s, and high V_p/V_s 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 V_p, low V_s, and low V_p/V_s. 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-V_p, low-V_s, and low V_p/V_s features in the upper crust suggest the existence of H₂O (rather than melt) right beneath active volcanoes.