抄録
Three dimensional P and S waves velocity structure of the crust and the uppermost mantle beneath the northeastern Japan arc is investigated by an inversion of P and S waves arrival times observed at the seismic network of Tohoku University from local earthquakes.
The results show that, in the uppermost mantle, there exists a dipping high velocity zone of P and S waves which corresponds to the descending Pacific plate. Beneath active volcanoes low velocity zones of P and S waves are clearly seen, which are dipping westward and extend to a depth of about 100km.
Although the pattern of P wave velocity distribution is very similar to that of S wave velocity distribution, change in absolute value of S wave slowness perturbation is bigger than that of P wave slowness perturbation, indicating the spatial change in VP/VS value. Within the Pacific plate VP/VS values are generally low, whereas the low velocity zones existing beneath active volcanoes have high VP/VS values.
