Upper-mantle seismic discontinuities in a subduction zone (Japan) investigated from P to S converted waves

The lateral variations of the upper mantle discontinuities in a subduction context are investigated by the use of broadband wave data provided by the GEOSCOPE network at the Inuyama station (Japan). Differential arrival times of P and P-to-S conversions on the discontinuities are used. These converted phases, though of weak amplitude, can be observed by stacking many seismograms. A stack of 50 data with high signal-to-noise ratio is performed by different techniques. The dominant period is around 7-10 s in this study, and the lateral resolution is estimated to be of the order of 300 km. This is an intermediate scale between short wavelength studies, which can resolve very small details, and long wavelength studies, for which the Fresnel zone is wider than about 1000 km. The first stacking technique allows to retrieve the mean depth of the discontinuities below the station. The main result is that the well-known discontinuity close to 660 km is depressed down to about 700 km, while the 410-km discontinuity is observed close to its usual depth, and that a discontinuity close to 520 km is visible. However, in such a heterogeneous region, the different traces are not coherent, and it is useful to account for the variability of arrival times of converted phases. Using alternative stacking techniques, we show that the 410-km discontinuity is elevated in the slab area of about 60 km, and the 660-km discontinuity is depressed by more than 50 km. This depression of more than 50 km found for the 660-km discontinuity yield to a value of -3 GPa K-1 of the Clapeyron slope for the phases change Olivine-gamma-Magnesiowustite + Perovskite. Although the 520-km discontinuity clearly shows up with all the techniques used, it is difficult to reliably access its depth variation in the slab area. The evidence for a 520-km discontinuity in this period range, while it is not observed at shorter periods, suggest that this discontinuity has a thickness between 15 and 25 km. The large elevation of the 410-km discontinuity suggests that the thermally controlled depth of the phase change may affect the dynamic behaviour of the slab. (C) 1998 Elsevier Science B.V. All rights reserved.