Precursors to SS and structure of transition zone of the north‐western Pacific

In the records of the Grafenberg observatory and of a few other stations we have detected long-period SH reflections at the underside of mantle discontinuities. The bounce points of the reflections are located in the northwestern margins of the Pacific. The data are used to evaluate S660S - S410S differential times which are directly related to the thickness of the layer between the two discontinuities. Most of the bounce points are in the area of the Japan and Kuril - Kamchatka subduction zones where a large-scale depression of the 660-km discontinuity was previously found and attributed to a cooling effect of subduction [Shearer & Masters, 1992]. According to our data the differential time in this region is 3 s less than in the IASP91 model. This anomaly suggests a thinning of the layer between the two discontinuities instead of thickening which is expected from depression of the lower discontinuity. Our analysis also indicates that there is a strong reflector in the study region at a depth of 900 km.